Skip to content

Glossary

TWA

Also: Time-Weighted Average

Time-weighted average: airborne exposure averaged over a period, usually 8 hours. OSHA's asbestos PEL is 0.1 f/cc as an 8-hour TWA.

A time-weighted average (TWA) is an exposure level averaged over a defined period, weighting each concentration by how long it lasted. It smooths the short spikes and lulls of a shift into a single number.

OSHA’s asbestos permissible exposure limit is expressed as a TWA: 0.1 fiber per cubic centimeter of air averaged over an 8-hour day. The separate excursion limit — 1.0 f/cc over any 30-minute period — exists precisely because an 8-hour average can hide a dangerous short burst.

Personal air monitoring during asbestos work is what produces the TWA figures that determine whether exposures are within the limit and whether medical surveillance is triggered.

Build the vocabulary into your daily work.

Nexus uses these terms the way the rule uses them. No interpretation tax.

Start free trial Talk to a person