Time-Weighted Average (TWA)

A time-weighted average is a worker's average exposure over a set period, usually an 8-hour shift, with each concentration weighted by how long it lasted. OSHA's asbestos limit of 0.1 fibers per cubic centimeter is an 8-hour time-weighted average, so short bursts of very high exposure can be averaged out by quieter periods.

Origin

Time-weighted averages became the standard way to express workplace limits in the mid-1900s, used in the American Conference of Governmental Industrial Hygienists' threshold limit values and later in OSHA's permissible exposure limits from 1971. OSHA added a separate short-term excursion limit for asbestos in the late 1980s because averages can hide peaks.

Significance in Asbestos Context

Because a TWA spreads exposure across the whole shift, a worker who spent twenty minutes sawing asbestos board in a cloud of dust and the rest of the day doing other work might show an average that looks moderate. For a claim, this matters in several ways. Company records and defense experts may point to TWAs below the limit of the time to argue that exposure was small or lawful, while plaintiffs' experts look at the task-level peaks and at how many years the work went on. Compliance with a limit does not mean exposure was safe; OSHA itself has said there is no known safe level of asbestos exposure, and the limit has been lowered several times. Experts also use TWAs to build cumulative exposure estimates in fiber-years.

Example Sentence

The 8-hour TWA was below the 1976 limit, but the short-term samples taken while the men cut the pipe covering were many times higher.

Related Terms

Permissible Exposure Limit (PEL), Threshold Limit Value (TLV), Excursion Limit, Fiber Year, Cumulative Exposure, Personal Breathing Zone Sample, No Safe Level of Exposure

Notes

• If records show your exposure was "within limits," ask whether that was an 8-hour average and what the short-term readings showed.
• The legal limit changed several times; a level that met the limit in the 1970s would exceed today's.