Extended-Shift Noise Exposure and a Questionable Standard Threshold Shift
A foundry runs four 10-hour shifts per week. Personal noise sampling on a furnace operator gives 95 dBA for 5 hours, 88 dBA for 3 hours, and 82 dBA for 2 hours. The OSHA PEL is 90 dBA as an 8-hour time-weighted average; the action level is 85 dBA. Hearing protection is available but not enforced. The baseline audiogram shows 10 dB at 4000 Hz in the left ear and 5 dB at 2000 and 3000 Hz. The annual audiogram shows 20 dB at 4000 Hz and 5 dB at 2000 and 3000 Hz. The employer proposes relying on hearing protection and skipping engineering controls because the worker is "only 3.5 dBA over" the PEL.
1. Compute the 8-hour TWA and determine whether the worker is over the PEL. Then determine whether the audiogram change qualifies as an OSHA standard threshold shift.
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Using the OSHA 5-dB exchange rate, allowable time T = 8 / 2^((L - 90)/5):
- 95 dBA: T = 4 h
- 88 dBA: T = 10.56 h
- 82 dBA: T = 24.25 h
Dose = 100 x (5/4 + 3/10.56 + 2/24.25) = 100 x (1.25 + 0.284 + 0.0825) = 161.7%.
TWA = 90 + 16.61 log10(1.617) = 93.5 dBA.
The worker exceeds the 90-dBA PEL; the excess is about 3.5 dBA, which is not trivial because dose rises rapidly with level. Hearing protection is required, but the PEL is still exceeded without feasible controls.
The audiogram shift is 10 dB at 4000 Hz only. OSHA defines an STS as a 10-dB or greater average shift at 2000, 3000, and 4000 Hz. The average here is (0 + 0 + 10)/3 = 3.3 dB, so this is not an STS by the OSHA definition. It is still a reportable change for follow-up: retest within 30 days, review fit and attenuation, and evaluate progression.
2. Engineering controls reduce the levels to 90 dBA for 5 hours, 86 dBA for 3 hours, and 80 dBA for 2 hours. Recalculate the TWA and advise whether further controls are required.
Read the worked answer
New allowable times:
- 90 dBA: T = 8 h
- 86 dBA: T = 13.93 h
- 80 dBA: T = 32 h
Dose = 100 x (5/8 + 3/13.93 + 2/32) = 100 x (0.625 + 0.215 + 0.0625) = 90.3%.
TWA = 90 + 16.61 log10(0.903) = 89.3 dBA.
The exposure is now just below the PEL but still above the 85-dBA action level, so the hearing conservation program continues: annual audiograms, training, and hearing protection available or required by policy. The employer should not treat 89.3 dBA as a stable endpoint. The margin is less than 1 dBA, and day-to-day variability, overtime, or a second furnace could push the worker over the PEL. Continue feasible engineering controls, verify attenuation with fit testing, and use administrative rotation only as a supplement. The 10-hour shift also warrants a NIOSH-style adjustment for hearing-conservation planning, even though the OSHA PEL calculation uses the 8-hour TWA.