Chemical exposure · Single substance · 8 h
Calculate an eight-hour chemical TWA from exposure periods
Combine concentration readings and durations for one substance across a complete eight-hour period.
By HSECalc · Published
Check that this method fits your task
Account for exactly eight hours using measured or justified values. One substance and one unit only. No built-in exposure limit, mixture, peak, STEL, ceiling or extended-shift adjustment.
Choose your inputs and work through the steps
- Choose one substance and one concentration unit. Record representative concentration and duration for each distinct period.
- Account for all eight hours. Do not fill unsampled or unknown periods with zero merely to make the duration total work.
- Multiply each concentration by its hours, add the contributions, then divide by eight. Keep sampling assumptions with the result.
Eight-hour TWA = Σ(concentration × hours) ÷ 8. This implementation requires the entire eight-hour period to be accounted for.
A worked example
Illustrative values explain the tool; they are not prescribed inputs for your work.
- 150 ppm × 2 h = 300 ppm·h.
- 75 ppm × 2 h = 150 ppm·h; 50 ppm × 4 h = 200 ppm·h.
- (300 + 150 + 200) ÷ 8 = 81.25 ppm.
Read the result and decide what needs checking
The average is only one part of an exposure assessment. Applicable limits, short-term or ceiling conditions, mixtures, skin exposure and sampling quality need separate consideration.
This version does not adjust extended shifts or convert ppm to mg/m³. Get the correct assessment method rather than forcing those situations into an eight-hour entry.
Common questions
Can I leave unsampled hours blank?
No. Obtain representative data or an appropriately justified assessment. The calculator will not treat missing periods as zero.
Can I combine different chemicals?
No. Enter one substance. Mixture assessment requires a separate method.
Does the result show that exposure is acceptable?
No limit is assumed. A hygienist must check the applicable limit, sampling quality and other relevant exposure criteria.
References and scope notes
Use the current official guidance and the instructions for your actual equipment. HSECalc is independent; a reference is not an endorsement. Read how we use methods and sources or report a correction.