What is a time-weighted average (TWA)?
A time-weighted average (TWA) is the average concentration of a substance in the air you breathe, or the average noise level, over a fixed period of time. Occupational exposure limits are often expressed as a TWA. This means exposure may briefly be higher, as long as the average over that period stays below the limit.
Types
- 8-hour TWA: the average over an 8-hour working day. This is the most common form, because it matches a normal shift.
- 15-minute value (STEL): a short-term exposure limit for peak exposure, for example while sawing, sanding or mixing.
- Ceiling value (C): an absolute limit that must not be exceeded at any moment.
The unit depends on the substance: usually mg/m³ or ppm, and for asbestos the number of fibres per m³. For noise, the daily exposure level over 8 hours (LEX,8h) is used.
Use in construction
On site, the TWA matters for respirable crystalline silica (cutting and grinding concrete, stone and calcium silicate blocks), wood dust, solvents in paints and adhesives, asbestos and noise. Practical points:
- Measure, don’t guess: whether a limit is met can only be shown by an exposure measurement carried out by a competent person, such as an occupational hygienist.
- Tackle the source first: wet cutting and on-tool extraction reduce exposure far more than a dust mask. Respiratory protection is the last line of defence.
- Combined exposure: if you work with several substances that affect the same organ, their effects add up.
- An average is not a free pass: short, high peaks remain harmful, so also check the 15-minute value.
The legal limit values themselves are set by national authorities and differ from country to country. Always check the current limits that apply where you work.
Related terms
- Occupational exposure limit
- Asbestos
- Quartz
- Noise nuisance
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