A time-weighted average (TWA) is a worker's average exposure to a chemical, dust or noise over a set period, usually an 8-hour work shift. Each measured concentration is weighted by how long it lasted, then the results are added together and divided by the reference period. The TWA is then compared to an exposure limit such as an OSHA permissible exposure limit.
The TWA matters because exposure rarely stays constant. A worker might be exposed to high levels while mixing a product and low levels the rest of the day. The TWA turns those changing levels into a single number that can be compared with legal limits and guidelines, so employers know whether controls are needed.
What Is a Time-Weighted Average?
Most OSHA permissible exposure limits and ACGIH threshold limit values are written as 8-hour TWAs. A TWA allows exposure to rise above the limit for short periods, as long as lower exposures during the rest of the shift keep the average at or below the limit, and as long as any ceiling or short-term exposure limit (STEL) is also met.
TWAs are based on personal breathing-zone samples, collected by a qualified person such as an industrial hygienist using validated sampling and analytical methods.
The 8-Hour TWA Formula
OSHA's air contaminants standard at 29 CFR 1910.1000(d) gives the formula:
E = (CaTa + CbTb + ... + CnTn) ÷ 8
- E is the 8-hour TWA exposure.
- C is the concentration measured during each period, in ppm or mg/m³.
- T is the duration of each period, in hours.
Any part of the 8-hour shift that was not sampled is treated as zero exposure only if you are confident no exposure occurred. Otherwise, sample as much of the shift as possible.
Worked Example
A worker is exposed to a solvent vapor as follows during an 8-hour shift:
- 2 hours at 150 ppm
- 4 hours at 75 ppm
- 2 hours at 25 ppm
Calculation: (150 × 2) + (75 × 4) + (25 × 2) = 300 + 300 + 50 = 650 ppm-hours. Then 650 ÷ 8 = 81.25 ppm as an 8-hour TWA.
If the PEL for this solvent were 100 ppm as an 8-hour TWA, the worker would be below the limit, even though the first two hours were above 100 ppm. However, you would still need to check whether the substance has a STEL or ceiling limit that those peaks exceeded.
Mixtures
When a worker is exposed to a mixture of air contaminants, 1910.1000(d)(2) uses an additive formula: Em = C1/L1 + C2/L2 + ... + Cn/Ln, where C is each substance's exposure and L is its exposure limit. If the total exceeds 1, the mixture exceeds the limit, even if each substance is individually below its own limit.
Extended Shifts
Most OSHA PELs are not formally adjusted for shifts longer than 8 hours. In interpretation letters, OSHA has explained that for extended shifts it generally evaluates compliance using the worst 8 hours of exposure in the shift. The lead standards are the main exception, as they reduce the PEL for shifts over 8 hours.
Because longer shifts mean more total exposure and less recovery time, many industrial hygienists apply more conservative adjusted limits for 10- or 12-hour shifts using recognized models. A qualified industrial hygienist should decide which approach to use.
Noise TWA
Noise TWAs are calculated differently. OSHA's occupational noise exposure standard sets a PEL of 90 dBA as an 8-hour TWA and an action level of 85 dBA, using a 5-dB exchange rate. That means the allowed time halves for every 5-dB increase: 8 hours at 90 dBA, 4 hours at 95 dBA and 2 hours at 100 dBA. Noise dosimeters calculate a percent dose, and a 100 percent dose equals a 90 dBA TWA. NIOSH and ACGIH recommend a stricter 85 dBA limit with a 3-dB exchange rate.
How to Calculate and Use a TWA Step by Step
- Identify the agent and limit. Confirm the substance or noise source and its applicable TWA, STEL and ceiling limits.
- Plan sampling. Choose the workers, tasks and shifts most likely to have the highest exposure.
- Collect samples. Take personal breathing-zone samples, or wear a dosimeter for noise, covering as much of the shift as possible.
- Record times. Note the start and stop time of each sample and the tasks performed.
- Calculate the TWA. Multiply each concentration by its duration, add the results and divide by 8.
- Compare to limits. Compare to the PEL and any stricter limits your company uses, and check STEL and ceiling values separately.
- Act on results. Apply the hierarchy of controls if exposures are too high, notify employees as required and keep the records.
Common Mistakes to Avoid
- Dividing by the sampled time instead of 8 hours when calculating an 8-hour TWA against an 8-hour limit.
- Assuming unsampled time had zero exposure when it did not.
- Ignoring STEL and ceiling limits because the TWA is acceptable.
- Using area samples instead of personal samples.
- Ignoring extended shifts or overtime.
How SMS360 Helps
SMS360's Risk Assessment module helps teams document exposure hazards, monitoring results and controls for each task. The Safety Data Sheets module keeps exposure limit information accessible, and Actions Management tracks follow-up work such as re-sampling or new controls.
Frequently Asked Questions
Why is the TWA divided by 8?
Most exposure limits are based on an 8-hour workday. Dividing total exposure by 8 gives the average exposure over a standard shift, which can be compared directly with the limit.
Can exposure go above the PEL for part of the shift?
Yes, for a TWA limit, as long as the 8-hour average stays at or below the PEL and any STEL or ceiling limit is not exceeded.
What is the difference between a TWA and a STEL?
A TWA averages exposure over a full shift, usually 8 hours. A STEL averages exposure over 15 minutes and limits short, high peaks.
Is noise TWA calculated the same way as chemical TWA?
No. OSHA noise TWAs use a logarithmic decibel scale and a 5-dB exchange rate, so they are calculated from noise dose rather than a simple average.
