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Assignment #3: Calculation of Time Weighted Averages (TWA) and Conversions

Review the in-class calculation activity notes for assistance in completing these basic industrial hygiene calculations and conversions. Use the OSHA Annotated PELs (https://www.osha.gov/dsg/annotated-pels/) to look up the various PELs and TLV® TWAs.

1. A worker is exposed to four different airborne contaminants in the workplace. Samples have already been taken and the exposure concentrations for each of the four contaminants has been determined. The table below presents the results of these calculations. Determine the combined effect of these simultaneous exposures to determine if the worker exposure has exceeded the PEL. (6 points)

Table 1. Data for Problem #1

Substance	Observed Concentration	OSHA PEL for Substance
A	60 ppm	225 ppm
B	25 ppm	40 ppm
C	161 ppm	275 ppm
D	67 ppm	75 ppm

What is the equivalent exposure for this mixture of 4 substances? ____________________

Is this mixed exposure within acceptable limits, according to the OSHA PEL? __________

2. Cresols are widely occurring natural and manufactured aromatic organic compounds having a yellowish to brownish red tint, a distinctive “coal tar” aroma, and melting points near room temperature. The name cresol (creosol) reflects their traditional source, creosote. One isomer (m-Cresol) is a precursor to two commercially and widely used pesticides. A worker in the bagging plant for a pesticide manufacturer is exposed to airborne particulates containing cresol. (14 points)

The worker works five 8-hour days each week. An air monitoring device that sampled air in the breathing zone was attached to the worker during a typical work day and produced the results illustrated in Table 3.

Find the applicable occupational exposure limits (OELs) for creosol (all isomers) on the OSHA Annotated PELs table and populate Table 2.

Table 2. OELs for Creosol (all isomers)

Substance	CAS No.	Regulatory Limits	Recommended Limits
		OSHA PEL	Cal-OSHA PEL	NIOSH REL	ACGIH TLV
		ppm	mg/m3	8-hour TWA	Up to 10-hour TWA	8-hour TWA

Calculate the 8-hour time-weighted average (TWA) of worker exposure based on the information provided below to determine if the worker has been exposed above the OEL.

Table 3. Calculating the Time-Weighted Average

Time Period	Period Duration	Contaminant Concentration	Calculate Dose for each Time Period
1	90 minutes	19 mg/m3
2	30 minutes	32 mg/m3
3	15 minutes	39 mg/m3
4	120 minutes	23 mg/m3
5	180 minutes	20 mg/m3
6	45 minutes	21 mg/m3
Sum of Exposures	min	mg/m3	min x mg/m3

Use the TWA Calculation to determine the 8-hour TWA for this creosote exposure.

Was this worker exposed to this chemical above the OSHA PEL?

Was this worker exposed to this chemical above the TLV®-TWA?

Which time period(s) would you want to evaluate further? _______________________________

3. During painting operations, keeping oil-based paints the proper consistency is often accomplished through the addition of a thinning liquid. There are a variety of paint thinners, each utilizing a different mixture of solvents. During the mixing process and during application, particularly when an air applicator is used, there is the potential for workers to be exposed to vapors. A worker uses a combination of three different paint thinning agents; naphthalene, dimethylformamide (DMK), and ethylbenzene. The worker then applies the paint using a pneumatic spray gun inside a room that has been closed off from adjoining rooms by plastic sheets to prevent overspray and migration of odors, as well as to keep dust from entering the room. The observed concentrations of each thinning agent are identified in Table 4. (6 points)

Table 4: Finding the OSHA PEL Values in the Annotated Tables

Substance	Observed Concentration	OSHA PEL-TWA for Substance
Naphthalene	5 ppm
Dimethylformamide	6 ppm
Ethylbenzene	48 ppm

Perform the required calculations using the OSHA PEL-TWA values to determine the combined (mixture) effect so you will know if the worker was exposed above the PEL.

Was the worker’s exposure greater than the PEL? _____________________________

4. You reviewed a Safety Data Sheet for a new product and the main ingredient is toluene. (4 points)

Find the applicable occupational exposure limits (OELs) for toluene on the OSHA Annotated PELs table (Table Z-2) and populate the table shown below.

Table 2. OELs for Toluene

Substance	CAS No.	Regulatory Limits	Recommended Limits
		OSHA PEL	Cal-OSHA PEL	NIOSH REL	ACGIH TLV
		ppm	mg/m3	8-hour TWA	Up to 10-hour TWA	8-hour TWA

Sampling media: Charcoal Tube Maximum volume: 12 Liters Maximum flow rate: 0.05 L/min

How many minutes do you need to sample for to obtain the maximum allowed volume using the maximum allowed flow rate?

_________ min

How many samples will you need to take to represent an 8-hour (480 minute) work day?

_______ samples

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