Interpreting an eight-hour permissible exposure limit

October 6, 1995
Mr. Mark Nicas
University of California
Environmental Health Sciences
School of Public Health
[Recipient address omitted]
Dear Mr. Nicas:
This is in response to your letter of July 7, addressed to Mr. Joseph A. Dear, Assistant Secretary for the Occupational Safety and Health Administration (OSHA), requesting an interpretation on 8-hour total weight average (TWA) permissible exposure limit (PEL) as defined in 29 CFR 1910.1000 and in other substance-specific health standards.
The 8-hour TWA PEL is defined in the Federal Register, Vol. 57, No. 114, June 12 1992, pps 26539, 26556, 26572, 26573 and 26590 as follows:

"TWA is the employee's average airborne exposure in any 8-hour work shift of a 40-hour work week which shall not be exceeded."

The 8-hour TWA PEL is the level of exposure established as the highest level of exposure an employee may be exposed to without incurring the risk of adverse health effects. This should not be confused with short term exposure limits (STELs) or peaks. Therefore, your interpretation #1 comes closest to OSHA's definition of an 8-hour TWA PEL. OSHA would be justified in issuing a citation when personal air sampling indicates employee exposure to be in excess of the PEL on the day sampled.
It is well established by the industrial hygiene profession that a workplace survey should be conducted in a facility to determine the potential for exposure(s) in excess of established exposure levels such as the PEL and TLV's. This survey must include job tasks and the potential created for the exposure by these tasks over all operational shifts as well as the potential for increased exposure(s) as a result of production/work demands. The industrial hygiene survey will allow the employer to establish which shift(s), process(es), etc. need to be sampled in order to acquire a representative sampling of employee exposure. Good industrial hygiene practice dictates that significant changes in the work process would call for additional personal air monitoring thus acquiring revised data which will be representative of employee exposure(s).
As you may be aware, the State of California administers its own occupational safety and health program under a plan approved and monitored by Federal OSHA. Therefore, employers in California must comply with State occupational safety and health requirements. States are required to adopt and enforce occupational safety and health standards at least as effective as those promulgated by Federal OSHA and may be more stringent. If you would like further information regarding California occupational safety and health requirements, you may contact the California Department of Industrial Relations directly at the following address:

[Chuck Cake, Acting Director
California Department of Industrial Relations
Office of the Director
455 Golden Gate Avenue
San Francisco CA 94102
Telephone: (415) 703-5070]

We hope this information is helpful. If you have any further questions, please contact the [Office of Health Enforcement at (202) 693-2109].
Sincerely,
Joseph A. Dear
Assistant Secretary
(Corrections 5/23/2003) Please see the May 13, 1999 letter to Mr. Douglas S. Ellmann, Esq. for additional clarification on this issue.

[Requester letter reproduced on OSHA page; omitted here.]