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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
Chapter 3(b): Industrial Hygiene 3
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
Chapter 3(b): Industrial Hygiene 4
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
Chapter 3(b): Industrial Hygiene 5
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
Chapter 3(b): Industrial Hygiene 6
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
Chapter 3(b): Industrial Hygiene 7
Partial period samples
4 hours @ 11ppm
2 hours @ 14ppm
2 hours @ 20ppm
ppm14
hr8
hrppm112
hr2hr2hr4
ppm20hr2ppm14hr2ppm11hr4
TWA
Example of TWA Calculation
Chapter 3(b): Industrial Hygiene 8
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
Chapter 3(b): Industrial Hygiene 9
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:
Evaluating Exposures to Toxicant:TWA for Mixture
Chapter 3(b): Industrial Hygiene 10
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)
Evaluating Exposures to Toxicant:TWA for Mixture
Chapter 3(b): Industrial Hygiene 11
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.
Evaluating Exposures to Toxicant:TWA for Mixture
Chapter 3(b): Industrial Hygiene 12
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
Evaluating Exposures to Toxicant:TWA for Mixture
Chapter 3(b): Industrial Hygiene 13
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)
Chapter 3(b): Industrial Hygiene 14
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)
Chapter 3(b): Industrial Hygiene 15
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
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Estimating Worker Exposures to Toxic Vapours: Vapour Concentration
Chapter 1: Introduction
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
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Estimating Worker Exposures to Toxic Vapours: Vapour Concentration
Chapter 1: Introduction
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
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Estimating Worker Exposures to Toxic Vapours: Vapour Concentration
Chapter 1: Introduction
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.
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Chapter 1: Introduction
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
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Chapter 1: Introduction
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
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Estimating the Vaporisation Rate of a Liquid