Noise Exposure After a Compressor Upgrade
A metal fabricator installed two new rotary screw compressors in a room adjacent to a welding bay. Personal dosimetry on three welders over one 8-hour shift gives 8-hour time-weighted averages of 87, 91, and 94 dBA. The welders wear foam earplugs labeled NRR 29 dB, fitted by the worker in about 10 seconds with no fit check. Two workers report ringing after shifts. The plant has no audiometric testing, no noise signage, and no hearing conservation program. Management proposes issuing 'better' earplugs and re-checking in a year. The welding bay also runs a plasma cutter intermittently at about 105 dBA at the operator position.
1. Assess the exposure data against a hearing conservation trigger and evaluate the proposed earplug response. What is wrong with the plan?
Read the worked answer
All three dosimetry results exceed 85 dBA as an 8-hour TWA, which is the common trigger for a hearing conservation program; the 91 and 94 dBA results also exceed the 90 dBA level at which engineering and administrative controls become the priority, with hearing protection as the last resort. Ringing after shifts is a symptom consistent with temporary threshold shift and should be treated as a signal, not a complaint.
The earplug plan is inadequate for several reasons:
- The NRR is a laboratory rating, not a field estimate. Derating is required; a common approach applies roughly a 50% derating for foam plugs, which would put effective protection near 14-15 dB for the 94 dBA worker, not 29 dB.
- Fit is unverified. A 10-second roll-and-insert with no fit check commonly yields far less attenuation than the label, and poor fit is invisible to the worker.
- Protection alone does not address the source. The new compressors are a controllable engineering source; enclosure, isolation, or silencing should be evaluated first.
- There is no program: no baseline audiograms, no training, no signage, no re-evaluation schedule. Issuing plugs without these leaves the organization unable to detect or correct harm.
The correct response is a hearing conservation program with baseline and annual audiometry, training, signage, fit testing, engineering controls at the compressors, and periodic re-dosimetry.
2. The plasma cutter adds intermittent 105 dBA peaks. How does that change your exposure assessment and control priorities?
Read the worked answer
Intermittent high-level noise changes both the measurement approach and the control hierarchy. An 8-hour TWA can dilute short, intense events, so the assessment should capture peak levels and the duration and frequency of cutting, not just the shift average. If peaks reach the level at which impulse noise criteria apply, the exposure may be more damaging than the TWA suggests, and hearing protection rated only by NRR may not be adequate for impulsive or high-frequency sources.
Control priorities shift toward the source: enclose or relocate the plasma cutter, use water tables or local exhaust that also attenuates noise, and schedule cutting away from occupied workstations. Administrative measures such as rotating tasks can reduce individual dose but do not reduce the hazard and should not substitute for engineering controls.
Protection remains a layer, but selection should account for the peak character and be verified by fit testing; dual protection may be warranted for the operator during cutting. Re-dosimetry should include the cutter operator and nearby welders, and audiometric baseline should be established before further exposure. Document the rationale so the control decision is traceable.