Industrial Hygiene
ERT 312
Lecture 7 – Identification, Evaluation and Control
Identification
Able to identify the hazard from single exposure or
potential combined effects from multiple exposures
Require deep study on the chemical process, operating
conditions and operating procedures
Source of information;
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2
Process design descriptions
Operating instructions
Safety reviews
Equipment specs
Etc.
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4
Material Safety Data Sheets (MSDS)
Chemical Safety Data Sheets (CSDS)
MSDS lists the physical properties of a substance that may
be required to determine the potential hazards of the
substance
Manufacturer/supplier is responsible to provide the MSDS
to their customers
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* Example of MSDS
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Evaluation
To determine the extent and degree of employee
exposure to toxicants and physical hazards in the
workplace
Once exposure data obtained, comparison is being made
to acceptable occupational health standards eg: TLVs, PELs
and IDLH concentrations (page 56)
Then, the decision on proper control measure can be
made accordingly in order to reduce the risk
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6
Threshold Limit Value (TLV) of a chemical substance
is a level to which it is believed a worker can be exposed
day after day for a working lifetime without adverse
health effects
The Permissible Exposure Limit (PEL or OSHA
PEL) is a legal limit for exposure of an employee to a
substance or physical agent. For substances it is usually
expressed in parts per million (ppm), or sometimes in
milligrams per cubic metre (mg/m3)
IDLH is an initials for Immediately Dangerous to
Life and Health, and is defined by the NIOSH as
exposure to airborne contaminants that is "likely to cause
death or immediate or delayed permanent adverse health
effects or prevent escape from such an environment”
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Table 2.7 – established by ACGIH
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TLVs units – ppm, mg/m3,
For dust – mg/m3 or mppcf
For vapors, concentration in ppm;
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Cppm =
=
22.4 T 1
3
(
)( )(mg / m )
M 273 P
T
3
0.08205(
)(mg / m )
PM
Equation 1
Equation 2
T (temperature, Kelvin), P (absolute pressure, atm) M
(molecular weight, g/g-mol)

9
Problem 2.7 (Crowl & Louvar, 2002)

How much acetone liquid (ml) required to produce a
vapor concentration of 200 ppm in a room of dimension
3 x 4 x 10 m?
Given T is 25°C, P is 1 atm, molecular weight is 58.1 and
specific gravity is 0.7899.
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11
Evaluation Exposure of Organic
Toxicants
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The simplest way to determine worker exposures is
through continuous monitoring of the air concentrations.
For computation of continuous concentration data C(t)
the TWA concentration,
1
TWA   C (t )dt
80
tw

C(t)

tw
12
Equation 3
the concentration of the toxicant in the air,
ppm @ mg/m3
the worker shift time in hours

Sometimes, continuous monitoring is not feasible.
Therefore, intermittent samples representing worker
exposure at fixed points of time are obtained.
C1T1  C2T2  ...  CnTn
TWA 
8
Single Component Exposure, workers are
overexposed if the sum of conc. > permitted TWA
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Equation 4
Example 3.3 (Crowl & Louvar, 2002)

Determine the 8-hr TWA worker exposure if the worker
is exposed to toluene vapors as follows;
Solution:
Duration (h)
Concentration (ppm)
2
110
2
330
4
90
C1T1  C2T2  ...  CnTn
TWA 
8
Answer: 155 ppm
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Equation 5

For a case of more than 1 toxicant is present in the
workplace; the combined exposures from multiple
toxicants with different TLV-TWAs is determined by;
Ci

i 1 (TLV  TWA)
i
n

n
Ci

(TLV-TWA)i

15
If the sum of the equation > 1,
workers are overexposed
Equation 6
the total number of toxicants
the conc. of toxicant i with respect to
the other toxicants
the TLV-TWA for toxicant sp. i

The mixture also TLV-TWA can be computed using
equation below;
n
(TLV  TWA) mix 
C
i 1
i
Ci

i 1 (TLV  TWA )
i
n
If the total mixture conc. > (TLVTWA)mix , workers are overexposed
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Equation 7
Example 3.2 (Crowl & Louvar, 2002)
Air contains 5 ppm of diethylamine (TLV-TWA = 10 ppm),
20 ppm cyclohexanol (TLV-TWA = 50 ppm) and 10 ppm
of propylene oxide (TLV-TWA = 20 ppm). What is the
mixture TLV-TWA and has this level been exceeded?
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Evaluation of exposure to dusts
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Dusts particle size range of 0.2-0.5 µm
Particles > 0.5 µm unable to penetrate the lungs
Particle < 0.2 µm settle out too slowly, most exhaled with
the air
Units: mg/m3 @ mg/mppcf
n
TLVmix 
18
C
i 1
i
Ci

i 1 TLV
i
n
Equation 8
Example 3-5 (Crowl & Louvar, 2002)
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Determine the TLV for a uniform mixture of dusts
containing the following particles;
Type of dust
Concentration
(wt.%)
TLV (mppcf)
Nonasbestiform
70
20
Quartz
30
2.7
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Solution:
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Answer: 6.8 mppcf
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Evaluation of exposure to Noise
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Noise levels are measured in decibels (dB)
A dB is a relative logarithmic scale used to compare the
intensities of two sounds. If one sound is at intensity I and
another sound is at intensity Io, then the difference in
intensity levels in dB is given;
Noise intensity (dB) = - 10 log10(I/Io)
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Example 3.6 (Crowl & Louvar, 2002)
Determine whether the following noise level
permissible with no additional control features:
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Noise Level (dBa)
Duration (hr)
Max. allowed (hr)
85
3.6
No limit
95
3.0
4
110
0.5
0.5
is
Solution:
Ci

i 1 (TLV  TWA)
i
n
(TLV – TWA)mix, noise =
Ci
3.6
3 0.5

 
 1.75

i 1 (TLV  TWA)
no limit 4 0.5
i
n
The sum > 1.0, workers are immediately required to wear
ear protection. For long term plan, noise reduction
control should be applied.
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Evaluation of exposure to Toxic Vapors
Enclosure volume, V
Ventilation rate, Qv
(volume/time)
C ppm 
QmR gT
kQ v PM
Equation 9
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Volatile concentration, C
(mass/volume)
Volatile rate out, kQvC
(mass/time)
106
Evolution rate of volatile, Qm
(mass/time)
Assumptions
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The calculated concentration is an average concentration
in the enclosure. Localized conditions could result in
significantly higher concentrations; workers directly above
an open container might be exposed to higher
concentrations
A steady-state condition is assumed; that is, the
accumulation term in the mass balance is 0
The non-ideal mixing factor, k varies from 0.1 – 0.5 for
most practical situations. For perfect mixing, k = 1
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Example 3.7 (Crowl & Louvar, 2002)
An open toluene container in an enclosure is weighed as
a function of time, and it is determined that the average
evaporation rate is 0.1 g/min. the ventilation rate is 100
ft3/min. the temperature is 80oF and the pressure is 1 atm.
Estimate the concentration of toluene vapor in the
enclosure, and compare your answer to the TLV for
toluene of 50 ppm.
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Solution
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Use equation 9 to solve the problem
From data given;
Qm 0.1 g/min
Rg 0.7302 ft3.atm/lb-mol.oR
T
80oF = 540oR
Qv 100 ft3/min
M
92 lbm/lb-mol
P
1 atm
k
?
C ppm 
QmR gT
kQ v PM
106
Answer:
kCppm = 9.43 ppm
K varies from 0.1 – 0.5, therefore Cppm may vary from 18.9 – 94.3 ppm.
Actual vapor sampling is recommended to ensure that TLV is not exceeded
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Estimating the vaporization rate of a liquid
Qm
Qm
Open Vessel
Chemical Spill
Volatile Substances
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
General expression for vaporization rate, Qm (mass/time):
MKA( P  p)
Qm 
RgTL
sat
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M
K
Rg
TL
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Equation 10
Molecular weight of volatile substance
mass transfer coefficient (length/time) for an area A
ideal gas constant
absolute temperature of the liquid
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For most cases, Psat >> p;
MKAP
Qm 
RgTL

sat
Equation 11
The equation is used to estimate the evaporation rate of
volatile from an open vessel or a spill of liquid
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To estimate the concentration of volatile in enclosure
resulting from evaporation of a liquid;
Equa. 11 used
in Equa .9
Most
events,
T = TL
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K
31
sat
C ppm
KATP
6

10
kQv PTL
Equation 12
sat
C ppm
KAP
6

10
kQv P
gas mass transfer coefficient
Equation 13
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Estimation of K, gas mass transfer coefficient;
K  aD
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a
D
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constant
gas-phase diffusion coeeficient
2/3
Equation 14
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To determine the ratio of the mass transfer coefficient
between species K and a reference species Ko;
K D
  
K o  Do 

2/3
Equation 15
The gas-phase diffusion coefficients are estimated from
the molecular weight, M of the species;
D
Mo

Do
M
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Equation 16

Combined equation 15 & 16, simplified;
 Mo 
K  Ko 

M 

Kwater
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0.83 cm/s
1/ 3
Equation 17
Example 3.8 (Crowl & Louvar, 2002)
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A large open tank with a 5-ft diameter contains toluene.
Estimate the evaporation rate from this tank assuming a
temperature of 77oF and a pressure of 1 atm. If the
ventilation rate is 3000 ft3/min, estimate the
concentration of toluene in this workplace enclosure.
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Evaluation of exposure during vessel
filling operations
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For this case, volatile emissions are generated from 2
sources:
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Evaporation of a liquid, (Qm)1
Displacement of the vapor in the vapor space by the liquid
filling the vessel, (Qm)2
Therefore, the net generation of volatile;
(Qm) = (Qm)1 + (Qm)2
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Equation 18
Total Source = Evaporation + Displaced Air
Volatile in
Vapor
Evaporation
Liquid
Vessel
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MKAP
(Qm )1 
RgTL
(Qm ) 2  rf Vc v
rf
v
38
sat
Equation 19
Equation 20
constant filling rate of the vessel (time-1)
density of the volatile vapor

Hence, the net source term;
Equation 20
sat
MP
Qm  (Qm )1  (Qm ) 2 
(rf Vc  KA)
RgTL
Equation 21
sat
C ppm
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P
6

(rf Vc  KA) 10
kQv P
Problem 3.24 (Crowl & Louvar, 2002)
55-gallon drums are being filled with 2-butoxyethanol. The
drums are being splash-filled at the rate of 30 drums per
hour. The bung opening through which the drums are
being filled has an area of 8 cm2. estimate the ambient
vapor concentration if the ventilation rate is 3000 ft3/min.
the vapor pressure for 2-butoxyethanol is 0.6 mm Hg
under these conditions.
2-butoxyethanol chemical formula: HOCH2C2HOC4H9
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Solution:
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Appendix A
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Appendix B

Conversion of Fahrenheit (°F) to Rankine (°R)
1st step
Convert Fahrenheit to Celcius
2nd step
Convert Celcius to Kelvin
K
3rd step
Convert Kelvin to Rankine
TF  32
TC 
1 .8
T  TC  273.15
TR  1.8TK
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