TWA (Time-Weighted Average)
TWA is the standard averaging period for workplace airborne exposure. Most WHS limits are set as 8-hour TWA: how it works and why it matters on construction.
Ask Chalkline about this →A time-weighted average (TWA) is the standard averaging period used to measure a worker’s exposure to an airborne hazardous substance over a defined timeframe, almost always 8 hours (a normal work shift). Australian WHS regulations express most exposure limits as an 8-hour TWA: the average concentration of a substance in the worker’s breathing zone over the shift must not exceed the limit, even if peak concentrations during specific tasks were higher.
How it works
If a worker is exposed to a substance at varying concentrations across an 8-hour shift, those concentrations are mathematically averaged (weighted by the time at each level) to produce a single TWA number. For example, a worker exposed to a substance at 0.10 mg/m³ for 4 hours and 0 mg/m³ for the remaining 4 hours has an 8-hour TWA of 0.05 mg/m³.
This matters because a single brief spike does not automatically breach the limit, and a low-level continuous exposure can. The TWA measures the cumulative dose over the shift rather than the worst moment.
Where it shows up
Most Australian airborne exposure limits are expressed as 8-hour TWAs (verified 2026-05-29, see respirable crystalline silica):
- Respirable crystalline silica (RCS): 0.05 mg/m³ 8-hour TWA, set by Safe Work Australia under the model WHS Regulation Schedule 14. From 1 December 2026 the WES becomes a Workplace Exposure Limit (WEL); the headline number stays the same.
- Asbestos fibres: 0.1 fibres/mL 8-hour TWA.
- Lead dust, solvent vapours, welding fume, and other airborne contaminants: each with their own 8-hour TWA, all listed in the Workplace Exposure Standards document.
Some substances also carry a short-term exposure limit (STEL) for 15-minute peaks and a peak limit for instantaneous exposure; the 8-hour TWA is the baseline.
Why it matters on site
For a builder running construction work that generates dust or fume (cutting concrete, welding, painting, demolition), TWA is the metric an air-monitoring assessment is measured against. Three practical points:
- Personal sampling: TWA is measured on the worker (lapel sampler) over the shift, not on a fixed point in the room.
- Controls hierarchy: TWAs above the limit trigger engineering controls (local exhaust ventilation, wet-cutting), administrative controls (job rotation, scheduling), and finally respiratory PPE.
- Documentation: any monitoring done as part of an exposure assessment becomes part of the WHS record for the workers involved.
Also known as: time-weighted average, 8-hour TWA.
Category: WHS / exposure measurement
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Last updated: 2026-05-29. Verified: 2026-05-29. Quarterly review for currency.