Understanding Combined Chemical Exposure in the Workplace
April 25th, 2023
In the field of industrial hygiene, it is important to consider that an employee may be simultaneously exposed to several different chemical substances in the workplace. Most occupational exposure limits, including that of the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs), are based on a single chemical substance exposure. Therefore, it is often necessary to evaluate the combined effects of an exposure to determine if additional protection is needed. If the toxicological effect of the substances is similar (i.e., each substance has a similar toxicological effect on the same target organ/system), it can be considered that the combined effect of the chemicals will be the sum of individual effects. A common source of information to assist with the determination of combining effects is provided within the ACGIH TLV & Biological Exposure Indices (BEI) guidebook (“TLV Basis” column found in the table of Adopted Values). It is important to consult the most up to date version of the TLVs and BEI’s for toxic effects information.
A common example of additive effects and application of the TLVs for mixtures would be simultaneous exposure to several organic solvents in a workplace.
In order to evaluate this as a mixture, ACGIH recommends the following calculation: C1/T1 + C2/T2 +…. Cn/Tn*
*C is the concentration in the air of the individual substance and T is the associated threshold limit.
If the sum of equation is higher than 1, the TLV of the mixture is considered exceeded.
(Note that there are several possible combinations of the Threshold Limits as shown in Table E-1 of the TLV & BEI guidebook based on the duration of the sample collection period and assigned exposure threshold limit type)
Assessing the combining effects of a chemical exposure can be a complex task which requires an understanding of the different possible modes of chemical interaction. In addition to the additive interaction discussed above, there are also possible synergistic and antagonistic effects, each of which requires a different approach. For certain categories of carcinogens (i.e., A1, A2, or A3), one must be cautious when applying this method. This approach should not be used for complex mixtures (e.g., certain combustion exhaust).
Moris N. Ayamba, MS, CSP, Senior Project Consultant at HRP Associates, Inc.
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