Determination of Atmospheric Workplace Exposure Limits for Mixtures

When conducting air monitoring as part of workplace exposure assessments, situations involving multiple chemicals with additive health effects are assessed differently than for a single chemical exposure. Both the OSHA Air Contaminant Regulations (29 CFR 1910.1000) and the ACGIH TLVs^® and BEIs^®publication provide guidance on calculating exposure limits for such mixtures.

When health effects are additive, individual chemical concentrations within the mixed exposure are expressed as a fraction of the individual chemical exposure limit, and the individual fractions are then added together. If the result is greater than 1 (or unity), then the exposure limit for that mixture combination was effectively exceeded.

For example, consider the measurements below from a workplace atmosphere that contained methyl ethyl ketone, toluene, methanol, and 2-butoxyethanol. All are identified as affecting the central nervous system.

Individually, none of the substance concentrations exceeded its specific TLV. However, applying the mixture formula where the fractional values of each substance’s air concentration divided by their respective TLVs are summed, the result is 1.15.

Since the resulting value exceeds 1 (or unity), measurements indicated the TLV for this atmospheric mixture was exceeded. The OSHA calculation for mixtures, which uses the same formula, is detailed at 29 CFR 1910.1000 (d)(2)(i). The ACGIH TLVs^® and BEIs^® publication contains considerably more detail on applicability of this approach.