Hearing safety · mixed exposure
Noise dose and TWA calculator.
List the noise levels you were exposed to and for how many hours each. Get the percent dose and the 8-hour time-weighted average under both NIOSH and OSHA, with the limit status.
Inputs
Result
Enter at least one level and number of hours.
How noise dose and TWA are computed
Dose adds up the fraction of the allowed time used at each level: D = 100 x (C1/T1 + C2/T2 + ... ), where C is the hours spent at a level and T is the allowed hours at that level. 100% is the limit. NIOSH's criteria document and OSHA 1910.95 Appendix A both give this form. The 8-hour TWA is then NIOSH: TWA = 10 x log10 (D/100) + 85 and OSHA: TWA = 16.61 x log10 (D/100) + 90. OSHA's action level, where a hearing conservation program begins, is a 50% dose (85 dBA TWA) counted from 80 dBA. OSHA's permissible exposure limit (PEL) is a separate 90 dBA TWA, and the OSHA Technical Manual says PEL compliance uses data from a 90 dBA threshold. So this tool shows two OSHA doses: the action-level dose (every level from 80 dBA) and the PEL dose (only levels of 90 dBA and above).
Worked example. 4 hours at 92 dBA, 3 hours at 85 dBA and 1 hour at 70 dBA. NIOSH: 4 h at 92 dBA is 2.52 of the allowed time (allowed 1.6 hours), 3 h at 85 dBA is 0.375, so the dose is 289% and the TWA is 89.6 dBA, well over the 85 dBA recommended limit. OSHA action-level dose (80 dBA threshold): 0.66 + 0.188 gives 85%, a TWA of 88.8 dBA, past the 50% action level. OSHA PEL dose (90 dBA threshold): only the 92 dBA period counts, so 66% and a TWA of 87.0 dBA, under the 90 dBA limit. The 70 dBA hour is not counted in either.
Levels under 80 dBA are ignored because OSHA integrates sound from 80 dBA up for the action level and the NIOSH exposure table starts at 80 dBA; levels under 90 dBA are also left out of the OSHA PEL dose. The result is only as good as the levels you enter: use a dosimeter for a real exposure assessment.
Frequently asked questions
What is a 100% noise dose?
It means the full allowed daily exposure has been used. Under NIOSH that is an 85 dBA 8-hour average; under OSHA it is a 90 dBA 8-hour average.
What is the OSHA action level?
An 8-hour TWA of 85 dBA, which is a 50% dose. At that point the employer must run a hearing conservation program.
Why do the two methods give different doses for the same day?
They use different criterion levels (85 versus 90 dBA) and exchange rates (3 versus 5 dB). The same noise day uses up a larger share of the NIOSH limit.
Are quiet periods counted?
Anything under 80 dBA is left out of every dose shown, so a quiet break neither adds nor removes dose. The OSHA PEL dose also leaves out levels from 80 to under 90 dBA, which count only toward the OSHA action-level dose and the NIOSH dose.
Sources
Checked
- NIOSH 98-126: Criteria for a Recommended Standard, Occupational Noise Exposure, Revised Criteria 1998
- OSHA 29 CFR 1910.95: Occupational noise exposure
- OSHA 1910.95 Appendix A: Noise exposure computation
- OSHA Technical Manual, Section III Chapter 5: Noise (80 dBA action-level and 90 dBA PEL thresholds)
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Scope
Hearing safety guidance, not medical or occupational-health advice. These are published limits and rules of thumb applied as arithmetic, not a noise survey. Measure real levels with a sound level meter or dosimeter, follow the rules that apply to your workplace, and see an audiologist if you have ringing, muffled hearing or hearing loss.