How the dose works
OSHA allows 8 hours at 90 dBA, and every 5 dB louder cuts the allowed time in half: 4 hours at 95, 2 at 100, 1 at 105. For each period, divide the hours by the allowed hours at that level, then add them up. That sum is what 1926.52 calls the equivalent noise exposure factor. Over 1, or 100% dose, the exposure is over the limit. The allowed time at any level is 8 ÷ 2(L − 90) ÷ 5 hours, and the 8-hour TWA is 16.61 × log₁₀(dose ÷ 100) + 90.
Check it against OSHA's example
The calculator opens on the example in 1926.52(d)(2)(iii): 15 minutes at 110 dBA, 30 minutes at 100 and 90 minutes at 90. That's 0.25 ÷ 0.5 + 0.5 ÷ 2 + 1.5 ÷ 8 = 0.938, a 93.8% dose, just under the limit.
The other two numbers
- Action level (general industry): 1910.95 requires a hearing conservation program at an 8-hour TWA of 85 dBA, a 50% dose. For that dose, every level from 80 dBA up counts. Construction's rule, 1926.52, requires a hearing conservation program whenever exposure is over the limit.
- NIOSH: NIOSH recommends 85 dBA over 8 hours with a 3 dB exchange rate. That's tighter than OSHA's limit and not a regulation, but many safety programs plan to it.
Limits of a hand calculation
Use levels from a sound level meter on slow response, or better, a dosimeter on the worker for the full shift. A dosimeter catches the moments a task-by-task estimate misses. Sound over 115 dBA isn't allowed for any length of time under Table D-2, and impact noise shouldn't exceed 140 dB peak.