Safety Tidbit 3.11 - Exposure Calculations for Extended Work Shifts

Safety Tidbit 3.11 – Exposure Calculations for Extended Work Shifts

Reference:    OSHA RA Letter dated November 10, 1999

Last Friday, a fellow consultant pitched me one of their random phone call questions. She says the client has a problem with air monitoring. Not sure, could she have been any vaguer?  I called the client, and after a few questions and humble curiosity, I learned that the client’s concern dealt with determining a worker’s exposure when the shift is more than the traditional eight hours. More specifically, how would OSHA determine the worker’s exposure when they work a 12-hour shift.

Two methods dominate how to determine worker exposure when working extended shifts: the conservative way and the OSHA way.  The prudent technique is known as the Brief and Scala Model and has been around for a long time and is detailed in Patty’s Industrial Hygiene and Toxicology. A letter dated November 10, 1999, to the Regional Administrators details the OSHA way.

We’ll take the OSHA way first. OSHA says the compliance officer has two approaches to sample extended work shifts. First, the compliance officer tests what they believe to be the worst 8-hour work period. The second method involves the compliance officer being more creative and taking multiple samples that encompass the worst 8-hours of exposure.

The Brief and Scala Model adjusts the exposure limit based on how many hours worked and how many hours left for recuperation.

Reduction Factor = (8/h) * ((24-h)/16)

Then multiply the Permissible Exposure Limit (PEL) by the reduction factor yielding an adjusted PEL.

Scenario: A worker puts in a 12-hour shift and is exposed to substance X. How does the employer determine if the employee’s exposure is below the OSHA 8-hour time-weighted average Permissible Exposure Limit (PEL)?

OSHA will try to determine the worst eight hours of exposure and the sample during that time and compare the level to the Permissible Exposure Limit. Brief and Scala model gives a reduction factor of 50%. In other words, the employer will sample for the entire 12 hours and then compare the exposure level to half of the PEL.

When I explained this to the client, he said, so I just need to figure out what the worst 8 hours of exposure are, right?  Well, I tried.

Hope this was helpful and thanks for reading my Safety Tidbits! Comments and questions are always welcome. ~ Bryan

P.S. If you have an interesting safety or health question please let me know.