IRSST - Institut de recherche Robert-Sauvé en santé et en sécurité du travail

Guide for the adjustment of permissible exposure values (PEVs) for unusual work schedules


Current knowledge on the adjustment of PEVs to unusual work schedules, meaning schedules other than eight hours per day, five days per week, was recently summarized in several publications particularly relevant to the Québec context3,4,5,6,7,8,9. These publications describe the main methods for calculating PEV adjustment factors in the case of substances that require an adjustment.

The PEV adjustment process in this guide is based on a guiding principle that was agreed on in Technical Committee 3.33.1 for Schedule I of the ROHS7. In toxicological terms, for many chemical contaminants, an equilibrium is established between the accumulation of a contaminant in the body during the time at work and the elimination of the contaminant during the time away from work (this period is assumed to be exposure-free) until the maximum body burden or accumulation plateau in the body is reached. The time-weighted average exposure value (TWAEV) applicable to workers exposed to these contaminants during unusual work schedules must therefore be modified to ensure that the maximum body burden does not exceed the maximum body burden reached by a worker with a conventional work schedule. As a corollary, no adjustment of the standard is necessary for any means of exposure or any toxic action of a contaminant that is unrelated in any way to the body burden.

With this guiding principle as a basis and using the logic of the Occupational Safety and Health Administration3 (OSHA), as inspiration, a group of toxicologists met at the IRSST to propose adjustment categories9 (I, II, III and IV) for each of the substances found in Schedule I of the ROHS as well as a method for calculating adjustment factors supported by toxicokinetic modeling11,12. This group of experts also defined the conditions and limitations of application of the adjustment procedure.

Using these recommendations, the members of Technical Committee 3.33.1 for Schedule I of the ROHS of the Commission de la Santé et de la Sécurité du Travail (CSST, Québec workers’ compensation board) ont established a consensus on the conditions of application of PEV adjustment (see Appendix I). The present guide is the tool for facilitating the application of PEV adjustment for unusual work schedules using toxicological considerations as a basis as reviewed by consensus by the members of the Schedule I committee. It provides several definitions and conditions of application, explains the PEV adjustment process and the interpretation of the adjusted average exposure value (AAEV), and presents some application examples in the Appendix II.

Additional Information

Category: Technical guide
  • Daniel Drolet
Research Project: 0099-6920
Online since: February 19, 2008
Format: Text