Toxic Gas Exposure TWA Calculator
Calculate an 8 hour time weighted average (TWA) exposure from several periods at different gas concentrations, and compare it with the occupational exposure limit, for gas detector alarm setting and exposure checks.
- TWA = Σ(Ci × ti) ÷ 8 h = 40 ÷ 8 = 5 ppm
- Unexposed time counts as zero concentration
8 Hour TWA Formula
C is the concentration during each period and t its duration in hours. The total is always divided by 8 hours, so time without exposure counts as zero. Exposure limits come from your national authority (for example OSHA PEL, ACGIH TLV or EU IOELV); short term (15 minute) limits are checked separately.
Worked example
2 h at 12 ppm and 4 h at 4 ppm give TWA = (24 + 16) ÷ 8 = 5 ppm, which is 50 % of a 10 ppm limit.
Frequently Asked Questions
What is TWA?
The time weighted average concentration a worker is exposed to over an 8 hour shift.
What is STEL?
The short term exposure limit, a 15 minute average that must not be exceeded at any time in the shift.
Where do exposure limits come from?
From occupational health regulators and bodies such as OSHA, NIOSH, ACGIH or national authorities. Use the limit that applies at your site.
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