As the environmental health and safety (EHS) manager, you have been asked to evaluate an existing operation to determine employee’s current exposure to hexane, which is a chemical used in painting processes.You are to complete an evaluation of the employee’s current exposure by determining the employee’s exposure to hexane (toxic contaminant) by calculating time-weighted average (TWA) and compare it to the permissible exposure limit (PEL) and modify the PEL, if necessary. If it is necessary to modify the PEL, explain the reasoning why it is to be modified. Once the current exposure has been determined, select the appropriate ventilation system between dilution and local exhaust that is the best choice to control the concentration of the contamination. Ensure that you provide the reasons why you think your selection of ventilation system is the best choice. You are to provide the results in a three-page project report using the
Unit III Consulting Report template
.ScenarioAn employee working in a room measuring 15 ft x 20 ft x 15 ft sprays paint assembly parts using a paint containing hexane. The PEL for hexane is 500 ppm. The employee wears a half-face, air-purifying respirator, but management would like to install engineering control measures to reduce or eliminate the employee’s exposure to hexane. The options are to:(1) install a dilution ventilation system, or(2) install a local exhaust ventilation system.The employee works a 9-hour shift and has the following exposures:For the general dilution ventilation system to work, there must be at least 20 room air changes per hour. The fan which supplies air to this area produces 1,500 cubic feet per minute (cfm) (Q).In order for the local exhaust ventilation system to remove the contaminant to acceptable concentrations, the capture velocity must be at least 140 feet per minute (fpm). The proposed local exhaust system ventilation system uses a 1.0 feet diameter duct and a fan that provides a total volume of 1,275 cubic feet per minute (cfm). The source of the contamination is located 1.5 feet from the opening of the duct. Determine the appropriate ventilation control and calculate the capture velocity. UNIT III STUDY GUIDE
Particulates, Gases, and Ventilation
Course Learning Outcomes for Unit III
Upon completion of this unit, students should be able to:
1. Evaluate occupational hazards in the workplace.
1.1 Determine employee exposures to toxic contaminants by calculating time-weighted averages
(TWAs).
1.2 Identify when it is necessary to modify the permissible exposure limit (PEL) for a specific
contaminant.
1.3 Calculate permissible exposure limit (PEL) modifications when it is determined appropriate.
5. Evaluate the appropriate calculations to control occupational hazards.
5.1 Determine the appropriate ventilation control for a given scenario (general, dilution, or local
exhaust ventilation).
5.2 Calculate the capture velocity to remove a contaminant at its source using specific data.
Required Unit Resources
Chapter 4: Particulates and Gases
Chapter 7: Ventilation
Unit Lesson
This unit is designed to provide a basic review and understanding of chemistry that is essential for the
environmental, health, and safety professional. It will cover the periodic table, chemical bonding, moles,
molecules and compounds, mixtures and formulas, along with pH. The textbook will delve further into the gas
laws, calculating permissible exposure limits (PELs) and threshold limit values (TLVs). In addition, this unit
discusses the use of ventilation to reduce or eliminate harmful chemicals or toxicants to acceptable
concentrations. It is one thing to understand how to complete an equation from a textbook, it is another to put
those equations into practical application. This Unit III Study Guide will demonstrate the practical uses of many
of the equations.
Practical Exercise
As the safety manager for a manufacturing facility, you have been asked to evaluate an employee exposure to
styrene and compare it to the PEL. Styrene is a lubricating spray component used in the fiberglass molding
process. The lubricating material is sprayed onto a heating element where it becomes volatized. The PEL for
styrene is 100 ppm. The employee is working a 12-hour shift. As such, the PEL needs to be modified. As part
of the exposure evaluation, you have collection three samples, the results are as follows:
Sample #
1
2
3
Sampling Minute
330 minutes
185 minutes
205 minutes
Sample Results (ppm)
90 ppm
75 ppm
70 ppm
The first step in determining the overall daily exposure for this employee is to calculate the time-weighted
average (TWA), using the following equation:
𝑇𝑊𝐴 =
(𝐶1 𝑇1 ) + (𝐶2 𝑇2 ) + (𝐶𝑛 𝑇𝑛 )
total hours workes
OSH 4308, Advanced Concepts in Occupational Safety and Health
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UNIT x STUDY GUIDE
Where:
Cn = Concentration
Tn = Time
Title
TWA =
(90 ppm)(5.5 h) + (75 ppm)(3.1 h) + (70 ppm)(3.4 h)
12 h
TWA =
(495 ppm/h) + (232.5 ppm/h) + (238 ppm/h)
12 h
TWA =
965.5 ppm
12 h
𝐓𝐖𝐀 = 𝟖𝟎. 𝟓 𝐩𝐩𝐦
Since all PELs are based on 8-hour shifts, it must be modified to reflect a 12-hour shift.
Note: Only when a shift exceeds 8 hours must the PEL be modified. The PEL cannot be modified when a shift
is less than 8 hours.
The following equation is used to modify a PEL:
PEL𝑛 hours =
PEL(or TLV) × 8 h
(actual shift worked in hours)
The 8-hour PEL for Styrene is 100 ppm.
PEL12 hours =
100 𝑝𝑝𝑚 × 8 h
12 h
PEL12 hours =
800 𝑝𝑝𝑚/ℎ
12 h
𝐏𝐄𝐋𝟏𝟐 𝐡𝐨𝐮𝐫𝐬 = 𝟔𝟕 𝒑𝒑𝒎
Comparing the actual employee exposure to styrene at 80.5 ppm for a 12-hour shift to the modified PEL, it is
determined that the employee’s exposure exceeds the 12-hour PEL. Since this exposure exceeds the PEL,
steps must be taken to mitigate (i.e., reduce or eliminate) the employee’s overall exposure.
There are several methods that can be taken to reduce or eliminate the employee’s exposure including
administrative controls such as working 8-hour shifts, using a less hazardous chemical lubricant, or removing
the chemical through means of mechanical ventilation. In the current case, there is no acceptable replacement
chemical and management has decided that it is necessary to work 12-hour shifts. As such, the chemical can
be removed at the source using mechanical ventilations. For this scenario, the best means of removing the
contaminant is by the use of local exhaust ventilation (LEV) with a side draft illustrated below in Figure 1.0.
OSH 4308, Advanced Concepts in Occupational Safety and Health
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UNIT x STUDY GUIDE
Title
Figure 1.0
Since this scenario is one that requires the removal of the contaminant at its source, we use the capture velocity
equation. The side slot opening is located 1.5 feet from the contaminant source and the fan is pulling (negative
pressure) at a volume (Q) of 1,450 cubic feet per minute (cfm). Using the data in the scenario and the drawing,
calculate whether the side slot is properly sized?
𝑉=
𝑄
10𝑋 2 + 𝐴
Where:
V = velocity (fpm)
Q = flow rate (cfm)
X = source distance from hood opening (ft)
Note: The equation is only accurate for a limited distance of 1.5 times the diameter of a round duct or the
side of a rectangle or square duct.
A = area (square feet [ft2])
The first step would be to determine the area of the duct opening, using the rectangle area equation.
𝐴=𝐿𝑥𝑊
Where:
A = area
L = length
W = width
𝐴=𝐿𝑥𝑊
𝐴 = 3 𝑓𝑡 𝑥 1 𝑓𝑡
𝐴 = 3 𝑠𝑞𝑢𝑎𝑟𝑒 𝑓𝑒𝑒𝑡 (𝑠𝑓)
Next, we insert the known data into the equation as follows:
1,450 cfm
10(1.5 ft)2 + (3 ft 2 )
1,450 𝑐𝑓𝑚
𝑉=
25.5 𝑠𝑞. 𝑓𝑡
𝑉 = 56.86 ( 𝑜𝑟 57) fpm
𝑉=
Therefore, the capture velocity under this given scenario is 57 fpm. The opening of the slot is inadequate and
must be modified (Yates, 2020).
OSH 4308, Advanced Concepts in Occupational Safety and Health
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These calculations are useful tools in controlling workplace exposures to toxicUNIT
chemicals.
TheGUIDE
results can
x STUDY
provide a quantitative analysis of actual employee exposures and whether or not
a proposed solution can
Title
meet the requirements prior to a costly implementation that may or may not work.
Reference
Yates, W. D. (2020). Safety professional’s reference and study guide (3rd ed.). CRC Press.
https://online.vitalsource.com/#/books/9781000029789
Learning Activities (Nongraded)
Nongraded Learning Activities are provided to aid students in their course of study. You do not have to submit
them. If you have questions, contact your instructor for further guidance and information.
Complete this Unit III Chapter 4 Matching Activity to reinforce your understanding on particulates and gases
as discussed in Chapter 4.
PDF version of Unit III Chapter 4 Matching Activity
OSH 4308, Advanced Concepts in Occupational Safety and Health
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