Occupational Exposure Limits: An Approach and Calculation Aid for Extended Work Schedule Adjustments

Past reviews of occupational exposure limit (OEL) adjustments have covered both decision logic and calculation methods to derive factors to assure protection of workers on extended (also known as unusual) work shifts. The approaches reviewed included several Haber's rule based methods, several variants of single compartment toxicokinetic (TK) models, and physiologically based pharmacokinetic modeling. These models calculate OEL adjustment factors based on the work shift and the uptake and elimination of the toxicant. A key parameter of the TK models is the biologic half-life of the toxicant, but reliable data for the half-life are not available for all substances of concern. A spreadsheet is presented that implements TK calculations, with one of the presented TK calculation alternatives not dependent on half-life data. This half-life data independent approach is suggested as a viable option for situations when the toxicant's half-life is unknown or uncertain.

Keywords:

• adjustment factors
• compressed work week
• extended work shifts
• occupational exposure limits
• toxicokinetics
• unusual work schedule

ACKNOWLEDGMENTS

ExxonMobil Occupational Exposure Limits Committee funding provided support for Armstrong and Caldwell's (while with ExxonMobil) contributions to this manuscript.

Notes

^A Adapted from Laperé et al. showing adjustment based on maximum blood concentration and (in parentheses) area under the blood concentration curve. See Ref. 11 for further discussion.

^B This is the maximum recommended work week cycle for using the Brief and Scala2 method.

^C NA = Not applicable or not available. Brief and Scala's publication2 discusses extension to 1 week. Further extension raises difficulties about the work/rest cycle, and may give inappropriate results in such cases. Lapare et al.11 did not report values for the shifts noted NA in the last column.

^A For this example, it is presumed that the half-life is not available.

^B Not calculable without biologic half-life data.

^C A combined exposure limit greater than 1.0 for the mixture indicates that the additive effects combined TLV is exceeded.