0131382268

CHAPTER 3(b)

Industrial Hygiene

Chapter 3(b): Industrial Hygiene 2

Instructional Learning Objectives

After completing this chapter, students should be able to :

1. Define Industrial Hygiene (IH) and explain the scope of IH

2. Estimate TWA and TLV-TWA

3. Evaluate worker exposure to toxicants and noise

4. Assess worker exposure to toxic vapors

5. Determine the vaporization rate of a liquid


Industrial hygiene is the science of anticipating, recognizing, evaluating, controlling workplace conditions and preventing workplace environmental stressors that can cause sickness or serious discomfort to workers.

Industrial Hygiene


ANTICIPATION Expectation of hazard existence

IDENTIFICATION Presence of workplace exposure

EVALUATION Magnitude exposure

CONTROL Reduction to acceptable levels

Scope of Industrial Hygiene (IH)

For this topic, the focus will be on the

Evaluation aspect

Evaluation phase determines the extent and

degree of employee exposure to toxicants and

physical hazards in the workplace


Acute Exposure: Usually Minutes, Hours or Several Days

characterized by sudden and severe exposureand rapid absorption of the substance. Normally, a single large exposure is involved. Acute health effects are often reversible

Chronic Exposure: Regular Exposure Over Months, Years, or a Lifetime

characterized by prolonged or repeated exposures over many days, months or years. Symptoms may not be immediately apparent. Chronic health effects are often irreversible.

Evaluation : Measurement of Exposure


Determination of exposures can be done by monitoring the concentrations of toxicant in air continuously

For continuous concentration C(t), the TWA (time-weight average) concentration is computed:

wt

dttCTWA0

)(8

1

C(t) is the concentration of a toxicant in the air in ppm or mg/m3

tw is the worker shift time in hours

TWA typically for 8 hrs exposureTLV-TWA typically for 12 hrs exposure

hrs

TCTCTCTWA nn

8

......2211

Evaluating Exposures to Toxicant:Time Weighted Average


Partial period samples

4 hours @ 11ppm

2 hours @ 14ppm

2 hours @ 20ppm

ppm14

hr8

hrppm112

hr2hr2hr4

ppm20hr2ppm14hr2ppm11hr4

TWA

Example of TWA Calculation


Exposure limits (TLVs) are meant for single substances, but multiple simultaneous exposures always happened in industry

If more than one chemical is present, the effects of toxicants can be assumed to be additive.

Combined exposure limit can be calculated if:

• Components have similar toxicological effects

• Combined effect is assumed to be additive

Evaluating Exposures to Toxicant:TWA for Mixture


The combination of exposures :

where n is the total number of toxicants,Ci is the concentration of chemical i

(TLV-TWA)i is the TLV-TWA for chemical species i

If the sum exceeds 1, the workers are overexposed

n

i i

i

TWATLV

C

1 )(

0.1)TWATLV(

C...

)TWATLV(

C

)TWATLV(

C

n

n

2

2

1

1

Mixture in compliance if:



Mixture:

Methyl isopropyl ketone @ 100 ppm

(TLV = 200ppm)

Methylcyclohexane @ 300ppm (TLV = 400ppm)

Both TLVs set for protection against anesthetic

(CNS) effects

25.1ppm400

ppm300

ppm200

ppm100Limit Exposure

This mixture is NOT in compliance

(workers are OVEREXPOSED)



The mixture TLV-TWA can be computed from:

n

i i

i

n

ii

mix

TWATLV

C

C

TWATLV

1

1

)(

)(

If the sum of the concentration of the toxicants in the mixture exceeds this amount, the workers are overexposed.

For mixtures of toxicants with different effect (acid vapour mixed with lead fume), the TLV-TWAcannot be assumed to be additive.



Air contains 5 ppm of diethylamine (TLV-TWA of 10 ppm), 20 ppm of cyclohexanol (TLV-TWA of 50 ppm), and 10 ppm of propylene oxide (TLV-TWA of 20 ppm). What is the mixture TLV-TWA and has this level been exceeded?

Solution

ppm25

ppm20

ppm10

ppm50

ppm20

ppm10

ppm5

ppm10ppm20ppm5)TWATLV( mix

The total mixture concentration is 5 + 20 + 10 = 35 ppm. The workers are overexposed



Noise levels are measured in decibels (dB)

010

I

Ilog10 (dB)intensity Noise

Source of noise Sound intensity Maximum exposure level (dBA) (hr)

An absolute sound scale (in dBA), the hearing threshold is set at 0 dBA.

Factory 90 8Passing truck 100 2Punch press 110 0.5

Refer to Table 3-11 (Crowl & Louvar – pg. 90) for the Permissible Noise Exposure from OSHA CFR1910.05

Evaluating Exposures to Noise: Noise Intensity (decibel)


Determine whether the following noise level is permissible:

Noise level Duration Max allowed(dBA) (hr) (hr)

85 3.6 16 95 3.0 4110 0.5 0.5

Solution

n

i i

i

TWATLV

C

1 )(97.1

5.0

5.0

4

3

16

6.3

The sum exceeds 1.0, employees are immediately required to wear ear protection. Control methods should be developed for long-term exposure.

Evaluating Exposures to Noise: Noise Intensity (decibel)


Estimating Worker Exposures to Toxic Vapours: Vapour Concentration

The best way to determine exposures to toxic vapour is measuring the vapour concentrations directly.

Commonly, estimates of vapour concentrations are required in enclosed spaces, above open containers, where drums are filled and in the area of spills.

Volatile rate OutkQvC (mass/time)

Concentration of volatile, C in enclosed volume, V(mass/volume)

Ventilation rate, Qv

(volume/time)

Evolution rate of volatile, Qm

(mass/time)

Chapter 1: Introduction

C is the concentration of volatile vapor in the enclosure (mass/volume)

V is the volume of the enclosure (volume)Qv is the ventilation rate (volume/time)k is the non ideal mixing factor (unitless), and Qm is the evolution rate of volatile material (mass/time)

Perfect mixing k = 1, for non ideal mixing, k varies from 0.1 to 0.5

dt

dCV

dt

)VC(d volatile of onAccumulati

VC volume in volatile of mass Total

mQ evolution from volatile of rate Mass

CkQv out volatile of rate Mass

16



CkQQdt

dCV vm species volatile the on balance massdynamic The

v

m

kQ

QC0

dt

dC

hence term onaccumulati the state,steady At

Let m represent mass, ρ represents density, subscripts v and b denote the volatile and bulk gas

66 1010

PM

TR

V

m

V

VC g

b

v

b

vppm

whereRg is the ideal gas constant T is the absolute ambient temperatureP is the absolute pressure, M is the molecular weight of the volatile

17



610PMkQ

TRQC

v

gmppm

The term mv /Vb (mass/volume) is identical to Qm / kQv (mass/volume)

The equation is used to determine the average concentration (ppm) of any volatile species in an enclosure system.

Assumption:A steady-state condition is assume, the accumulation term in the mass balance is

zero

18



Estimating Worker Exposures to Toxic Vapors

EXAMPLE 3-7

An open toluene container in an enclosure is weighed as a function of time, and its is determined that the average evaporation rate is 0.1 g/min. The ventilation rate is 100ft3/min. The temperature is 80°F and the pressure is 1 atm. Estimate the concentration of toluene vapor in the enclosure.

610PMQ

TRQkC

v

gm

ppm

Solution: The value of k is not known directly, use as a parameter

Qm = 0.1 g/min = 2.20 × 10-4 Ibm/minRg = 0.7302 ft3 atm/lb-mol°R,T = 80°F = 540°R,M = 92 lbm/lb-mol,P = 1 atm

ppm43.9

10)mollb/lb92)(atm1min)(/ft100(

)R540)(Rmollb/atmft7302.0min)(/lb1020.2(kC 6

m3

oo3m

4

ppm

k varies from 0.1 to 0.5, the concentration is expected to vary from 18.9 to 94.3 ppm.

19


Estimating the Vaporisation Rate of a Liquid

Liquids with high saturation vapour pressures evaporate faster.For a vaporization into stagnant air, the vaporization rate is generally expressed via:

For many situation, P sat ≥ p

Lg

sat

mTR

MKAPQ

where

Qm = evaporation rate (mass/time)

M = molecular weight of the volatile substance

K = mass transfer coefficient (length/time) for an area A

Rg = ideal gas constant, and

TL = absolute temperature of the liquid

Psat = saturation vapour pressure of pure liquid at temperature of the liquid

p = partial pressure of the vapour in the bulk stagnant gas above the liquid

Lg

sat

mTR

pPMKAQ

20


610PkQ

KAPC

v

sat

ppm

The vaporization rate of a volatile in an enclosure resulting from evaporation of a liquid

The gas mass transfer coefficient is estimated :

31

M

MKK o

o

21

Estimating the Vaporisation Rate of a Liquid

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