Time-weighted average (TWA)
An exposure averaged over a work shift, usually 8 hours, weighting each level by how long it lasted.
Definition, in the source's words
29 CFR 1910.1000(a)(2)(opens ecfr.gov in a new tab)“Other substances—8-hour Time Weighted Averages. An employee's exposure to any substance in Table Z-1, the exposure limit of which is not preceded by a “C”, shall not exceed the 8-hour Time Weighted Average given for that substance in any 8-hour work shift of a 40-hour work week.”
What it means
Most PELs are 8-hour TWAs. Short peaks above the limit can be allowed if the shift average stays under it, unless the substance also has a ceiling or short-term limit. Noise uses its own TWA method in 1910.95.
Example
A worker spends 2 hours at 100 ppm, 2 hours at 80 ppm and 4 hours at 50 ppm. TWA = (2 × 100 + 2 × 80 + 4 × 50) ÷ 8 = 560 ÷ 8 = 70 ppm.
Where it appears on the exam
ASP: ASP11 Domain 1, Mathematical Calculations
Practice: Exposure Assessment and Storage Capacity Calculations
CSP: CSP11 Domain 6, Occupational Health and Applied Science
Practice: Air Contaminants and Silica (General Industry), Exposure, Noise, Containment and Lifting Calculations
CHST: CHST5 Domain 1, Hazard and Risk Identification and Control
Practice: Respirable Crystalline Silica in Construction, Lead in Construction
Questions
- Why divide by 8 even if the sample was shorter?
- The limit is an 8-hour average. Unsampled time is either sampled too or, where that's justified, counted as zero exposure; the sum of concentration × time is divided by 8 hours.
Related
- Ceiling limit (C)
- Action level
- Formula: 8-hour TWA exposure (air contaminants)
- Formula: Mixture of air contaminants
- Formula: Noise TWA from dose
- Regulation: 29 CFR 1910.1000
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.