Electromagnetic fields at workplaces

Federal Ministry of Labour and Social Affairs

Forschungs penavi Sozialforschung

400-E

Electromagnetic fields at workplaces

Final Report

ISSN 0174-4992

Impressum:

Herausgeber: Bundesministerium furArbeit und Soziales

Referat Information. Publikation, Redaktion

53107 Bonn

Stand: NOvember 2011

Artikel-Nr.: FB 400-E

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Die Durchfiihrung der Untersuchungen sowie die Schlussfolgerungen aus den Untersuchungen sind son den Auftragnehmern in eigener wissenschaftlicher Verantwortung vorgenommen worden. Das Bundesministerium fiir Arbeit und Soziales ithernimmt insbesondere keine Gewahr fiir die Richtigkeit, Cenauigkeit und Vollstandigkeit der Untersuchungen.

Der Umwelt zuliebe gedruckt auf100%Recyclingpapier.

Bericht der EMF-Arbeitsgruppe desBundesministeriums fur Arbeit und Soziales

Elektromagnetische Felder am Arbeitsplatz

Ein neuer wissenschaftlicher Ansatz fur die Sicherheitund den Gesundheitsschutz der Beschaftigten

Electromagnetic fields at workplacesA new scientific approach to occupational health and safety

F. BornerH. Bruggemeyer

S. EggertM. FischerH. Heinrich

K. HentschelH. Neuschulz

Stand: November 2011

Executive summary

This report provides an in-depth analysis of the physical and physiological background for aneffective protection of the health and safety of workers with respect to occupational exposureto electric, magnetic and electromagnetic fields (EMF), based on current scientific knowledge.Answers are given to the concerns being raised by stakeholders and to shortcomings within Directive2004/40/EC. Therefore, information provided in this report, especially the figures and tables insection 4.1 and 4.2, can serve as a sound base for a review of the risk-related provisions of Directive2004/40/EC.

A revised concept of exposure limit values for the low frequency electric and magnetic fields isbased on the physiologically relevant parameter of the peak electric field strength in the tissue andrepresents common scientific understanding. Based on this concept a set of exposure limit valueshas been laid down guaranteeing the health and safety of workers without the need for unnecessaryand costly measures or unduly impacting the use of certain technologies or industrial processes. Foran easy and also cost-effective assessment of the risks due to the exposure to low frequency electricand magnetic fields and in order to avoid unnecessary complex and time-consuming calculationscurrently necessary to show the compliance of an exposure situation with the exposure limit values,two sets of easier-to-implement action levels are given. These action levels can be compared directlywith measurable electric field strengths or magnetic flux densities.

Because all EMF-related biological effects in the low frequency range are linked to peak valuesof the internal electric field strength in the tissue, all exposure limit values and lower and upperaction levels are given as peak values and not as rms-values as in Directive 2004/40/EC.

The report also addresses the risks of workers with respect to the movement and the projectile riskin static magnetic fields. For the low frequency range it provides a sound solution on how to dealwith pulsed electric and magnetic fields, multi-frequency electric and magnetic fields and contactcurrents. Contact currents are now classified as exposure limit values because of the biologicalrelevance.

For both the static and the low frequency range, effects of localized exposure and time or spatialaveraging are considered in the report. So far, no changes have been proposed for frequencieshigher than 100 kHz.

Preface

Due to the ongoing technological development and scientific research regarding occupational ex-posure to electric, magnetic and electromagnetic fields, this report presents the current knowledgeand understanding of open questions and concerns on a solid and well established scientific founda-tion. This report provides an in-depth analysis and the most up-to-date information available forthe ongoing discussion concerning occupational health and safety with regard to workers exposureto static and low frequency electric and magnetic fields. If necessary, it will be updated when newtechnologies emerge, new studies and results become available or new questions and concerns arebeing raised.

Apart from the considerations in this document, additional guidance and information may benecessary to assist the employer in risk assessment, thus saving time and money while guaranteeingthe safety and health of workers at the same time.

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Physiological effects of EMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Direct effects of electric, magnetic and electromagnetic fields . . . . . . . . 2

2.1.1 Electric fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1.1.1 Static electric fields . . . . . . . . . . . . . . . . . . . . . 2

2.1.1.2 Low-frequency electric fields . . . . . . . . . . . . . . . . 3

2.1.2 Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.2.1 Static magnetic fields . . . . . . . . . . . . . . . . . . . . 4

2.1.2.2 Low-frequency magnetic fields . . . . . . . . . . . . . . . 5

2.1.3 High-frequency electromagnetic fields . . . . . . . . . . . . . . . . 6

2.2 Indirect effects of electric and magnetic fields . . . . . . . . . . . . . . . . . 6

2.2.1 Electric fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.2 Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 Body models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Neurophysiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Mechanisms and facts for the creation of action potentials . . . . . . . . . . 8

3.2 Electrical stimulation of excitable tissues . . . . . . . . . . . . . . . . . . . 12

3.2.1 Basic facts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2.2 Long stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2.3 Short stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2.4 CNS tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2.5 Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4 Limiting occupational exposure to static and low frequency electric and magneticfields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1 Exposure limit values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1.1 Static electric fields . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1.2 Static magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1.3 Low frequency electric and magnetic fields . . . . . . . . . . . . . 20

4.1.4 Contact currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.2 Upper and lower action levels . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.2.1 Upper action level . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

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4.2.1.1 Electric fields . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.2.1.2 Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . 26

4.2.2 Lower action level . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2.2.1 Electric fields . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2.2.2 Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . 29

5 Special exposure situations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.1 Simultaneous exposure to electric and magnetic fields . . . . . . . . . . . . 31

5.2 Simultaneous exposure to multiple field sources operating with the samefrequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.3 Simultaneous exposure to multiple frequency fields . . . . . . . . . . . . . . 31

5.3.1 Summation formulae . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.3.2 Assessment of fields with arbitrary temporal behaviour . . . . . . 32

5.3.3 Harmonic content . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.4 Localized exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.5 Movement in static magnetic fields . . . . . . . . . . . . . . . . . . . . . . . 34

5.6 Interference with active implanted medical devices (AIMD) . . . . . . . . . 34

5.7 Projectile risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Annex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

A Quantities, variables, abbreviations and SI-units . . . . . . . . . . . . . . . . . . . 44

B Tissue data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

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1 Introduction

On 29 April 2004, the European Parliament and the Council adopted Directive 2004/40/EC on theminimum health and safety requirements regarding the exposure of workers to the risks arising fromphysical agents (electromagnetic fields). This directive is commonly referred to as the EMF Direc-tive. It establishes minimum health and safety requirements for the protection of workers againstthe risks arising from exposure to static and time-varying electric, magnetic and electromagneticfields (EMF). The frequency range extends from 0 Hz to 300 GHz.

Directive 2004/40/EC obliges the employers to assess the risks arising from electric, magnetic andelectromagnetic fields at the workplace and to take adequate measures to eliminate or to minimizesuch risks where necessary. The Directive refers to a set of exposure limit values (ELV) listed intable 1 in its Annex. The exposure to electromagnetic fields cannot be measured directly becausethe physiologically relevant physical quantities, e.g. current density and specific absorption rate,only exist inside the human body. In order to facilitate the application of the directive a set ofso called action values (AV) was given to simplify the determination of the level of exposure at aworkplace. If these action values are not exceeded, an inherent compliance with the exposure limitvalues is guaranteed.

However, the exceedance of the action values does not automatically lead to an exceedance of theexposure limit values. Where action values are exceeded, employers can make further efforts toassess and, if necessary, prove that the exposure is still below the exposure limit values.

Since the adoption of Directive 2004/40/EC scientific knowledge with regard to

• the concept of limit values,

• risks related to the movement in a static magnetic field,

• the projectile risk,

• risks related to pulsed electric and magnetic fields,

• risks related to multi-frequency electric and magnetic fields,

• risks related to contact currents,

• risks related to implanted medical devices

has significantly improved.

This report will address these aspects on a solid and well established scientific and technologicalbasis.

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2 Physiological effects of EMF

The physiological effects of electrical, magnetic and electromagnetic fields on the human body aredependent on the frequency.

The effects of static electric fields are limited to the surface of the human body and can causemotion of body hair and corona discharges.

Static magnetic fields exert forces on ferro- and dia-magnetic materials as well as charged movingparticles. This may lead to acceleration, torque effects and the induction of electric fields in thetissue.

In the low-frequency range up to some 100 kHz the main physiological effect is the electricalstimulation of excitable body tissues like muscles, nerves and sensory organs.

In the frequency range between several 100 kHz and some MHz electrical stimulation and tissueheating occurs. The higher the frequency, the more the tissue heating effects increase and thestimulation effects decrease. Tissue heating effects are dominant for frequencies above severalMHz.

A further distinction is made with regard to the interaction with the human body. If there is adirect interaction between EMF and the human body, e.g. stimulation of muscles, nerves or sensoryorgans or tissue heating, this type of interaction is called an direct effect.

If there is an interaction between EMF and objects outside the human body, e.g. contact currents,projectile risk or the interference with implanted medical devices, this type of interaction is calleda indirect effect.

There are no confirmed long-term health effects related to the exposure to EMF.

2.1 Direct effects of electric, magnetic and electromagnetic fields

2.1.1 Electric fields

The relationship between the external undisturbed electric field strength E0 and the electric fieldstrength hereby induced in the body tissue Ei is established through the condition that the normalcomponent of the displacement current must remain steady at the surface boundary of the humanbody [30, 41].

For a simple homogeneous ellipsoid model of the body it is expressed by:

E0 · k · ε0 · 2π · f = κ · Ei (2.1)

with k field distortion factor; for human beings k ≈ 13 . . . 18

ε0 permittivity of free space (vacuum); ε0 = 1µ0·c20

≈ 8, 854 · 10−12 A2·s4kg·m3

f frequency of the fieldκ (mean) conductivity of the body tissue(s)

In principal eqn. 2.1 remains valid even for more natural and anatomically correct body models.However, the variables k and κ become parametric functions.

2.1.1.1 Static electric fields

As an immediate result of eqn. 2.1 for static fields (f = 0) it follows that the electric field strengthinside the tissue Ei is (nearly) zero, regardless of the electric field strength of the external staticelectric field. The external static electric field breaks down completely at the surface of the hu-man body, the inner body is totally shielded from any effect of the external static electric field.Therefore, no direct physiological effect can occur inside the human body.

External static electric fields, exceeding E0 ≈ 30 kV/m, can cause corona discharges at the surfaceof the human body, e.g. fingers, nose, ears, hairs [41, 63]. Those corona discharges depend on the

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external field strength, posture, the size and form of the body and climatic factors, e.g. relativehumidity. Such discharges can be annoying, startling or even painful.

Significant external static electric fields can only occur where high DC-voltage is used (DC-powerlines, including switchyards and inverter stations) or can be produced by triboelectricity,e.g. plastics production and other industrial processes where highly insulating solids or liquids arehandled and charge separation can occur.

2.1.1.2 Low-frequency electric fields

External low frequency (LF) electric fields can generate internal electric fields in the tissue. Con-sulting eqn. 2.1 the relationship between external and internal field strength can be rewrittenas:

Ei =k · ε0 · 2π · f

κ· E0 (2.2)

As (k ·ε0 ·2π ·f) is very small compared to κ in the low frequency range, there still exists a shieldingeffect from the outside to the inside of the human body. However, it is not a complete shieldinglike in static electric fields.

Therefore there is the potential for adverse effects inside the human body. However, externalelectric fields used by technological processes or near electric powerlines are generally not strongenough to cause adverse health effects.

Especially for power frequency fields with an external electric field strength of some (kV/m) theelectric field strength in the body tissue is in the range of some (mV/m). With very high electricfield strengths currently in use by technological processes (mean value≈ 180 kV/m for experimentalhigh-voltage transmission lines with voltages >1500 kV [46]) and limited by the breakdown fieldstrength of air (≈ 3000 kV/m for homogeneous fields [41, 46]), it is not possible to generateelectrical fields in the tissue of the human body that can trigger any adverse physiological effectslike electrophosphenes (E0 >200 kV/m at 50 Hz) or peripheral nerve stimulation (E0 >4000 kV/mat 50 Hz).

Very strong electric fields higher than 30 kV/m can also cause corona discharges on the surface ofthe human body.

However, these electrical field strengths in the tissue can interfere with the proper operation ofactive medical implants, e.g. pacemakers or cardioverter defibrillators, see section 5.6.

2.1.2 Magnetic fields

Magnetic fields exert physical forces on electric charges, but only when such charges are in motion.There are three physical effects of magnetic fields on biological tissues:

• Electrodynamic forces and magnetic induction

• Magneto-mechanical effects

• Electron spin interaction

The main effect of a magnetic field is the Lorentz force ~F on a point charge q moving with velocity~v as described by:

~F = q(~v × ~B

)(2.3)

Due to these forces charge transfers develop in the biological tissue. They generate differences inthe electrical potential and thus an electrical field strengths in the tissue of the human body. The

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connections between electrical field strength and the magnetic flux density describes Faraday’s lawof induction [30]. ∮

~E · d~l = − d

dt

∫~B · d ~A (2.4)

The left-hand side of eqn. 2.4 is a line integral over a closed loop and the right hand side is thetime derivative of a surface integral of the normal component of the magnetic flux. The equationonly calculates the average electric field over the loop, but it is often the only measure availablewhen the actual local field in a complex system can only be estimated with numerical methodsthat require very detailed knowledge about the fields on the system boundary and its materialproperties. If we assume that the bulk conductivity of the material is relatively homogeneous, wecan also infer the average induced current by Ohm’s law.

The equation will register an average electric field when the integral changes with time. If weconsider the loop to be of fixed dimensions, this can happen in several different ways:

• the magnetic field itself varies with time. This is the typical situation for many field studiesin which a spatially homogenous field is modulated, e.g. with a sine wave;

• by motion in a field that has spatial variation. This is, for instance, relevant when trans-porting a person into or out of a magnetic resonance imaging (MRI) machine that has verystrong spatial gradients at the opening to the bore;

and

• the relative orientation between the loop and the field vector is changed. This happens whenwe rotate the loop in a static field.

The field gradients are decisive and can enhance the induction effect.

It makes no difference whether a person is stationary in a field changing over time or whether aperson moves in a constant magnetic field. In both cases the effect is the same: Induction of anelectrical field in the body tissue.

2.1.2.1 Static magnetic fields

Current data suggests possible pathological effects of static magnetic fields, e.g. induced blood flowpotentials around the heart which might interfere with the autonomous heart action, increasesin blood flow resistance due to magneto-hydrodynamic effects, can occur only in magnetic fluxdensities exceeding 10 T [54, 55, 56, 71, 99, 112, 107].

However, it must he noted, that the actual data base is very small for flux densities exceeding 8T.Available studies for flux densities exceeding 8 T are often not replicated.

Detailed information of direct effects of static magnetic fields, especially for MRI and magneticresonance spectroscopy (MRS) applications can be found in [54, 55, 56, 99, 112]. However, allconclusions must be carefully examined to determine if they are really due to the direct physiologicaleffects of static magnetic fields, because they are sometimes mixed up with effects from time-varyingmagnetic fields or movements in static magnetic fields.

Changes in a static magnetic field, e.g. time-variation or movement, induces electric fields in bodytissues. The induced field may interact with the human body by several mechanisms. The mainmechanisms are sensory or nerve stimulation – see section 3. The occurrence of these effects isdependent on the temporal gradient of the field or the spatial gradient of the field and the movementspeed of the subject.

Movement in a static magnetic field – see section 5.5 – causes a low frequency internal electric fieldin the tissue.

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The strongest static magnetic fields are currently used in MRI with flux densities up to 14 T andMRS with magnetic flux densities up to 25 T.

Other sources of strong static magnetic fields are thermonuclear reactors, magnetohydrodynamicsystems, particle accelerators, and superconducting generators. Industries where strong magneticfield exposure can occur are those involving electrolytic processes such as chlorine or aluminumproduction and in the manufacture of permanent magnets and magnetic materials. The typicalexposures in these industries are a few mT of the working day with peak exposures up to severaltens of mT.

2.1.2.2 Low-frequency magnetic fields

The main physiological effect of low-frequency (LF) magnetic fields is the induction of electricalfields in the human body and the stimulation of excitable body tissues, like sensory organs, nervesand muscles.

Because different excitable body tissues have different maximum sensitivities with respect to thefrequency, the major points of interaction change with the frequency. Table 2.1 shows for somephysiological effects their major point of interaction and their frequency range of maximum sensi-tivity.

Maximum sensitivity Physiological effect Point of interaction

1 Hz Metallic taste Various receptors in the tongue(shift in ion gradients)

< 0.1 . . . 2 Hz Vertigo, nausea Inner ear (vestibular system)Blood flow induced electric Nerve, muscle excitationfields in tissue (interference with heart action)

≈ 20 Hz Magnetophosphenes Retina

≈ 50 Hz Tactile and pain sensations Peripheral nervesLoss of muscle control Peripheral nerves, musclesInterference withautonomous heart action Heart

Table 2.1: Frequency range of maximum sensitivity and major point of interaction forsome physiological effects

Extremely low frequency sensory effects caused by movements in a strong static magnetic field areexperienced with flux densities above 2 - 3 T [26, 36, 56]. The maximum sensitivity is expected atfrequencies around 0.1 Hz. Sometimes these effects last longer than the actual field exposure andcan be detrimental to work performance and quality.

Pathological effects of blood flow induced electric fields in the tissue causing nerve and muscle ex-citation in the immediate vicinity of these blood vessels or that may interfere with the autonomousheart action are expected for flux densities exceeding 8 - 10 T [99].

However, exposure during typical industrial processes, including e.g. electrolysis, electroplating orwelding is well far below these threshold values.

Magnetophosphenes give the magnetically evoked appearance of light spots in vision. They have avery sharp response peak (maximum sensitivity) at ≈ 20 Hz. For lower frequencies the sensitivitydecreases approximately proportional with f , for higher frequencies the decrease in sensitivity isproportional to nearly f3.

At the frequency 50 Hz there is the maximum sensitivity for nerve and muscle stimulation. How-ever, the response curve is very flat in the frequency range from 10 Hz to some hundreds of Hz.

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Only for frequencies higher than 3 - 5 kHz the sensitivity decreases approximately proportionalwith f .

In general, the thresholds for the direct stimulation of muscles are much higher than for nervousstructures. However, the exposure at most industrial workplaces is far below both threshold values.

All these physiological effects have a clearly defined threshold. Any stimuli below the thresholdvalue will not cause an adverse effect, even when applied for a long time [50] – see also section 3.

A comprehensive compilation of direct physiological effects of LF magnetic fields can be found in[51, 54, 55, 57, 63, 64, 68, 70, 82, 89, 90, 91, 108].

Low frequency magnetic fields are ubiquitous at workplaces where electric energy is used. Mag-netic fields will be produced e.g. by transmission lines, underground cables, distribution lines,transformers, electric railway systems, household appliances, resistance and induction heating sys-tems, hand-held electric tools and arc, spot and resistance welding equipment.

Exposure to low frequency magnetic fields at workplaces in terms of magnetic flux density rangesfrom some nano- or microtesla, e.g. in office buildings, up to several tens or hundreds of millitesla,e.g. at industrial workplaces.

The frequency range covered reaches from fractions of 1 Hz, e.g. movement in static magneticfields, up to some tens or hundreds of kHz, e.g. induction heating.

2.1.3 High-frequency electromagnetic fields

The direct effect of high frequency (HF) electromagnetic fields is the penetration of HF-electromagnetic fields in the body and the absorption of energy in tissues. The energy absorptioncauses an increase of temperature in the tissue which could lead to an increase in body temperature.

To prevent adverse health effects the increase in tissue and body temperature must be limited. Acommonly used value is to limit the temperature increase in the tissue caused by an electromagneticfield to a maximum value of 1 [51, 106].

The penetration depth into the biological tissue depends on the frequency of the electromagneticfield and the electric properties of the body tissue. The higher the frequency of the electromagneticfield and the electrical permittivity of the tissue, the shorter the penetration depth.

For continuous-wave exposures with frequencies exceeding 10 GHz the penetration depth is veryshort and the total energy is absorbed in the top layers of the skin.

2.2 Indirect effects of electric and magnetic fields

2.2.1 Electric fields

Static electric fields can accelerate dust particles towards the worker and therefore enhance thedust deposition on the worker. This can lead to allergic and inflammatory reactions in sensitivepersonnel.

Movement or vibration of body hair can also occur in static and time-varying electric fields, creatinga possible annoyance. However, the perception threshold of hair vibration shows a wide individualvariation [10, 108].

Contact currents occur, if a worker touches a charged object or touches a grounded object whilebeing charged himself, due to exposure to a electric field or due to triboelectricity. The resultantphysiological effect is largely dependent on the size of the contact area, e.g. touch or grasp contact,and on the amount of discharge energy and transferred charge, as well as the amplitude andfrequency of the continuously flowing contact current. These effects can be annoying, painful orcan have life threatening consequences [18, 58, 59, 60].

In general, two different phases of a contact current event can be distinguished:

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• a spark discharge, i.e. an initial discharge current impulse

• a continuous contact current

Depending on the specific exposure scenario only one or both phases of the contact current eventmight be present. Usually, the initial discharge current with a duration in the sub-millisecondrange is only present for exposure situations involving either a static or time-varying electric field.

In general, a continuous contact current is linked to time-varying electric or magnetic fields, butcan also occur in conjunction with ongoing triboelectric processes. The frequency of the continuouscontact current depends on the frequency of the causal time-varying electric field, but can also bea DC current in case of triboelectric processes.

Therefore it is necessary to limit both phases of the contact current event.

The thresholds for perception and pain are lower for touch contact when compared to graspingcontact. For a frequency of 50 Hz the perception thresholds for such touch and grasp currents arein the range of 1 . . . 3.5 mA (rms). For frequencies in the 100 kHz and MHz range, the thresholdsare up to 40 . . . 50 mA (rms) [3, 10, 13, 21, 22, 39, 58, 59, 60, 106, 108].

If in a certain workplace environment, e.g. high-voltage switchyards, spark discharges or contactcurrents cannot be avoided by technical measures, workers should be trained to always make graspcontact or instructed to use special work techniques, e.g. equalization of potentials, or work gear,e.g. insulating or conductive gloves.

2.2.2 Magnetic fields

Indirect effects of static and time-varying magnetic fields are translational and rotational forceson ferromagnetic and conductive objects, interference with AIMDs and the heating of conductiveobjects.

A quantitative solution for the translational and rotational forces on a ferromagnetic object beingplaced in a static magnetic field can be found in chapter 5.7.

For the magnetic field characteristic (spatial magnetic gradient) of an unshielded magnet a mini-mum magnetic flux density of Bz ≈ 60 mT is needed to overcome the initial frictional force, whichin turn makes it possible that a sphere is accelerated in the magnetic field and a so-called projectilerisk can occur. This result is in good agreement with the value given in [21].

In general, shielded superconducting magnets have higher spatial gradients at their openings tothe bore. This leads to a lower minimum magnetic flux density which could constitute a so-calledprojectile risk. Current data for shielded systems indicates a minimum magnetic flux density inthe central axis of a superconducting cylindrical magnet in the range from 30 . . . 40 mT necessaryfor a projectile risk to occur.

For non-spherical objects not only a translational force can exist, but a torque as well. Needle-shaped rotational ellipsoids try to turn their long axis parallel to the direction of the field. Themagnitude of the torque is proportional to the square of the static magnetic flux density B2

z , sothe maximum torque is to be expected in the center of the magnet and can be higher than themaximum translational force. Personnel working in areas with high static magnetic fields, e.g.MRI, MRS, electrolysis, electroplating, particle accelerators, superconducting generators, shouldbe informed that these torques can occur and trained to avoid any interference with the properhandling of tools and material.

Interference mechanisms for static and time-varying magnetic fields with AIMDs are covered insection 5.6.

High time-varying magnetic fields can also heat up conductive objects, e.g. passive medical implantsand tools. A detailed risk assessments needs to be carried out for workers with passive medicalimplants who are exposed to high time-varying magnetic fields or who need to handle conductiveobjects in these fields.

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2.3 Body models

Directly measurable external quantities, e.g. electric field strength, magnetic flux density or con-tact current, are linked to the exposure-limiting body internal quantities, e.g. peak electric fieldstrength in the tissue, by using analytical and numerical body models with different resolution andcomplexity. All calculations throughout this report are done using simple ellipsoid models [63, 75]– mainly used for validation purposes –, detailed anatomical models based on the Visible Humandata set [81] and on CAD models of the Virtual Family [23] with voxel sizes in the range from 1 to5 mm3. For calculations inside of the eye and the inner ear custom made high resolution modelswith spatial resolutions of up to 0.1 mm3 were used.

3 Neurophysiology

3.1 Mechanisms and facts for the creation of action potentials

The main physiological effect of electrical fields in the body tissue created by low-frequency electricor magnetic fields is electrical stimulation of excitable body tissues, like sensory organs, nerves andmuscles.

It is therefore of utmost importance to understand the underlying neurophysiological processeswhich lead to the generation of action potentials, their thresholds, time behavior and other impor-tant parameters, in order to limit the exposure to low frequency electric and magnetic fields, thusprotecting the health and safety of workers while being exposed to these physical agents.

Figure 3.1: Schematic structure of a typical CNS or motor neuron(In part from [111])

Fig. 3.1 shows the simplified structure of a typical neuron. Basic components of a neuron are one ormore dendrites, a single soma with the cell nucleus, a single axon and one or more axon terminals.The information is passed in form of an electrical signal, i.e. the action potential, between thedendritic inputs and the axon terminal outputs. Coupling to other neuronal structures usuallyhappens in the form of neurotransmitters, i.e. chemical agents, which are released at the axonterminals and picked up by receptor sites on the postsynaptic dendritic spines.

The axon hillock is the anatomical part of a neuron that connects the cell body, i.e. the soma,to the axon. It is described as the location where the summation of inhibitory and excitatorypostsynaptic potentials from numerous synaptic inputs on the dendrites or cell body occurs. Theaxon hillock also has a high concentration of voltage-gated ion channels, which are also commonon the surface of the soma and at the nodes of Ranvier, but not on the dendritic spines. Most ofthe length of the axon is insulated by a myelin sheath, i.e. Schwann cells in the peripheral nervous

8

system and oligodendrocytes in the central nervous system, which wrap themselves around theaxonal segment forming a thick fatty layer that prevents ions from entering or leaving the axon.The internodal distance d between two nodes of Ranvier lies in the range of 0.2 . . . 2 mm and islinked to the fiber diameter D by the empirical equation:

d ≈ 100 ·D (3.1)

The length of the uninsulated gap G at a node of Ranvier usually is only a few micrometerswide (G ≈ 1 . . . 2 µm) [100]. This myelin insulation increases both the energy efficiency of thepropagation process since the ionic currents are confined to the nodes of Ranvier – see fig. 3.1 –and the conduction velocity of an action potential va through so-called saltatory conduction – seetable 3.2.

Table 3.1 gives some rough estimates for the electric properties of the cell membrane of a nervefiber at a nodal gap and the cell membrane plus the Myelin sheath between two nodes of Ranvier[84].

Specific leakage resistance Specific capacitance[kΩ · cm2] [µF/cm2]

Cell membrane 1 1Myelin sheath 100 0.01

Table 3.1: Electrical properties of cell membrane and Myelin sheath

A classification of peripheral nerve fibers according to Erlanger and Gasser [29] together with somebasic fiber properties is given in table 3.2. The autonomic, motor and sensory nervous system usedifferent kinds of peripheral nerve fibers.

Fiber class Diameter D Conduction velocity va Myelin sheath[µm] [m/s]

Aα 10 - 20 60 - 120 very thickAβ 7 - 15 40 - 90 thickAγ 4 - 8 15 - 30 normalAδ 3 - 5 5 - 25 thin

B 1 - 3 3 - 15 partial

C 0.3 - 1 0.5 - 2 none

Table 3.2: Classification and properties of peripheral nerve fibers

Class B and C fibers are found in the autonomic nervous system.

Class C fibers can also be found in the sensory nervous system innervating nociceptors for slowpain and warmth receptors. Class Aδ fibers are associated with touch and pressure receptors (freenerve endings) as well as thermoreceptors for cold and nociceptors for slow pain. Aα and Aβ fibersof the sensory nervous system are the primary and secondary connections of proprioreceptors, e.g.muscle spindles, with the CNS. Aβ fibers also innervate all cutaneous mechanoreceptors.

The lower motor neurons of the motor nervous system consist of Aα and Aγ fibers which innervatethe extrafusal and intrafusal muscle fibers, respectively.

The distribution of peripheral nerve fibers in the human body comprises fiber diameters in therange from 0.3 . . . 17 µm with relative maxima in the fiber number at diameters of 0.6 µm forunmyelinated fibers and 2.3 µm, 3.8 µm, 6.3 µm, 8.6 µm and 12.8 µm for myelinated fibers

9

[15, 80, 90, 94]. The distribution of myelinated fiber diameters in the central nervous system hassignificantly different numbers. In the human pyramidal tract more than 89 % of nerve fibers arein the diameter range from 1 . . . 4 µm, approximately 9 % in the diameter range from 5 . . . 10 µmand less than 2% in the diameter range from 11 . . . 20 µm [74, 90].

Another important component of the neuron is its cell membrane.

Figure 3.2: Schematic structure of a cell membrane(Adapted from [110])

Fig. 3.2 shows the schematic structure of a cell membrane and its basic components. A keycomponent is the phospholipid bi-layer which prevents molecules and ions from leaving or enteringthe cell through uncontrolled diffusion. Channel proteins form controlled gateways for substancesentering or leaving the cell. For neurons two ionic pathways through the membrane are of specialinterest:

• Active ion pumps create and maintain an ionic concentration gradient between the inside ofthe neuron, i.e. the cytoplasm, and the outside of the neuron, i.e. the extracellular fluid

• Voltage-gated ion channels use this concentration difference to selectively transport ions alongtheir concentration gradients

Directly linked to these ionic concentration differences between the cytoplasm and the extracellularfluid, i.e. the inside (index ’i’) and the outside (index ’e’) of the neuron, is the existence of a potentialdifference UM = Φi −Φe or an electric field EM across the cell membrane. Any transport of ions –and therefore charges – across the membrane by pumps or channels changes the difference of theelectric potentials and the electric field across the membrane.

The concentration of potassium (K+) ions inside the neuron is approximately 20-fold larger thanthe outside concentration, whereas the concentration of sodium (Na+) ions on the outside is roughly9-fold larger than on the inside of the neuron. Similarly, ionic gradients across the cell membraneof a neuron also exist for calcium (Ca++), chloride (Cl−) and magnesium (Mg++) [48].

The equilibrium membrane potential – the resting potential Ur – at which the net flow of all ionsacross the membrane is zero can be calculated with the Goldman equation [38] and leads to atypical electrical potential difference of Ur ≈ −70 . . . 80 mV across the membrane. Membranepotentials are always measured relative to the exterior of the cell. This membrane potential inturn leads to a strong directional electrical field EM across the membrane.

Fig. 3.3 shows the various phases of an idealized action potential passing a single point on thecell membrane of an axon. As soon as a stimulus increases the transmembrane potential to morepositive values both the voltage-gated sodium and potassium channels begin to open, leadingto an increase of both the inward sodium ionic current, causing further depolarization, and the

10

Figure 3.3: Phases of an idealized action potential(In part from [109])

outward potassium ionic current, responsible for repolarization/hyperpolarization. If the changein membrane potential is only small and does not exceed the threshold, the higher potassiumionic current is counterbalancing the lower sodium ionic current, thereby returning the electricalpotential across the membrane to its resting value. These so-called failed initiations of a actionpotential describe one part of the fundamental “all-or-none” principle which is a key element tothe behavior of excitable structures. In other words, action potentials either occur fully or do notoccur at all. That means that larger stimuli do not create higher action potentials than smallerstimuli. Instead, the frequency of the action potentials is used to encode the intensity of a stimulus.

However, if the change in membrane potential is large enough to exceed a typical threshold levelof about 15 . . . 25 mV above the resting voltage, a positive feedback from the already open sodiumchannels opens even more sodium channels and in rapid succession leads to a runaway condition,where the electrical potential difference across the membrane nearly reaches the levels of the sodiumequilibrium potential UNa ≈ +55 mV. Because some of the slower acting potassium channels arealso open at this point in time, the peak membrane potential is lower than the sodium equilibriumpotential UNa and reaches typical values of approximately +40 mV. This rising phase of the actionpotential has a time duration of typical 1 ms.

The positive feedback of the rising phase finally slows, comes to a stop and is finally transformedinto a negative feedback by a special behavior of the sodium channels. Every sodium channel hasa built-in shut-off feature which automatically closes an open channel after a certain amount oftime. The probability for a sodium channel to stay open decreases with higher potentials acrossthe membrane. This inactivation of the sodium channels occurs much slower than the transitionfrom a closed to open state and takes some additional time for being reset to a normal closed stateof the channel. The inactivation of the sodium channels lowers the membrane’s permeability tosodium, thus driving the membrane potential back down. At the same time, the slower actingpotassium channels, which lack an automatic inactivation feature, become fully open causing themembrane potential to drop quickly, thus repolarizing the membrane and creating the falling phaseof the action potential.

Because the potassium channels act much more slowly than the sodium channels, it takes sometime to close them again, resulting in a hyperpolarization of the cell membrane (undershoot). Onlywhen the membrane’s permeability to potassium returns to its usual value, the potential acrossthe membrane assumes the resting value again.

A previous action potential leaves many sodium and potassium channels in a refractory state,

11

in which they are unable to open again, regardless of any stimulus being present. This absoluterefractory period, where no action potential can be created, is maintained until the membranepotential reaches sufficient negative values or even is hyperpolarized for a certain length of time.In the relative refractory period enough ion channels have recovered that a new action potential canbe created, however requiring a stimulus, i.e. an initial depolarization of the cell membrane, muchlarger than usual. These refractory periods guarantee that the action potential usually travelsonly in one direction along the axon, but also limits the maximum frequency of generating actionpotentials.

For mammalian nerve fibers the absolute refractory period is in the range of 0.4 . . . 1 ms for class Afibers and ≈ 2 ms for class C fibers, whereas the relative refractory period is in the range of severalmilliseconds. Under lab conditions the maximum repetition rate for action potentials created byexternally applied electric stimuli is ≈ 2000 per second. However, the maximum repetition rate foraction potentials in the human body is typically in the range of 10 . . . 100 per second and rarelyexceeds a value of 500 action potentials per second [16, 90].

The ions exchanged during an action potential make only a negligible change to the total internaland external ionic concentrations. Even with blocked sodium-potassium-pumps a typical axoncan generate up to hundreds of thousands of action potentials before a degeneration in amplitudeoccurs.

Because of the thermal motion it is not possible to predict whether a certain channel will be openor closed at any given time. However, the laws of probability allow to make certain predictions ofthe average behavior of a channel. Typically, a large number (≈ 102) of channels contribute to thegeneration of an action potential.

Sodium channel Potassium channel

Faster than potassium channel Slower than sodium channel(up to a factor of ten) (probability of being openTime constant: ≈ 10 µs (range: 5 . . . 200 µs) increases with depolarization)

Automatic inactivation No automatic inactivation(slow recovery, ≈ 10 ms at -70 mV)

3 distinct states: 2 distinct states:open, closed, inactivated open, closed

9 internal states (1 open / 8 closed) 16 internal states (1 open / 15 closed)when not inactivated

Table 3.3: Fact sheet for sodium and potassium ionic channels

Some important data for sodium and potassium channels is summarized in table 3.3 and can alsobe found in [44, 47, 84].

As already shown in table 3.2 and discussed in the previous paragraphs, myelinated class A fibershave higher conduction rates, shorter action potential durations, shorter refractory periods andlower electrical stimulation thresholds when compared to unmyelinated class C fibers [94].

3.2 Electrical stimulation of excitable tissues

3.2.1 Basic facts

Because of the lower electrical stimulation thresholds of myelinated class A fibers, due to the longerinternodal distance d, these fibers are an excellent choice for studying their behavior with regardto setting safety limits.

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A quantitative solution for the generation and propagation of action potentials as well as a de-scription of the underlying ionic mechanisms in an unmyelinated nerve fiber, e.g. squid giant axon,was first given by Hodgkin and Huxley [49]. Frankenhaeuser and Huxley reformulated the classicalHodgkin-Huxley equations, in terms of electrodiffusion theory, and computed action potentialsspecifically for saltatory conduction in myelinated axons [31].

The whole mathematical framework is well beyond the scope of this report but some key equationswill be presented, which give a very detailed insight into the whole process of electrical stimulationof excitable tissue and the generation of action potentials in nerve fibers. Additional backgroundinformation can be found in the literature [24, 31, 47, 49, 84, 90, 94, 103, 104].

For a first approach, an individual nerve fiber of infinite length, the center of which is orientedalong the spatial z-axis lying in an unbounded extracellular medium (conductivity κe) is selected.For subthreshold conditions where the excursion of the transmembrane voltage uM = UM − Ur

from its resting value are small, the electric properties of the membrane are those of a passiveadmittance described as a parallel RC network with constant R and C values. Assuming steadystate conditions, i.e. ∂uM/∂t = 0, the relationship between the membrane potential uM and thepotential of the external stimulus ϕe normalized to their respective baselines is expressed by thedifferential equation:

∂2uM∂x2

− uMλ2

= −∂ϕe

∂z2(3.2)

with λ =√rM/ri, where rM is the membrane resistance per unit length and ri the resistance of

the intracellular medium per unit length.

With the electric field being the negative spatial derivative of the corresponding function for theelectric potential, eqn. 3.2 can be rewritten as

∂2uM∂x2

− uMλ2

=∂Ez

∂z(3.3)

The term ∂Ez/∂z on the right hand side of eqn. 3.3 is often called the activation or forcing functionin the differential equation.

Eqn. 3.3 describes some important facts for changes in the membrane potential (hyperpolarization,depolarization) and in the second case the successful initiation of an action potential:

• A gradient along the fiber axis in the electric field of the external stimulus must exist.This finding is proven by experimental results, that the electric stimulation of excitabletissue is facilitated, if the electric field of the stimulus is parallel to elongated cells or fibers.A perpendicular field orientation is rather inefficient and requires a much higher stimulus inorder to be successful [9, 65, 79, 84, 86, 87, 88, 90].

• The spatial field gradient does not necessarily have to originate from the external stimulus butcan also be created by boundary conditions, e.g. beginning, termination, bends or branches,changes in diameter of the fiber, adjacent tissues and structures with different electricalproperties.This fact is especially of interest when studying complex excitable tissue structures, e.g.brain, retina.

• Peak depolarization or hyperpolarization are expected at locations where ∂Ez/∂z attains itsmaximum value.

• The overall reaction of the cell membrane depends on the entire course of the functionE(t, z, . . . ) not just the location or amplitude of its initial or peak values.It is therefore expected that different forms of stimuli, e.g. rectangular, trapezoid, triangular,sinusoidal, exponential, mono- or biphasic, even those having the same amplitude, will havedifferent effects on the overall behavior of the nerve cell membrane.

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The following sections apply these basic findings to the mechanism of electrical stimulation ofperipheral nerves (PNS) and central nervous system of the head (CNS).

A review of the current literature reveals that these factors are often not well controlled and arenot sufficiently documented. Especially for experimental data it is very difficult to find relevantparameters in the published documents.

3.2.2 Long stimuli

However, a careful review of the literature and additional numerical simulations at membrane level,including parameter variation studies, reveals a threshold for the electric field strength in the tissuefor the onset of peripheral nerve stimulation (PNS) in the range of 6 . . . 7 V/m for stimulationpulses longer than 1 . . . 2 ms [14, 17, 25, 42, 66, 83, 90, 101]. This value is quite conservative becausemany experiments and calculations use point sources for the stimulation current or voltage, whichcan cause a high spatial field gradient in the tissue, especially for small distances between the fieldsource and the axon under investigation. Because these high spatial field gradients in the tissueare difficult to obtain by using external electric or magnetic fields for stimulation, even higherthreshold levels for peripheral nerve stimulation are to be expected in those cases.

Lapicque’s law [72, 73], also known as the modified Weiss equation [105], gives the fundamentalrelationship between the stimulation strength – historically given as a rectangular stimulationcurrent Is – and the duration of the stimulus T with respect to physiological parameters like therheobase – also historically given as a current IR – and an empirical time constant τe which is linkedto the membrane time constant τM = RM · CM (approximately in the range of 1 ms), defined bymembrane resistance RM and membrane capacity CM, and the spatial distribution of the stimuluscurrent or the spatial gradient of the electrical field strength in the tissue:

Is =IR

1− e−T/τe(3.4)

Figure 3.4: Graphical representation of Lapicque’s law given by eqn. 3.4

As shown in fig. 3.4 and according to eqn. 3.4 Is, the minimum stimulation strength (or current)with duration T , is required to reach the stimulation threshold. For long stimuli (T → ∞) thevalue for Is is identical to the rheobase value IR, which defines the stimulation threshold. The timeT = τc where the minimum stimulation strength required is twice the rheobase value Is = 2 · IRwas named chronaxie by Lapicque. It must be noted that the rheobase value is dependent onphysiological parameters and individual exposure conditions.

Three fundamental statements can be derived from eqn. 3.4:

1. Stimuli must exceed a threshold, i.e. a minimum stimulation current or minimum electricfield strength in the tissue, in order to create an action potential

14

2. Stimuli below the threshold, i.e. the rheobase value, cannot create an action potential evenif they are of very long duration

3. Stimuli with shorter durations must be of higher intensity in order to be effective, i.e. createan action potential

Some documents [3, 51, 57] present a U-shaped stimulation threshold or exposure limit value curve,which allows for higher values of the current density or electric field strength in the tissue due toaccommodation of the nerve fiber for frequencies below 10 Hz. However, this is not endorsed byLapicque’s law or eqn. 3.4 and is based on a misinterpretation of physiological data as explainedbelow.

If a stimulus is constant at a sub-threshold value or increases only slowly with time, e.g. sinusoidalwaveform at a low frequency starting at a zero amplitude value, the sodium channels can opengradually which leads to a small rise in membrane voltage and also to an increase in stimulationthreshold. This creates a chase condition between the stimulus and the stimulation thresholdwhich can only be overcome with a higher amplitude of the stimulus or a faster rate of change.The behavior of a nerve to adapt to a constant or slowly varying stimulus is called accommodation.However, this behavior is only present if there is a slowly changing stimulus, e.g. sinusoidal ortriangular waveforms beginning at a zero value. It is not encountered with long rectangular,exponential or even trapezoid waveforms with steep rising and falling slopes. It is also absent ifthe sinusoidal waveform starts at its peak value. Therefore, the usage of those higher values mustbe restricted to certain waveforms and should not be given as a general option without stating thelimitations.

3.2.3 Short stimuli

When it comes to short stimulus durations (T → 0), the stimulus charge or the integral of electricalfield strength in the tissue ET over the stimulus duration T becomes the new threshold value:ET · T ≥ cs for rectangular stimuli, where cs is a constant threshold value. It must also be notedthat the threshold value cs is nearly invariable to dET/dt and therefore does not largely depend onthe form of the stimulus, i.e. rectangular, trapezoid, triangular, sinusoidal and exponential stimulinearly give the same results.

From analytical calculations and numerical parameter variation studies and for stimulus durationsof less than 10 µs (T ≤ 10µs) a value cs > 2 · 10−3 Vs/m can be obtained, which translates to anelectric field strength in the tissue in excess of 200 V/m for a 10 µs stimulus.

3.2.4 CNS tissue

Experimental data in the literature gives lower threshold rheobase values when it comes to thestimulation of CNS tissue of the head, e.g. electro- and magnetophosphenes [6, 76, 77, 90, 91, 95]. Itmust be noted that this data are not highly reliable because of incomplete dosimetric documentationand often gives only average values for current densities or electric field strength in the tissue ordoes not take spatial gradients of the electric field in the tissue into account.

However, from smaller fiber diameters and shorter fiber lengths higher threshold values would havebeen expected. As already pointed out in section 3.2.1 and shown in eqn. 3.3, high spatial fieldgradients resulting from boundary conditions, neighboring tissue structures with different electricalproperties and a possible influence from highly specialized receptors, e.g. the rods of the retina ofthe eye and their neural interface, can make up for a seemingly lower total stimulation thresholdvalue. As a preliminary result and a rough estimate, simulations indicate a total factor in the orderof 20 . . . 40 when comparing the threshold levels in the frequency range of maximum sensitivity –see table 2.1 – with those for peripheral nerve stimulation.

15

Applying this factor to the threshold value for the electric field strength in the tissue for the onsetof peripheral nerve stimulation for long stimuli – see section 3.2.2 – in the range of 6 . . . 7 V/mgives a threshold value for CNS tissue in the range of 0.15 . . . 0.35 V/m for the same type of stimuli.

Similar results are expected when it comes to vertigo and nausea, but high-resolution numericalmodels, needed to link any exposure to extremely low frequency magnetic fields to those effects,are sparse or not existent.

Other CNS tissue, e.g. spinal cord, can be disregarded in this context, because due to the electricproperties of the surrounding tissues an ’electric shielding effect’ occurs which results in generallyhigher threshold values for electrical stimulation [12, 43].

3.2.5 Uncertainties

A reduction factor fr =√

10 is introduced in order to address uncertainties

• in modeling, e.g. body models [23, 81]

• physiological data, e.g. tissue data [33, 34, 35]

• due to individual health status and possible pathological conditions

3.2.6 Summary

Summarizing the findings of this section on neurophysiology, mechanisms and electrical stimulation,some important facts for threshold-level stimuli have to be noted:

• The relevant physiological parameter to describe the electrical stimulation of excitable bodytissues, like sensory organs, nerves and muscles is the peak electric field strength in the tissuetogether with its spatial and temporal derivatives.

• The location where an electric field ET in the tissue, caused by an external (low frequency)electric or magnetic field, depolarizes or hyperpolarizes the cell membrane of an axon isdependent on its gradients in space and time. This important fact means that exposures todifferent sources of electric and magnetic fields and to different frequencies in general havedifferent points of interaction with the cell membrane and are therefore independent of eachother. In other words, there is hardly any additivity of the different spectral componentsunder practical exposure conditions.

• In the case of stimuli with repetition frequencies of less than 300 . . . 800 Hz, every peak valueof the electric field strength in the tissue can create an instantaneous action potential. Forthese stimuli, a threshold for the electric field strength in the tissue for the onset of peripheralnerve stimulation in the range of 6 . . . 7 V/m applies. Due to high spatial field gradientsresulting from boundary conditions and a possible influence from highly specialized receptors,the thresholds for CNS tissue stimulation appear to be lower than those for peripheral nervestimulation by a factor in the order of 20 . . . 40.

• For stimuli with repetition frequencies exceeding several kHz many stimuli, e.g. periods ofa sinusoidal waveform, are necessary in order to create an action potential. This behavioris caused by a slow drift in the membrane potential due to subsequent stimulation and isattributed to the different time behavior of the sodium and potassium channels, leading toa so-called delayed action potential. Published data [44, 90] shows that for stimuli with arepetition frequency of 5 kHz a delayed action potential is evoked after 5 . . . 10 stimuli orperiods, whereas for a repetition frequency of 50 kHz approximately 50 . . . 100 stimuli orperiods are necessary to create a delayed action potential.

16

• The probability of creating an action potential is very small for frequencies exceeding≈100 kHz and requires a high electric field strength in the tissue (>200 V/m). Especiallyfor continuous-wave signals these field strengths in the tissue can lead to significant tissueheating effects, which must be controlled.

• The generation of action potentials is instantaneous or nearly instantaneous. However, withtime frames of less than 1 . . . 2 ms, no time averaging can be justified. This also means thatroot-mean-square (RMS) values, which by definition are an average, are a poor metric andshould be avoided. The usage of peak values for measurement and calculation purposes ishighly recommended. However, for single-frequency, continuous-wave sinusoidal waveforms,the peak values can be derived from RMS values by multiplication with a factor of

√2.

• The geometric dimensions of the main areas of field interaction and the neurological structuresinvolved in the generation of an action potential are very small and therefore do not allowfor any spatial averaging. However, from a practical point of view, e.g. for measuring andcalculation purposes, some spatial averaging is inevitable, but has to be controlled carefully.A detailed analysis of this issue has to take into account several parameters, e.g. location (inthe tissue or outside the body) and source (dimension, distance), and is beyond the scope ofthis document.

All these important facts need to be taken into account when limiting the exposure to low frequencyelectric and magnetic fields for the protection of the health and safety of workers.

17

4 Limiting occupational exposure to static and low fre-quency electric and magnetic fields

The major goal of Directive 2004/40/EC is the protection of the health and safety of workers. Thismeans, that any physiological effects caused by an exposure to EMF must be limited in such away, that they do not pose a potential threat to the health and safety of workers.

Any

• interference with autonomous heart action

• loss of muscle control

• significant pain

• severe form of vertigo and nausea

• whole-body heat stress and excessive localized tissue heating

qualifies as a potential threat to the health and safety of workers and the risk of such an occurrenceshould therefore be controlled.

Other effects, like phosphenes, may or may not pose a potential safety threat, depending on theworking environment and the duty of the worker. The same is true for effects like metallic tasteand minor tactile sensations at threshold level.

The proposed exposure limit values and action values presented in the next subsections of thisdocument are based on this valuation.

4.1 Exposure limit values

As described in section 3 the relevant metric to quantify physiological effects based on electricalstimulation of excitable body tissue is the electric field strength in the tissue together with itsspatial and temporal derivatives.

Because these effects are threshold-based, the peak value of the electric field strength in the tissueis the relevant parameter which needs to be limited. If this peak field strength in the tissue remainsbelow the identified stimulation threshold at all times, no stimulation will occur [42, 43, 44, 63,89, 90].

4.1.1 Static electric fields

The exposure limit values for static electric fields are indicated in table 4.1.

As stated in section 2.1.1.1, the external static electric field cannot penetrate the body surface.Therefore the exposure limit value is solely based on indirect effects of the static electric field andis indicated as direct measurable external field quantity.

4.1.2 Static magnetic fields

The exposure limit values for static magnetic fields are indicated in table 4.2.

Sections 2.1.2.1, 2.2.2 and 5.7 give the rationale for setting these exposure limit values stated asdirectly measurable external field quantities.

It has to be noted that these exposure limit values only apply if the worker is stationary withrespect to the static magnetic fields. For all time-varying exposures, including movements in staticmagnetic fields, the exposure limit values as indicated in section 4.1.3 do also apply. Additionalinformation can be found in section 5.

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External electric field strength (a,b)

[kV/m]

30

Note: (a) Value refers to the spatial maximum

(b) If there is a risk that the worker touches any grounded or ungrounded object,additional restrictions due to contact currents – see section 4.1.4 – may apply

Table 4.1: Exposure limit value for static electric fields

Maximum magnetic flux densityExposure of head and trunk (a,b,c,d) Exposure of limbs (a,b,c,e)

[T] [T]

2 8


(b) Personnel with active medical implants, e.g. pacemakers, cardioverterdefibrillators, should not be exposed to static magnetic fields with fluxdensities higher than 0.5 mT at the location of the implant. For additionalinformation see section 5.6

(c) Magnetic flux densities in excess of 30 mT are allowed if any projectile riskor any risk from translational or rotational forces on metallic objects orimplants can be excluded

(d) For controlled environments where access is limited to specially instructedand trained workers, where special work practices and measures are in forceand where a detailed risk analysis shows that any risks to the health andsafety of the workers or any negative impact on their duties or the safetyof others with regard to vertigo, nausea and phosphenes can be excluded,magnetic flux densities up to 8 T are allowed

(e) For controlled environments magnetic flux densities in excess of 8 T are ac-ceptable for a limbs only exposure

Table 4.2: Exposure limit values for static magnetic fields

19

4.1.3 Low frequency electric and magnetic fields

0.01

0.1

1

10

100

<0.01 0.1 1 10 100 1 k 10 k 100 k

Pea

k el

ectr

ic fi

eld

stre

ngth

in th

e tis

sue

[V/m

]

Frequency [Hz]

Exposure of the trunk / Controlled environmentWhole body exposure / Exposure of the head

Figure 4.1: Exposure limit values for time varying, low frequency electric and magneticfields given as peak electric field strength in the tissue

The exposure limit values are indicated as peak electric field strength in the tissue and are basedon the results presented in section 3 and the valuations in section 4 of this document.

As outlined in section 3.2.5 uncertainties in modeling, physiological data and due to individualhealth status and possible pathological conditions are addressed by applying a reduction factorfr =

√10 to the values derived in sections 3.2.2, 3.2.3 and 3.2.4. The resulting exposure limit

values are given in fig. 4.1 and table 4.3.

Applying the reduction factor fr to the threshold for peripheral nerve stimulation of 6 . . . 7 V/m– see section 3.2.2 – gives 2 V/m as the exposure limit value in the frequency range up to 3 kHz.For short stimuli, i.e. frequencies exceeding 100 kHz – see section 3.2.3 –, with a threshold valueof at least 200 V/m, the application of the reduction factor fr yields for an exposure limit valueof ≈ 67 V/m for a frequency of 100 kHz.

According to section 3.2.4 the thresholds for the stimulation of CNS tissue, e.g. magnetophosphenes,vertigo, nausea, appear to be a factor of 20 . . . 40 lower than those for peripheral nerve stimulationdue to boundary and other special conditions. Dividing the exposure limit value for peripheralnerve stimulation and long stimuli of 2 V/m by 40 gives the exposure limit value for CNS tissueof 0.05 V/m. According to section 2.1.2.2 and tab. 2.1 magnetophosphenes have a very sharpmaximum sensitivity peak at ≈ 20 Hz, which decreases rapidly for higher frequencies. Thereforethe corner frequency of 25 Hz for the exposure limit value curve for exposure of the head or wholebody exposure together with a frequency proportional behavior is chosen quite conservatively.

In order to keep the exposure assessment as simple as possible, only two intermediate data pointsare chosen in order to describe the frequency behavior of the exposure limit values in the frequencyrange up to 100 kHz. However, this leads to larger reduction factors, especially for frequencies inthe range between approximately 100 Hz and several kHz.

Fig. 4.1 – lower (green) curve – gives the exposure limit values for whole body exposures to timevarying, low frequency electric and magnetic fields in the frequency range up to 100 kHz as peakelectric field strength in the tissue. These exposure limit values address all direct adverse effects

20

Peak electric field strength in the tissue

Frequency range Whole body exposure Exposure of the trunk /or exposure of the head (a,b,d) Controlled environment (a,b,c,d)

f / Hz [V/m] [V/m]

0 < f ≤ 25 0.05 225 < f ≤ 1000 f/500 2

1000 < f ≤ 3000 2 23000 < f ≤ 100 · 103 f/1500 f/1500


(b) Value given is the peak permissible electric field strength in the tissue andmust not be exceeded. Uncertainties linked to measurement or calculationprocedures must be subtracted

(c) For controlled environments where access is limited to specially instructedand trained workers, where special work practices and measures are in forceand where a detailed risk analysis shows that any risks to the health andsafety of the workers or any negative impact on their duties or the safety ofothers with regard to vertigo, nausea and phosphenes can be excluded, thisvalue also applies to whole body exposures

(d) Peak electric field strength in the tissue exceeding 49 V/m must alsobe checked for compliance with the exposure limit values for whole- andpartial-body SAR in order to prevent inadmissible tissue heating

Table 4.3: Exposure limit values for time varying, low frequency electric and magneticfields given as peak electric field strength in the tissue

based on electrical stimulation of body tissues and do also apply to partial exposures of the head,which is the main area of interaction for effects like vertigo, nausea and phosphenes.

For partial body exposures of the trunk of the human body and for controlled environments,where access is limited to specially instructed and trained workers, where special work practicesand measures are in force and where a detailed risk assessment shows that any risks to the healthand safety of the workers or any negative impact on their duties or the safety of others, with regardto vertigo, nausea and phosphenes are controlled, the use of the exposure limit values shown in fig.4.1 – upper (red) curve – could be allowed. However, due to the possibility that annoying indirecteffects, e.g. movement or vibration of body hair, sparc discharges and contact currents, might occurmore frequently at these exposure levels, their use should be time-restricted to fractions of a wholework shift.

Both the exposure limit values for time varying, low frequency electric and magnetic fields for wholebody exposures and for partial body exposures of the head, applicable in general, and the exposurelimit values for partial body exposures of the trunk and for special controlled work environmentsare summarized in table 4.3.

4.1.4 Contact currents

If a worker touches a charged object or touches a grounded object while being charged himself dueto exposure to a electric field or due to triboelectricity, a contact current will flow. The same canhappen, if the worker closes an induction loop when touching a conductive object in a time-varyingmagnetic field.

The physiological effect is largely dependent on the size of the contact area, e.g. touch or graspcontact, and on the amount of discharge energy and transferred charge, as well as the amplitudeand frequency of the continuously flowing contact current. These effects can be annoying, painfulor can have life threatening consequences [18, 58, 59, 60, 106].

21

In general, two different phases of a contact current event can be distinguished:

• an initial discharge current impulse, e.g. spark discharge

• a continuous contact current

Depending on the specific exposure scenario, only one or both phases of the contact current eventmight be present. Usually, the initial discharge current is only present for exposure situationsinvolving either a static or a time-varying electric field. In general, a continuous contact current islinked to time-varying electric or magnetic fields, but can also occur in conjunction with ongoingtriboelectric processes.

The initial discharge current usually is a very fast event, present only in the sub-millisecond range.According to the results from section 3.2.3, these effects are best described and limited by theintegral of electrical field strength in the tissue over the duration of the initial discharge or thetransferred charge. If the voltage difference between the object and the worker is known, thedischarge energy can also be used.

The frequency of the continuous contact current depends on the frequency of the causal time-varying electric or magnetic field, but can also be a DC current in case of triboelectric processes.Again, the limiting value is the electric field strength in the tissue at the contact site – see section3.2 –, is directly related to the contact current for touch and grasp contact. Presenting the limitin form of a contact current is preferred, because this quantity is directly measurable.

Maximum discharge energy (a) Maximum transferred charge (a)

[mJ] [µC]

350 50

Note: (a) Continuous contact current, if any, needs to be limited according to the valuesgiven in table 4.5

Table 4.4: Exposure limit values for the initial discharge pulse of a contact current

Peak contact current (a)

Frequency range Grasp contact (b) Touch contactf / Hz [mA] [mA]

0 ≤ f ≤ 3000 5 13000 ≤ f ≤ 45000 f/600 f/3000

45 · 103 < f ≤ 100 · 103 75 15

Note: (a) Initial discharge impulse, if any, needs to be limited according to the valuesgiven in table 4.4

(b) In order to avoid shocks and burns, contact currents exceeding touch currentlimits are permitted only, if the workers are properly trained to always makegrasp contact or instructed to use special work techniques or work gear

Table 4.5: Exposure limit values for continuous touch and grasp contact currents

Therefore it is necessary to provide limits for both phases of the contact current event. Theseare given in table 4.4 for the initial discharge current impulse and in table 4.5 and fig. 4.2 for thecontinuous contact current.

Higher values for grasp contact currents can be allowed, because both the peak electric field strengthin the tissue and its spatial gradient are lower due to the larger contact area. However, these valuesshould only be used if the workers are properly instructed and trained.

22

1

10

100

<0.01 0.1 1 10 100 1 k 10 k 100 k

Pea

k co

ntac

t cur

rent

[mA

]

Frequency [Hz]

Grasp contactTouch contact

Figure 4.2: Exposure limit values for touch and grasp contact currents

If in a certain workplace environment, e.g. high-voltage switchyards, spark discharges or contactcurrents cannot be avoided by technical measures, workers should be trained to always make graspcontact or instructed to use special work techniques, e.g. equalization of potentials, or work gear,e.g. insulating or conductive gloves.

4.2 Upper and lower action levels

The metrics, e.g. basic restrictions and exposure limit values that best describe the onset of adversephysiological reactions [3, 57], are mainly quantities that only exist in the biological tissue, e.g.peak electric field strength in the tissue for stimulation effects and specific energy absorption rate(SAR) for tissue heating, and are therefore not directly measurable.

EMFs are the only physical agent where this problem exists and therefore require a special solution.For the assessment of possible health effects of electromagnetic fields a differentiation must be madebetween basic restrictions (connected with exposure limit values) and reference levels (connectedwith action levels) [3, 57].

According to [57] basic restrictions are defined as ’mandatory limitations on the quantities thatclosely match all known biophysical interaction mechanisms with tissue that may lead to adversehealth effects’. [3] calls these values exposure limit values and states: ’Compliance with these limitswill ensure that workers exposed to electromagnetic fields are protected against all known adversehealth effects’. Because these exposure limit values mainly represent physical parameters thatexist only inside the human body, thus making them unavailable for direct measurements, a set ofreference levels [57] or action values [3] is derived from these basic restrictions or exposure limitvalues, which is given as directly measurable field quantities.

Reference levels are defined as ’the . . . peak electric and magnetic fields and contact currents towhich a person may be exposed without an adverse effect and with acceptable safety factors. Thereference levels for electric and magnetic field exposure . . . may be exceeded if it can be demonstratedthat the basic restrictions are not exceeded. Thus, it is a practical or ’surrogate’ parameter thatmay be used for determining compliance with the basic restrictions’ [57]. Directive 2004/40/EC

23

calls these values action values. Compliance with the action values also guarantees compliancewith the exposure limit values.

Lower Action Level

Un

acce

pta

ble

he

alth

an

dsa

fety

rela

ted

risks

Upper Action Level

Exposure Limit ValueLevel,

dura

tion

and

type

of

exposure

Sa

fety

rela

ted

risks

Compliance check with exposurelimit values required

No further action/measures required

Safety measuresrequired

Figure 4.3: Schematic relationship between exposure limit value and the upper andlower action levels with respect to the level, duration and type of exposure

Fig. 4.3 shows an extension to this concept, by introducing an upper action level and a loweraction level. Compared to the existing situation, this concept will allow a higher flexibility andthe reduction of unnecessary costs for employers for determining workers’ exposure to EMF.

Compliance with the lower action level ensures that all direct and indirect effects of EMFs, in-cluding phosphenes, which may represent a potential threat to the health and safety of workers –see section 4 – are safely avoided. At the same time minor indirect effects at threshold level, e.g.touch currents, are also eliminated as far as possible.

Because this lower action level will not be exceeded for approximately 90 % of all workplaces,there is also no need for additional measures, thus reducing costs while guaranteeing the healthand safety of workers at the same time. This is a very important fact for employers, especiallywith regard to small and medium-sized enterprises (SMEs).

However, health risks associated with the interference of EMFs with the proper function of activeimplanted medical devices (AIMD), e.g. pacemakers, must always be considered, even if there iscompliance with the lower action values.

The upper action level is installed to simplify the determination of compliance with the exposurelimit value. At an exposure level connected with the upper action level, mildly annoying field

24

effects, e.g. phosphenes, vertigo and contact currents, are possible but adverse health effects areexcluded. The workers have to be informed on how to avoid or reduce these effects by using properworking techniques and tools. However, measures have to be installed in order to avoid potentialthreats from some indirect effects, e.g. projectile risk.

If the upper action level is exceeded, health and safety related risks can no longer be excluded. Inthose cases it is mandatory to check whether or not the exposure limit values are exceeded and totake corrective action, if necessary, in order to prevent exposures that might exceed the exposurelimit values.

This is in accordance with the current concept of Directive 2004/40/EC and means: If the exposureis below the exposure limit value, workers will be protected against the established adverse healtheffects of EMFs on the human body.

4.2.1 Upper action level

The upper action level is given in order to make the exposure assessment simpler and cheaper.This level is derived by converting the body-internal exposure limit values into directly measurableexternal field quantities, e.g. external electric and magnetic field strength and magnetic flux den-sity, assuming worst case exposure conditions. Therefore compliance with the upper action levelguarantees that the exposure limit values are not exceeded.

The mandatory action to check whether or not the exposure limit values are exceeded – and totake corrective action, if necessary – is coupled with the instance that the upper action level isexceeded.

4.2.1.1 Electric fields

100

1 k

10 k

<0.01 0.1 1 10 100 1 k 10 k 100 k

Pea

k ex

tern

al e

lect

ric fi

eld

stre

ngth

[V/m

]

Frequency [Hz]

Whole body

Figure 4.4: Upper action level for occupational exposures of the whole body to externalstatic and time-varying electric fields

Fig. 4.4 and table 4.6 show the upper action level for the external electric field strength. All valuesrefer to the spatial maximum and are given as the peak external electric field strength.

25

Peak external electric field strength (a,b,c)

Frequency range Whole body exposuref / Hz [V/m]

0 ≤ f ≤ 300 30000300 < f ≤ 3000 9 · 106/f

3000 < f ≤ 100 · 103 3000


(b) The peak values given in this table can be exceeded if compliance with theexposure limit values given in tables 4.1 or 4.3 is shown. However, a peakelectric field strength of 30000 V/m should never be exceeded because of therisk of severe indirect effects

(c) If there is a risk that the worker touches any grounded or ungrounded object,additional restrictions due to contact currents – see section 4.1.4 – may apply

Table 4.6: Upper action level for occupational exposures of the whole body to externalstatic and time-varying electric fields

It has to be noted that the electric field strength in the tissue given in fig. 4.1 and table 4.3 doesnot impose a practical limit on the peak external electric field strength due to the shielding effectof the body – see section 2.1.1.2 –, but does provide information on the frequency dependency.

The upper action level is defined by the exposure limit values for static electric fields and extrap-olated values from the high frequency range.

Because no different values for partial body exposures can be allowed, the upper action level givenfor whole body exposures to external electric fields do also apply to partial body exposures.

The peak values indicated in fig. 4.4 and table 4.6 can be exceeded if compliance with the exposurelimit values listed in tables 4.1 or 4.3 is shown.

Additional restrictions due to contact currents – see section 4.1.4 – may apply, if the worker cantouch any grounded or ungrounded object.

4.2.1.2 Magnetic fields

Fig. 4.5 and table 4.7 show the upper action level for the static and time-varying magnetic fields.All values refer to the spatial maximum and are given as the peak magnetic flux density.

This upper action level for magnetic fields is derived from the exposure limit values for staticmagnetic fields given in table 4.2 and the exposure limit values for the electric field strength in thetissue given in fig. 4.1 and table 4.3 assuming worst-case exposure conditions.

The upper action level given in fig. 4.5 and table 4.7 can be exceeded if compliance with theexposure limit values given in tables 4.2 or 4.3 is shown.

For controlled work environments where any negative impact on the worker’s duties or the safety ofothers with regard to vertigo, nausea and magneto-phosphenes can be excluded, higher exposuresto magnetic fields in a certain frequency range can be justified. These values also apply to trunkonly exposures and are shown in fig. 4.5 as the curve marked ’Trunk only / Controlled environment’.

Exposures of the whole body or the head to static or time-varying magnetic fields are limited bythe upper action level shown in fig. 4.5 as the curve marked ’Whole body / Head’.

Magnetic flux densities in excess of 30 mT are only allowed, if any projectile risk or any risk fromtranslational or rotational forces on metallic objects or implants can be excluded.

Workers with active implanted medical devices, e.g. pacemakers or cardioverter defibrillators,should not be exposed to static magnetic fields with flux densities in excess of 0.5 mT at thelocation of the implant. Additional information is given in section 5.6.

26

100 µ

1 m

10 m

100 m

1

<0.01 0.1 1 10 100 1 k 10 k 100 k

Pea

k m

agne

tic fl

ux d

ensi

ty [T

]

Frequency [Hz]

Trunk only/Controlled environmentWhole body/Head

Figure 4.5: Upper action level for occupational exposures to static and time-varyingmagnetic fields of the whole body, head or trunk and for controlled environ-ments

Peak magnetic flux density (a,b,c,d,e)

Whole body exposure Exposure of the trunk /Frequency range or exposure of the head Controlled environment

f / Hz [T] [T]

0 ≤ f ≤ 0.024 2 20.024 ≤ f ≤ 0.96 48 · 10−3/f 20.96 ≤ f ≤ 25 48 · 10−3/f 1.92/f

25 < f ≤ 1000 1.92 · 10−3 1.92/f1000 < f ≤ 3000 1.92/f 1.92/f3000 < f ≤ 100 · 103 0.64 · 10−3 0.64 · 10−3


(b) The peak values given in this table can be exceeded if compliance with theexposure limit values given in tables 4.2 or 4.3 is shown

(c) Magnetic flux densities in excess of 30 mT are allowed, if any projectile riskor any risk from translational or rotational forces on metallic objects orimplants can be excluded

(d) Workers with active implanted medical devices, e.g. pacemakers or car-dioverter defibrillators, should not be exposed to static magnetic fields withflux densities in excess of 0.5 mT at the location of the implant – see alsosection 5.6

(e) The values for the magnetic field strength H can be calculated from thevalues of the magnetic flux density B by using the formula H = B/µ0 withµ0 = 4π · 10−7 T·m

A

Table 4.7: Upper action level for occupational exposures to static and time-varyingmagnetic fields of the whole body, head or trunk and for controlled environ-ments

27

4.2.2 Lower action level

If the lower action level is not exceeded, no further actions or measures are required. Exceptionsto this rule are safety measures for workers with AIMD – see section 5.6.

Compliance with the lower action level excludes the occurrence of any adverse direct or indirecteffects – apart from disturbance of AIMD – and does not require a detailed exposure assessment forrelated workplaces, thus avoiding unnecessary actions and measures and therefore reducing costs.

4.2.2.1 Electric fields

100

1 k

10 k

<0.01 0.1 1 10 100 1 k 10 k 100 k

Pea

k ex

tern

al e

lect

ric fi

eld

stre

ngth

[V/m

]

Frequency [Hz]

Whole body

Figure 4.6: Lower action level for occupational exposures of the whole body to externalstatic and time-varying electric fields

Peak external electric field strength (a,b)

Frequency range Whole body exposuref / Hz [V/m]

0 ≤ f ≤ 600 5000600 < f ≤ 3000 3 · 106/f

3000 < f ≤ 100 · 103 1000



Table 4.8: Lower action level for occupational exposures of the whole body to externalstatic and time-varying electric fields

Fig. 4.6 and table 4.8 show the lower action level for the external electric field strength. All valuesrefer to the spatial maximum and are given as the peak external electric field strength.

28

Compliance with the lower action level for occupational exposures to external static and time-varying electric fields avoids most indirect effects, e.g. hair movement, micro shocks and touch orgrasp currents.

4.2.2.2 Magnetic fields

100 µ

1 m

10 m

100 m

1

<0.01 0.1 1 10 100 1 k 10 k 100 k

Pea

k m

agne

tic fl

ux d

ensi

ty [T

]

Frequency [Hz]

Whole body

Figure 4.7: Lower action level for occupational exposures of the whole body to staticand time-varying magnetic fields

Fig. 4.7 and table 4.9 show the lower action level for the static and time-varying magnetic fields.All values refer to the spatial maximum and are given as the peak magnetic flux density.

For static and extremely low frequency (f < 1 Hz) magnetic fields the lower action level for themagnetic flux density of 30 mT ensures that projectile (translational) or rotational risks fromferromagnetic objects in these magnetic fields, as well as effects like vertigo and nausea will notoccur. For higher frequencies (f > 20 Hz) the lower action level for magnetic fields also ensuresthat other adverse or annoying direct and indirect effects are safely avoided.

Workers with active implanted medical devices, e.g. pacemakers or cardioverter defibrillators,should not be exposed to static magnetic fields with flux densities in excess of 0.5 mT at thelocation of the implant. Additional information is given in section 5.6.

29

Frequency range Peak magnetic flux density (a,b,c,d)

f / Hz [T]

0 ≤ f ≤ 0.55 30 · 10−3

0.55 ≤ f ≤ 25 16.5 · 10−3/f25 < f ≤ 1000 660 · 10−6

1000 < f ≤ 3000 660 · 10−3/f3000 < f ≤ 100 · 103 220 · 10−6



(c) Workers with active implanted medical devices, e.g. pacemakers or car-dioverter defibrillators, should not be exposed to static magnetic fields withflux densities in excess of 0.5 mT at the location of the implant – see alsosection 5.6

(d) The values for the magnetic field strength H can be calculated from thevalues of the magnetic flux density B by using the formula H = B/µ0 withµ0 = 4π · 10−7 T·m

A

Table 4.9: Lower action level for occupational exposures of the whole body to staticand time-varying magnetic fields

30

5 Special exposure situations

5.1 Simultaneous exposure to electric and magnetic fields

Workplaces with simultaneous whole body exposures to both external electric and magnetic fieldsexceeding the upper action levels are rarely found in the work environment.

Referring to eqn. 2.2, the contribution of the external electric field component to the electric fieldstrength in the tissue is very small in general. Furthermore, for most cases, the external electricand magnetic field component have different points of interaction within the tissue and are notadditive with regard to the electric field strength in the tissue.

Therefore it is sufficient to show compliance with the lower or upper action levels for both theelectric and magnetic field component separately.

If both the external electric and magnetic field components exceed the upper action levels, or ifcompliance with the exposure limit values is shown directly, both the external electric and magneticfield component should be used in order to calculate the electric field strength in the tissue correctly.

5.2 Simultaneous exposure to multiple field sources operating with thesame frequency

This exposure situation is covered by measuring the combined peak electric and magnetic fieldstrength or magnetic flux density of all simultaneously used field sources at the workplace. Themeasurement time must be sufficiently long to cover the worst-case exposure scenario, especially ifthe operation of the different field sources is not continuous or they are operated under changingconditions, e.g. loads, cycles, settings, parameters.

Alternatively it is possible to take measurements of the peak electric and magnetic field strengthor magnetic flux density for the worst case exposure condition of each field source independentlyand sum up the results before comparing them with the lower or upper action values, respectively.

Both procedures, especially the second one, introduce an overestimation of the exposure situation.However, they are easy to apply and in most cases sufficient to show the compliance of a workplacewith the lower or upper actions levels.

The correct procedure would require to perform a vector addition for the external electric ormagnetic field vectors, respectively, of all field sources for each point within the dimensions of thehuman body and compare the worst case result with the lower or upper action values. However,the use of this solution is limited to numerical calculations of the electric field strength in the tissueusing anatomical body models.

5.3 Simultaneous exposure to multiple frequency fields

5.3.1 Summation formulae

When it comes to the assessment of simultaneous exposures to multiple frequency fields currentsafety standards often refer to summation formulae [51, 53, 57, 63]. Both [51] and [57] state, that’it is important to determine whether, in situations of simultaneous exposure to fields of differentfrequencies, these exposures are additive in their effects’. However, neither of these documentsprovides any guidance whether or not this is the case for a certain exposure scenario. As alreadyshown in [44] there is no additivity associated with simultaneous exposures to multiple frequencyfields for exposure situations at workplaces in general.

The use of summation formulae [51, 53, 63] or the weighted filter approach [57], which relies on thesame mathematical principle, for the assessment of simultaneous exposures to multiple frequencyfields introduces by default a large overestimation of the exposure situation at the workplace.Because both assessment methods are easy to apply, they can only be used to show compliance

31

of an exposure situation with the reference or action levels, respectively. However, if the exposuresituation is deemed non-compliant when using these methods, this need not be the case at all. Forthose situations a more physiological based assessment method must be used.

5.3.2 Assessment of fields with arbitrary temporal behaviour

The procedure outlined in the following section can be used for the assessment for all kinds of fieldsindependently from their temporal course. These assessment procedures are especially useful for,but not limited to, the assessment of non-sinusoidal or pulsed fields.

As already pointed out in section 3 and summarized in section 3.2.6, the area of interaction withexcitable tissue is dependent, among other parameters, on both the direction and the value of thevector of the electric field strength in the tissue.

The signum function sgn(x) is defined as:

sgn(x) =

+1 x > 00 for x = 0−1 x < 0

(5.1)

With this function the effective duration of a pulse or stimulus can be defined as the timeframe τPwhere the signum function of the electric field strength in the tissue Ei is constant but differentfrom zero: Either sgn(Ei) > 0 or sgn(Ei) < 0.

According to eqn. 2.2 the electrical field strength in the tissue Ei is proportional to the externalelectric field strength E0

Ei ∼ E0 (5.2)

proportional to the contact current Ic as given by Ohm’s law

Ei ∼ Ic (5.3)

and also proportional to the time derivative ddt of the external magnetic field B as is shown by

eqn. 2.4

Ei ∼dB

dt(5.4)

and therefore allows that this concept can be extended to both external electric and magnetic fieldsand contact currents, if necessary. However, within this document only pulsed magnetic fields willbe covered.

Magnetic fields with exponential waveforms require some special consideration because the timederivative of an exponential function reaches zero only for infinite time durations. Therefore theeffective duration of a magnetic field with an exponential waveform is defined as the timeframe τPwhere it rises between zero and (1− e−π/2) of its peak value or where it decays from its peak valueto a value of e−π/2 of its peak value.

If the values for τPi differ significantly over time or are different for rising and falling slopes aconservative approach is to base all further assessment on the smallest value for all τPi:

τP,min = min (τPi) (5.5)

In every case the frequency fP can be calculated as:

fP =1

2 · τP,min(5.6)

For an arbitrary time function of a magnetic field both the maximum and the mean rate of changeof the magnetic flux density need to be limited. However, for sinusoidal, triangular, trapezoid

32

and exponential waveforms it is sufficient to show compliance with the mean rate of change of themagnetic flux density only. The rationale for this assessment method can be found in [21, 42].

The maximum allowable rate of change of the magnetic flux density for a pulse with durationτP,min can be calculated as:∣∣∣∣dBdt

∣∣∣∣max

= ω · B = 2π · fP · B =π

τP,min· B (5.7)

Whereas the mean rate of change of the magnetic flux density for a pulse with duration τP,min canbe calculated as: ∣∣∣∣dBdt

∣∣∣∣mean

=B

τP,min= 2 · fP · B (5.8)

Compliance with the values for lower and upper action levels for magnetic fields with frequency fPgiven in tables 4.9 and 4.7 also ensures compliance with eqn. 5.8. This means that for all durationsτP,min the absolute value of the change in the magnetic flux density ∆B = |B(t+ τP,min)−B(t)|must be lower than the peak value B listed in tables 4.7 and 4.9 for the lower and upper actionlevels for magnetic fields with frequency fP.

For sinusoidal, triangular, trapezoid and exponential waveforms it is sufficient to check compliancewith eqn. 5.8. However, for arbitrary waveforms compliance with eqn. 5.7 must be checked, too.

If needed, the admissible values for the maximum and mean rate of change of the magnetic fluxdensity can be calculated for the peak values of the magnetic flux density for the lower and upperaction levels listed in tables 4.7 and 4.9 by using eqn. 5.7 and eqn. 5.8.

5.3.3 Harmonic content

In general, only a limited number of harmonics and, with rising ordinal numbers of the harmonics,i.e. higher frequencies, a decay in the harmonic amplitudes is usually present in electric powersystems.

[44] shows that any harmonic content shortens the effective duration of such a stimulus, which,according to Lapicque’s law, renders it less effective for stimulation. As already pointed out insections 3.2.2 and 3.2.6 hardly any additivity of the different spectral components exists underpractical exposure conditions.

In these cases it is sufficient to separately show compliance for each spectral component with thelower or upper action levels. If this compliance check fails, a more sophisticated method – seesection 5.3.2 – for exposure assessment should be used.

5.4 Localized exposure

The worst-case exposure conditions used to derive the lower and upper action levels from thecorresponding exposure limit values assume a homogeneous exposure of the whole body or thehead and trunk to an electric or magnetic field, respectively.

Especially magnetic field sources with small dimensions in the comparison to the human body orparts of it, which are used in close proximity to the workers’ body lead to highly localized exposureconditions. Because the magnetically induced electric field strength in the tissue is mainly confinedto the geometrical dimensions of the source itself, according to Faraday’s law stated in eqn. 2.4this leads to a smaller value of the surface integral for a given magnetic flux density. Togetherwith a given temporal derivative, e.g. frequency, this in turn leads to a lower electric field strengthin the tissue.

This means, that for localized exposures is it possible to use higher values for the external electricfield strength or magnetic flux density than those given by the lower and upper action levels,

33

respectively. Because a large number of parameters affect these permissible values, they have to becalculated on a case by case basis. However, for a given field source, e.g. device, cable, tool, at aworkplace, it is possible to assume worst-case exposure conditions again and give simple expressionsor even certain numbers for the external electric field strength or the magnetic flux density.

5.5 Movement in static magnetic fields

dB

dt=

dB

ds· ds

dt(5.9)

Eqn. 5.9 links the temporal derivative of a magnetic field to its spatial derivative, i.e. spatialgradient, and a velocity. For maximum effect the way element ds, i.e. the direction of movement,needs to be mutually perpendicular to the magnetic field vector.

With eqn. 5.9 and the results from section 5.3.2 it is possible to show if a movement with a givenvelocity v in a static magnetic field with a given spatial gradient dB/ds of the magnetic field iscompliant with the lower and upper action levels, respectively. However, in the work environment,neither the velocity of the movement nor the spatial gradient of the magnetic field will be constantfor long times or over large spatial areas [69]. Therefore it is necessary to break down the wholepath of movement into small distances for which both a constant velocity and a spatial gradientof the magnetic field could be assumed. Only those parts of the whole path need to be analyzedwhere either the velocity or the spatial gradient of the magnetic field or both reach a maximum.

Measurements of electric fields induced by typical human body movements such as walking orturning in the fringe magnetic field, e.g. of a whole body 3 T scanner gave 0.15 V/m for the upperabdomen, 0.077 V/m for head and 0.015 V/m for tongue [56].

5.6 Interference with active implanted medical devices (AIMD)

Static and time-varying electric and magnetic fields can influence the proper function of activeimplanted medical devices, e.g. pacemakers, implanted cardioverter-defibrillators (ICD) and insulininfusion pumps. The possibility of such interference depends on type, strength, frequency andpolarization of the field(s) and furthermore on the sensitivity of the AIMD and can impair thewell-being of the worker or can even have life-threatening consequences [11, 98].

Because such an interference may occur even if the lower action levels are not exceeded, specialattention must be given to all workplaces where workers with a AIMD are present in the workforce.

Whether or not a worker with a AIMD is fit for his or her job must be determined on a case bycase basis taking into account the exposure situation at the workplace, the type and location ofthe implant, its individual programming and, if applicable, the type and routing of the electrodes.Additional information and guidance for the assessment process is given in [11, 20, 28].

Static and extremely low frequency magnetic fields can trigger a reed switch inside the AIMD,which disables certain functions of the implant or causes it to change its mode of operation. Thiscan be safely avoided if workers with a AIMD are not exposed to static and extremely low frequencymagnetic fields with flux densities exceeding 0.5 mT.

The proper function of a AIMD may be impeded by time varying electric and magnetic fieldsinterfering with either the device circuitry directly or the measurement and detection of bodysignals and parameters, e.g. electrocardiogram (ECG) or blood sugar level. As already pointedout the interference threshold is dependent on many parameters and can only be determined onan individual basis.

[28] lists formulae which can be used to calculate peak electric and magnetic field strength for timevarying electric and magnetic fields for a given implant and its individual parameters. Complianceof a exposure situation with these calculated peak electric and magnetic field strength safely avoidsany interference of these fields with the proper function of the AIMD.

34

5.7 Projectile risk

Ferromagnetic materials, including so-called ’non-magnetic stainless steel’, can become dangerousobjects if exposed to strong static magnetic fields. Depending on their magnetic susceptibilityand their shape, the resulting translational forces and torques can range from negligible to lethalvalues. Current literature often refers to this effect as the so-called ’projectile risk’ [3, 21, 56, 99].However, the magnetic flux densities where these effects are deemed to occur, differ significantlyand range from 3 mT [3], ’in the order of several millitesla’ [56] to ’more than 67.9 mT’ [21].

Implanted devices like aneurysm clips, metal surgical clips or stents, metallic dental implants oreven tattoos and permanent makeup with magnetite or iron oxide based colors can be affectedby rotational or translational forces too, when being exposed to strong static magnetic fields withsometimes life threatening consequences.

A quantitative solution for the translational and rotational forces on a ferromagnetic object beingplaced in a static magnetic field can be obtained by solving Maxwell’s equations for static magneticfields in a Cartesian coordinate system [30]. By restricting the shape of the ferromagnetic objectfrom a general ellipsoid to a rotational symmetrical ellipsoid object and further to the shape ofa sphere, the number of independent principal axes can be reduced from three to one. Furthersimplifications can be reached by placing the ferromagnetic sphere in the static magnetic field atpoints located along the central axis of a cylindrical (superconducting) magnet. The unit vectors~ex, ~ey and ~ez form a right-handed coordinate system, with ~ez pointing to the inside of the magnetand the origin of the coordinate system being placed on the central axis of the magnet.

As is true for magnets commonly used in MRI, the only non-zero spatial magnetic component ata location with coordinates x = 0 and y = 0 is Bz.

The translational force on a ferromagnetic sphere is given by:

Fz ≈3V

µ0·Bz

∂Bz

∂z(5.10)

with V being the volume of the sphereµ0 permeability of free space; µ0 = 4π · 10−7 N

A2 = 4π · 10−7 T·mA

Bz · ∂Bz

∂z product of the z-component of magnetic flux density and its spatial gradientin the z-axis

According to eqn. 5.10 a translational force on the ferromagnetic sphere exists only, if the magneticflux density-spatial gradient-product Bz

∂Bz

∂z is different from zero. This means, that far away from

the magnet (Bz → 0 and ∂Bz

∂z → 0) and in the homogeneous region of the field (∂Bz

∂z → 0), usuallyinside the magnet, no translational force exists and therefore no so-called projectile risk couldoccur. The maximum translational force is to be expected near the opening to the bore for mostmagnets, where the product Bz

∂Bz

∂z reaches its maximum.

In order for the sphere to be accelerated it is necessary to overcome at least the sliding frictionforce Fsf :

Fz!= Fsf (5.11)

Fsf can be calculated as:Fsf = µsf · δ · V · g (5.12)

with µsf sliding friction coefficient; for steel on steel µsf = 0.06

δ mass density; for steel δ ≈ 8000 kgm3

g standard gravity; at sea-level g = 9.80665 Nkg

Solving3V

µ0·Bz

∂Bz

∂z= µsf · δ · V · g (5.13)

35

for the magnetic flux density-spatial gradient-product Bz∂Bz

∂z , the result becomes independent ofthe volume V of the sphere and the exact magnetic quantities of its material, as long as it is aferromagnetic substance with a magnetic volume susceptibility χmv 1:

Bz∂Bz

∂z= µsf · δ · g ·

µ0

3(5.14)

For non-magnetic materials χmv 1 the result is still independent of the volume V of the sphere,but depends on the exact magnetic quantities of the material of the sphere:

Bz∂Bz

∂z= µsf · δ · g ·

µ0

χmv(5.15)

As given by eqn. 5.15 so-called ’non-magnetic’ materials require a much higher magnetic fluxdensity-spatial gradient-product than ferromagnetics, in order to overcome the initial friction force.So, for a worst case assumption, it is safe to focus on ferromagnetic materials with a high suscep-tibility value.

Eqn. 5.14 gives for a ferromagnetic steel sphere:

Bz∂Bz

∂z≈ 2 · 10−3 T2

m(5.16)

Most unshielded superconducting cylindrical magnets used in MRI, independent of their absolutemagnetic field strength, have a ratio max

(B2

z

)/max

(∂Bz

∂z

)in the range of 1.8 . . . 2 m−1 [45, 97]

which can be derived from characteristic manufacturer data.

This magnetic field characteristic (spatial magnetic gradient) together with eqn. 5.16 gives a min-imum magnetic flux density Bz ≈ 60mT needed to overcome the initial frictional force, which inturn makes it possible that the sphere is accelerated in the magnetic field and a so-called ’projectilerisk ’ can occur. This result is in line with the value given in [21].

In general, shielded magnets have a smaller ratio max(B2

z

)/max

(∂Bz

∂z

)than unshielded magnets.

Because of the higher spatial gradients this leads to a lower minimum magnetic flux density whichcould constitute a so-called ’projectile risk ’. Current data for shielded systems suggests minimummagnetic flux densities in the central axis of a superconducting cylindrical magnet in the rangefrom 30 . . . 40 mT necessary for a projectile risk to occur.

For non-spherical objects not only a translational force can exist, but a torque as well. Needleshaped rotational ellipsoids try to turn their long axis parallel to the direction of the field. Themagnitude of the torque is proportional to B2

z , so the maximum torque is to be expected in thecenter of the magnet and can be higher than the maximum translational force.

36

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43

Annex

A Quantities, variables, abbreviations and SI-units

Quantity Symbol or UnitAbbreviation (Value)

Magnetic flux density B Tesla (T)Electric field strength E Volt per meter (V/m)Current I Ampere (A)Voltage, potential u, U , ϕ, Φ Volt (V)Force F Newton (N)Frequency f Hertz (Hz)Permittivity ε Farad per meter (F/m)Permeability µ Henry per meter (H/m)Permeability of free space µ0 Henry per meter (H/m)

(µ0 = 4 · π · 10−7 H/m)Conductivity κ Siemens per meter (S/m)Charge q Coulomb (C)Capacity C Farad (F)Resistance R Ohm (Ω)Distance s, d Meter (m)Diameter D Meter (m)Time T , t, τ Second (s)Volume V Cubic meter (m3)Velocity v Meter per second (m/s)Standard gravity g Newton per kilogram (N/kg)

(at sea level: g ≈ 9.80665 N/kg)Mass density δ Kilogram per cubic meter (kg/m3)

(for steel: δ ≈ 8000 kg/m3)Sliding friction coefficient µsf —

(for steel on steel: µsf = 0.06)Constant π —

π ≈ 3.14159Unit vector ~e –

44

B Tissue data

Tissue data necessary for numerical calculations using anatomical body models, e.g. Visible Human,were extracted from the body tissues database established by Gabriel et. al. [32, 33, 34, 35].

Table B.1 contains some sample tissue data used for calculations in this report. The full data setis listed in the body tissue database.

Mean tissue conductivities of the whole body or parts of the body in the low frequency range listedin table B.1 are obtained by integrating the individual tissue properties over the whole body orparts of the body using an anatomical body model.

Mean tissue conductivity [S/m]

Frequency Whole body Head Torso

50 Hz 0.216 0.254 0.23310 kHz 0.276 0.285 0.256100 kHz 0.288 0.300 0.332

Table B.1: Mean tissue conductivity in the low frequency range for the whole body andparts of the body

45

Affiliations

Borner, F.Institute for Occupational Safety and Health of the German Social Accident Insurance, Sankt Au-gustin, Germany.

Bruggemeyer, H.Lower Saxony Water Management, Coastal Defence and Nature Conservation Agency, Hildesheim,Germany.

Eggert, S.Federal Institute for Occupational Safety and Health, Berlin, Germany. (retired)

Fischer, M.German Social Accident Insurance Institution for the Energy, Textile, Electrical and Media Prod-ucts Sectors, Cologne, Germany.

Heinrich, H.2h–engineering & –research, Hausen, Germany.

Hentschel, K.Federal Institute for Occupational Safety and Health, Berlin, Germany.

Neuschulz, H.Federal Institute for Occupational Safety and Health, Berlin, Germany.

Acknowledgments

The research presented in this report was supported and funded by the German Federal Ministryof Labour and Social Affairs.

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Electromagnetic fields at workplaces

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