Occupational Noise Exposure and Hearing Conservation
This domain tests dose calculations, the structure of a hearing conservation programme, and the correct use of hearing protector ratings.
Core rules
- Under OSHA 29 CFR 1910.95 the permissible exposure limit is 90 dBA as an 8-hour TWA, with a 5 dB exchange rate and an action level of 85 dBA TWA.
- Dose: D = 100 Σ (Ci / Ti), where allowable time Ti = 8 / 2^((L−90)/5) hours. TWA = 16.61 log10(D/100) + 90.
- At and above the action level, a hearing conservation programme is required: exposure monitoring, audiometric testing, hearing protectors, training and recordkeeping. Baseline audiograms are due within 6 months of first exposure.
- A standard threshold shift is an average shift of 10 dB or more at 2000, 3000 and 4000 Hz in either ear.
- OSHA estimates protector performance as A-weighted exposure = dBA − (NRR − 7). NIOSH instead derates the NRR by 25 % for earmuffs and 50 % for foam plugs. Impulse noise is limited by a 140 dB peak criterion.
Worked example
A worker is exposed at 95 dBA for 6 hours. Allowable time at 95 dBA = 8 / 2^((95−90)/5) = 8/2 = 4 h, so D = 100 × 6/4 = 150 %. TWA = 16.61 log10(1.5) + 90 = 90 + 2.9 = 92.9 dBA - above the PEL, so feasible engineering controls come first, with protectors as an interim measure.
Traps that cost marks
- Applying a 3 dB exchange rate (ISO, ACGIH) to an OSHA question, or vice versa.
- Subtracting the full NRR without the −7 derating, or using NRR against C-weighted data.
- Treating 85 dBA as the PEL rather than the action level.
- Declaring a shift on a single frequency instead of the three-frequency average.