Exposure standards represent the airborne concentration of a particular substance or mixture that must not be exceeded to protect the health of workers. They are based on the airborne concentrations of individual substances which, according to current knowledge, should not cause adverse health effects nor undue discomfort to nearly all workers.
Exposure standards have been developed for conventional work shifts of five consecutive 8-hour work days, followed by two days off (40-hour work week).
Deviations from this traditional shift pattern are now the norm within the mining industry, and the potential for increased periods of contaminant exposure needs to be addressed through appropriate adjustment of 8-hour time-weighted exposure standards.
From trial and discussion there are four principal adjustment models that have emerged over time from the burgeoning number on offer:
- Brief and Scala Model: Accounts for increased exposure time and reduced recovery time, but does not consider the action of the agent within the body. This model is considered to be the most conservative of the four major models.
- OSHA Model: Categorises substances based on their toxic effect and assumes the magnitude of toxic response correlates to the concentration reaching the site of action of the agent.
- Pharmacokinetic Model: Uses a scientific, substance-specific methodology based around the concept of ‘body burden’. The model is considered to be a relatively accurate method of exposure standard adjustment, but involves complicated calculations and knowledge of biological half-lives.
- Québec Model: Developed from the OSHA model but attributed the most recent, available toxicological information to generate adjustment categories for specific contaminants. These categories further assign an adjustment type and associated reduction factor calculation.