Hi. My name is Chinomso Ibe.
I am a graduate research assistant at the University of Minnesota, and this is a hands-on
activity demonstration on "Choosing an Occupational Exposure Limit".
I developed this activity along with Tom Peters from the University of Iowa and Pete Raynor from the University of Minnesota.
An occupational exposure limit, also referred to as OEL, is the maximum concentration or quantity of a chemical, biological,
or physical agent that is considered safe or an acceptable risk in an environment.
OELs help us make decisions about exposure to these agents, especially in workplace environments.
There are three main types of OELs.
The time-weighted average OELs, or TWA, are established for exposures to substances
that can lead to chronic or long-term adverse effects if exceeded.
The TWA is the airborne concentration to which it is believed that nearly all workers may be
repeatedly exposed, day after day, for a working lifetime without adverse effects.
Thus, TWA exposures are usually measured over an 8-hour work day or 40-hour work week.
Short-term exposure limits OEL, or STEL, are applied to substances that cause acute or short-term adverse health effects if exceeded.
The STEL is the airborne concentration to which workers can be exposed continuously
for only a short duration of time, usually within a period of 15 minutes.
Ceiling (C) limit OELs are established for the maximum concentration of substances that will lead to acute adverse health effects.
These airborne concentrations must not be exceeded at any given time.
In the United States, there are several key groups that establish OELs.
The Occupational Safety and Health Administration (OSHA) issues and enforces
regulatory OELs called Permissible Exposure Limits, or PELs.
The National Institute of Occupational Safety and Health (NIOSH) sets Recommended Exposure Limits, or RELs.
The American Conference of Governmental Industrial Hygienists (ACGIH) provides technical guidance OELs called Threshold Limit Values,
or TLVs, as well as the Biological Exposure Indices, or BEIs.
The purpose of this activity is to determine and locate the appropriate OELs for 4 exposure scenarios and their corresponding data set
using several resources, one of which is the OSHA annotated Z table.
The annotated Z table contains details of the federal OSHA and the California State
OSHA PELs, NIOSH RELs, and ACGIH TLVs by substance on OSHA’s website.
For this activity, we will only consider the federal OSHA PEL, NIOSH REL, and ACGIH TLV.
Substances are listed on either Z-1, Z-2, or Z-3 tables. Each substance will have one or more of the various types of OELs.
The short-term OEL is denoted by the letters ST, and the ceiling OEL is denoted by the letter C.
For example, the California OSHA PEL for the substance acetic acid, has a
time-weighted average of 10 parts per million, a short-term limit of 15 parts per million, and a ceiling limit of 40 parts per million.
The NIOSH Pocket Guide is another good resource for this activity.
It contains RELs and PELs for various substances and also provides methods to measure exposures to those substances.
The Pocket Guide can be found on the Centers for Disease Control and Prevention website.
Let’s consider the first exposure scenario.
Hydrogen sulfide is produced naturally from decaying organic matter in locations such as manure waste pits or sewers.
Workers managing the St. Lawrence Seaway described "flu-like symptoms" and "general ill health", possibly due to exposure to
stagnant water and decaying marine life during the annual winter inspection, cleaning, and repairs of the locks.
Below are the maximum recorded exposure levels on personal breathing samples of 6 workers at various locations of the locks.
First, calculate the average concentration of hydrogen sulfide gas as measured from the 6 workers.
So, 18+87+68+5+11+2, divided by 6, equals a value of 32 parts per million or ppm.
Next, determine the type of OEL for the scenario.
In this case, the "maximum recorded exposure levels" were recorded and workers already showed signs of acute health symptoms.
Recall that the ceiling limit is the exposure to the maximum concentration of a substance that lead to acute adverse health effect.
Therefore, the type of OEL suggested here is the ceiling limit.
Given the situation, look up the ceiling limit OEL for hydrogen sulfide established by
OSHA, ACGIH, and NIOSH on the OSHA Annotated Z-2 Table.
Both OSHA and NIOSH have established ceiling limits for hydrogen sulfide, but ACGIH has not established one.
The PEL ceiling limit for hydrogen sulfide is 20 parts per million and the REL is 10 parts per million.
Enter the 20 parts per million and the 10 parts per million in the table.
In comparing the ceiling limit OEL values to the calculated average value of 32, it is apparent that
the hydrogen sulfide gas exposure levels are above the OSHA PEL and the NIOSH REL.
Let’s take a look at the second exposure scenario for methylene chloride in its liquid state.
Management at an analytical laboratory was concerned about multiple solvent exposure in the preparation of soil and water samples.
An environmental survey was conducted in the organic preparations area at the lab,
which included the measurement of short-term solvent concentrations in lab refrigerators using a Miran direct-reading instrument.
Below are 6 samples of methylene chloride concentrations measured by the Miran analyzer.
Here again, determine the average concentration of the methylene chloride measured.
So, add 4, 17, 8.7, 2.7, 5.9, and 1.1 parts per million together and divide the answer by 6 and the average is 6.6 parts per million.
The type of OEL reported here is the short-term exposure limit, also known as STEL.
Next, locate the STEL for methylene chloride on the OSHA Annotated Z Table or the NIOSH Pocket Guide.
Let’s try the NIOSH Pocket Guide.
Here, we find the OSHA PEL STEL for methylene chloride to be 125 parts per million. There are no STELs for ACGIH TLVs or NIOSH RELs.
Enter the 125 parts per million OEL value in the table and compare it to the average concentration of 6.6 parts per million.
It is apparent that the methylene chloride levels are below the OSHA PEL.
The third exposure scenario says: Managers at a building restoration and waterproofing company wanted to know if the employees were protected
against respirable silica particles during parking garage repair.
Below are full-shift air samples for respirable silica (quartz) collected on four employees over a day.
The dust contained 20% silica by weight.
The first step would be to calculate the average of the 4 air samples by adding 0.15, 0.13, 0.05, and 0.22 milligram per cubic meter together,
divide the answer by 4 to arrive at an average value of 0.14 milligram per cubic meter.
It is reported that the samples were collected over a full-shift, which is equivalent to an 8-hour work day.
Recall that a time-weighted average OEL is typically set for an 8-hr work day.
In this case, we can conclude that the type of OEL to locate for this substance is the time- weighted average OEL.
Given the situation, look up the time-weighted average for silica air particles provided by OSHA, ACGIH, and NIOSH.
The OSHA Annotated Table Z-3 shows a TLV of 0.025 milligram per cubic meter and an REL of 0.05 milligram per cubic meter.
The OSHA PEL requires the percentage of the respired silica from the air be factored when determining the time-weighted average.
Thus the formula: 10 milligrams per cubic meter divided by the summation of the percentage of the respired silica and 2.
Enter the 20 percent silica weight in the formula.
The answer, 0.45 milligram per cubic meter is the OSHA PEL time-weighted average for this specific exposure.
In comparing the three time-weighted average values of 0.45, 0.025, and 0.05 milligram per cubic meter to the calculated average value of
0.14 milligram per cubic meter, it is apparent that the silica particle exposure levels were above the
ACGIH TLV and NIOSH REL but below the OSHA PEL.
In the fourth exposure scenario, NIOSH collected manganese air samples from MIG welding
operations of a manufacturing company, with a sampling time of approximately 500 minutes.
The samplers can be considered to collect total or inhalable (IHL) samples of the particles.
First, determine the average concentration of manganese fumes by adding 110, 54, 270, 150, 110, and 240 micrograms per cubic meter by 6
to arrive at a value of 156 micrograms per cubic meter, which is equivalent to 0.156 milligram per cubic meter.
It is reported that the samples were taken over a 500-minute sampling time which is
approximately an 8-hour work day exposure or time-weighted average.
Given the situation, look up the time-weighted average for manganese fumes provided by OSHA, ACGIH, and NIOSH.
The OSHA Annotated Z-1 Table shows TLV and REL values for manganese fumes, but there is no time-weighted average provided for OSHA PEL.
Enter the OEL values for manganese fumes in the table. In comparing the time-weighted average TLV value of 0.1 milligram
per cubic meter and the time-weighted average REL of 1 milligram per cubic meter to the calculated average value of 0.156 milligram
per cubic meter, it is apparent that the manganese fume level at the
manufacturing company is above the ACGIH OELs but below the NIOSH OEL.
This the end of this activity. I hope this video has shown you how to determine and locate the various types of OELs to assess exposures.
Thank you for watching!