OSHA sets two key noise thresholds under 29 CFR 1910.95 — and each one triggers a different set of employer obligations. The action level (85 dBA as an 8-hour TWA) starts the clock on your hearing conservation program requirements. The permissible exposure limit (90 dBA) makes hearing protection mandatory. Exceed either without a documented program and you’re looking at a serious citation. Here’s exactly what each level requires.
Soundtrace measures individual worker TWAs using personal dosimetry, automatically determines enrollment status against the 85 dBA action level, and flags workers approaching or exceeding the 90 dBA PEL — linking each exposure reading to real-time noise data at the frequency level.
- 85 dBA TWA (Action Level): Begin noise monitoring, offer audiometric testing, make hearing protectors available, train employees
- 90 dBA TWA (PEL): All of the above + hearing protection use becomes mandatory
- 115 dBA: Maximum permissible exposure with hearing protection — no unprotected exposure allowed
- 140 dB peak: Absolute ceiling for impulse/impact noise
The action level (85 dBA) is where the hearing conservation program begins. The PEL (90 dBA) is where mandatory hearing protection and engineering control obligations kick in. Both thresholds are expressed as 8-hour TWAs. Most compliance errors happen when employers treat them as interchangeable — they are not.
The Two Thresholds: Action Level and PEL
What Each Threshold Triggers: A Complete Reference
| OSHA Requirement | 85 dBA (Action Level) | 90 dBA (PEL) |
|---|---|---|
| Noise monitoring | Required to determine exposure | Required (same program) |
| Audiometric testing | Baseline + annual required | Continues (same requirement) |
| HPD availability | Must be provided at no cost | Must be provided at no cost |
| HPD use | Voluntary (except STS workers) | Mandatory |
| Engineering controls | Not explicitly required | Required where feasible |
| Training | Annual training required | Continues (same requirement) |
| Recordkeeping | Full records required | Full records required |
| HPD attenuation target | 85 dBA (for STS employees) | 90 dBA (standard); 85 dBA (STS) |
How OSHA Noise Exposure Is Measured
OSHA requires measurement of an employee’s 8-hour time-weighted average (TWA) noise exposure using personal dosimetry. The dosimeter is worn on the worker’s collar during a full work shift, sampling noise continuously and integrating the results into a TWA value. This reflects actual exposure across all tasks, locations, and noise sources encountered during the shift — which is why personal dosimetry, not area monitoring, is the standard for making HCP enrollment decisions.
The TWA is calculated using OSHA’s formula: TWA = 16.61 log(D/100) + 90, where D is the noise dose percentage. An employee whose noise dose equals exactly 100% has a TWA of exactly 90 dBA (the PEL). A dose of 50% corresponds to a TWA of approximately 85 dBA (the action level).
The Exchange Rate and Permissible Durations
OSHA’s 5 dB exchange rate means that for every 5 dB increase in noise level, the maximum permissible exposure duration is cut in half. The reference point is 90 dBA for 8 hours (the PEL). Workers who spend portions of their shift in areas with different noise levels accumulate dose proportionally; the TWA is the integrated result of all exposures during the shift.
NIOSH uses a 3 dB exchange rate and a 85 dBA criterion level, producing a recommended exposure limit (REL) of 85 dBA for 8 hours. A dosimeter set to NIOSH settings will calculate a higher TWA than one set to OSHA settings for the same exposure. Always verify your dosimeter is set to OSHA’s 5 dB exchange rate and 90 dBA criterion before comparing results to OSHA’s action level and PEL.
Impulsive and Impact Noise
OSHA 1910.95(b)(2) sets a separate ceiling limit of 140 dB peak sound pressure level for impulsive or impact noise — the instantaneous peak generated by events like gunshots, hammer blows, or explosive sounds. This ceiling applies regardless of duration or how few impacts occur during the shift. Workers exposed to impulsive noise above 140 dB peak must wear hearing protection; there is no permissible dose calculation for peak levels above this ceiling.
Where NIOSH Differs from OSHA: The REL
NIOSH’s recommended exposure limit (REL) is 85 dBA as an 8-hour TWA using a 3 dB exchange rate — significantly more conservative than OSHA’s PEL. NIOSH recommends HPD use for all exposures above 85 dBA. OSHA’s standard is legally enforceable; NIOSH’s REL is a scientific recommendation with no enforcement mechanism. Programs that follow NIOSH criteria provide greater hearing protection but go beyond what OSHA strictly requires.
Frequently asked questions
Track Noise Exposures Against OSHA Thresholds Automatically
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