Adjusting OELs (Occupational Exposure Limits)

Occupational Exposure Limits or OELs are, by definition, based on the “standard” shift length of 8 hours. That is, the assigned OEL is the amount of chemical exposure deemed acceptable for the average male over an 8-hour period. What happens when your shift is not a standard length?

How to Adjust the OEL

Those who work shifts longer than 8-hours cannot compare their exposure to the OELs listed in the OHS Code (Schedule 1, Table 1). Instead, a bit of mathematical wizardry must be conducted in order to modify the OELs and thus ensure that all workers, regardless of shift length, are protected. 

Part 4 of the OHS Code provides a formula for this wizardry, which allows for calculation of a value called the “Daily Reduction Factor” or DRF. Once obtained, the DRF is multiplied by the provided 8-hour OEL to get an adjusted OEL.

Here's How it Works:

A worker works a 12-hour shift, with potential exposure to lead.

DRF = [(8/12) x (24 – 12)/16] = 0.5

Next, we multiply the DRF of 0.5 by the OEL for lead (0.05 mg/m3)

0.5 x 0.05 = 0.025 mg/m3

The adjusted OEL is 0.025 mg/m3, which can now be used in conjunction with occupational air monitoring to determine the workers’ level of exposure.

Is adjustment of OELs always required for shift longer than 8-hours?

Not always.

Some of the OELs listed in Schedule 1, Table 1 of the OHS are accompanied by a substance interaction value of 3. In those cases, OELs are based on irritation effects, and adjustment of OELs for unusual work schedules is not required. As example of this notation is seen for Acetaldehyde, as shown below:

oel chart

In other cases, OELs should not be adjusted because of laboratory analysis limitations – such as the case with Silica. The listed OEL is 0.025 mg/m3, which is just barely above the laboratory Limit of Detection (LOD, the lowest concentration which can be detected in the laboratory test for a given analyte). If an adjustment was performed, the adjusted OEL would be at or below the LOD, which would mean that every sample, regardless of silica concentration, would be above the OEL. 

Currently, no notation has been added to Silica in Schedule 1, Table 1, however, special permission from the Director of Occupational Hygiene in Alberta has been given to not adjust this OEL until the required amendments have been made to the OHS Code.

What about shifts shorter than 8 hours?

Is any adjustment required? The short answer is no – in these cases, exposure should be kept as low as reasonably achievable, but definitely below the OEL. However, short term exposure limits can be a bit more complicated:

For some contaminants, 15-minute (ceiling) exposure limits are listed in the OHS Code (Schedule 1, Table 1). As defined in the legislation, workers cannot be exposed to levels above the 15-minute ceiling limit at any time during the work shift, even if the total exposure throughout the shift (Time Weighted Average or TWA) is below the OEL. Additionally, workers cannot be exposed to levels between the OEL and 15-minute ceiling exposure limit for longer than 15-minutes, no more than 4 times a day, with at least 60 minutes between exposure occurrences.

For those contaminants with no assigned 15-minute ceiling exposure limit, exposure should not exceed 3x the 8-hour OEL for more than 30 minutes during a continuous 24-hour period. It should never exceed 5x the 8-hour OEL or the concentration that is immediately dangerous to life and health (IDLH).

Here’s an example:

Frank is exposed to Isopentyl Acetate while working an 8-hour shift. Air monitoring reveals exposure levels peak at 500 mg/m3 once during the shift, with a TWA (exposure normalized over the 8-hour work period) of 200 mg/m3. The 15-minute ceiling limit is 532 mg/m3, and the OEL is 266 mg/m3. Is Frank overexposed? 

As his peak exposure is below the 15-minute ceiling limit and only occurs once, this is acceptable. Additionally, the TWA is below the 8-hour OEL. Thus, Frank is not considered over-exposed in this scenario.

OELs chart two

Next week, Frank’s boss asks him to work a 12-hour shift. What now? Should the hygienist adjust the OEL to reflect the longer shift? 

Look at the substance interaction notation – for Isopentyl Acetate, the notation “3” is provided in the table, which means no adjustment is required. During this 12-hour shift, air monitoring reveals that Frank’s peak exposures occurred twice – the first was 520 mg/m3 and the second, which occurred 10 minutes later, was 345 mg/m3. Is Frank considered overexposed this time?

Both peak exposures were below the 15-minute ceiling limit, however, they were not 60 minutes apart. Even though there were only 2 events, the closeness of them means that Frank is considered overexposed in this scenario.

As you can imagine, many complex exposure scenarios can arise in a workplace. The best way to protect your workers is to ensure the air quality within your environment is regularly monitored and evaluated by a qualified industrial hygienist.

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