OSHA currently has two standards in which the PEL is adjusted based on the length of the work shift, both of which are lead standards:
All other PELs are based upon 8-hour time weighted average (TWA), a short term exposure limit (STEL), or a ceiling limit (C).
The lead PEL of 50 micrograms per cubic meter (ug/m3) is adjusted in accordance with the following formula:
New PEL = 400/hours worked during shift
e.g. If an employee works a 10 hour shift, the PEL is reduced from 50 ug/m3 to 40 ug/m3
Additionally, as discussed in OHShub.com‘s post PEL Adjustments for Extended Work Shifts – Noise, the Action Level for noise must be reduced for extended work shifts based on the following formula:
New AL (dBA) = 90 + 16.61 x log (50 / (12.5 x number of hours worked))
e.g. If an employee works a 10 hour shift, the PEL is reduced to 83 dBA
From a recommended practice standpoint, the Brief and Scala model provides a easy method of reducing exposure limits and is noted ACGIH’s TLVs and BEIs documentation. The Brief and Scala model provides a reduction factor via the following formula:
Reduction Factor = (8 x hours worked in shift) x ((24 – hours worked in shift) / 16)
It should be noted that the Brief and Scala model should not be used to justify very high exposures for short durations.
e.g. If an employee works a 10 hour shift, the reduction factor is 0.7. Multiply 0.7 times to the Exposure Limit to determine the recommended limit using the Brief and Scala model.
Additionally, it should be noted that OSHA does have PEL adjustments for cotton dust, but only when employees are required to wear respirators to reduce their respective exposures.