OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 4 Chemical Hazards, Biological Hazards and Harmful Substances

Section 18 Exposure during shifts longer than 8 hours

The amount of time a worker is exposed to a substance has a large effect on the total amount of material absorbed by the body. Non-traditional work schedules have become more common in the workplace. There is an increasing trend towards longer workdays with more days off between shifts. Many continuous process operations such as chemical manufacturing, oil refining, steel processing, oil and gas exploration and paper processing require two or three shifts in a 24 hour period to accommodate continuous production. Workers may routinely work overtime during periods of heavy demand. A second job may also result in workers being exposed to chemicals for extended periods.

OELs are based on the assumption that exposure occurs over an 8-hour period, following which the body is no longer exposed, but allowed to recover for the next 16 hours. When work shifts exceed 8 hours, these assumptions no longer hold true and the worker could be at increased risk of exposure. Although limits can be adjusted downward to accommodate longer periods of exposure, limits cannot be adjusted upwards to accommodate shorter periods of exposure. Numerous substances listed in Table 2 of Schedule 1 appear with the number “3” in the “Substance Interaction” column. For these substances, occupational exposure limits do not need to be adjusted to compensate for unusual work schedules.

The risk of an increased exposure to certain chemicals (body burden) has been recognized and several models have been proposed to modify the 8 hrs/day, 40 hrs/week standard to a “non-standard” workday. The intent of the models is to maintain the same overall body burden yet preserve the same margin of safety as the original standard. The Brief and Scala model used in this section is the simplest and most conservative model. It compensates for unusual work schedules by reducing the permissible concentration in proportion to both the increase in exposure time and the reduction in recovery time.

The employer may use other models to adjust exposure time as long as the models have been developed using recognized scientific principles approved by a Director of Occupational Hygiene. A Director of Occupational Hygiene is a staff member of Alberta Human Services, appointed by the Minister under section 5 of the OHS Act. It is recommended that a competent person be consulted to ensure that the adjustment method is appropriate and applicable since many models are theoretical and contain assumptions that may not apply to every chemical and work environment.

An understanding of the chemical is required and caution must be taken when limited toxicity data is available, the toxic effect is serious, or the chemical accumulates in the body following repeated exposure. The Director of Occupational Hygiene may accept alternate methods through approval of individual applications or publication of a Safety Bulletin. To obtain approval from the Director, the employer must show that

(a) the method is appropriate for the substance(s) used at the workplace. The employer will need to provide justification and a rationale for use of the particular method, and

(b) if the exposure is to more than one substance, possible potentiating and synergistic effects have been taken into account when applying the method.

For more information

The Effects of Unusual Work Schedule and Concurrent Exposures on Occupational Exposure Limits (OELs)
Bulletin CH055