Dosimetry, a recordable shift and a worker who removes his plugs
You are reviewing exposure data for a metal fabrication shop. A dosimeter worn by a press operator over a full shift logged 3.0 h at 92 dB(A), 2.0 h at 88 dB(A), 1.5 h at 95 dB(A) near the guillotine, and 1.5 h at 82 dB(A) in the office, with the balance of the eight-hour shift in a quiet crib room. The dosimeter was set to A-weighting, slow response, 3 dB exchange rate, and field-calibrated before and after the shift; drift was 0.4 dB. The operator's latest audiogram, compared with a baseline taken two years earlier, shows a shift of 5 dB at 2 kHz, 20 dB at 3 kHz and 15 dB at 4 kHz in the left ear, and a 5 dB average shift in the right. He shoots clay targets on weekends and does not always wear plugs because, he says, they cut out the machine noise so he cannot hear the warning alarms. Management wants to know whether the shop is compliant and what to do before the next audit.
1. Calculate the eight-hour equivalent level and state the compliance position.
Read the worked answer
Energy-average the partial exposures over eight hours:
Leq,8h = 10 log[(1/8)(3x10^9.2 + 2x10^8.8 + 1.5x10^9.5 + 1.5x10^8.2)]
= 10 log[(1/8)(4.755x10^9 + 1.262x10^9 + 4.743x10^9 + 0.238x10^9)]
= 10 log(1.375x10^9) = 91.4 dB(A).
Equivalently the dose is about 138% of an eight-hour 90 dB(A) criterion at a 3 dB exchange rate. On either reading the operator is overexposed: the result exceeds a 90 dB(A) eight-hour limit and is 6 dB above an 85 dB(A) action level, which is a fourfold dose. Calibration drift of 0.4 dB is within normal acceptance limits and does not change the conclusion.
Two cautions belong in the report. First, the exchange rate must be stated whenever a TWA is quoted: the same profile under a 5 dB exchange rate computes to roughly 91 dB(A) here, but the two rules diverge sharply for short, high-level or impulsive work, and this shop has impulsive work. Second, the crib-room time appears to have been entered as zero rather than measured; if the pattern repeats over a nine or ten hour shift the equivalent level rises by about 0.5-1 dB.
2. Interpret the audiometric result. Is it recordable, and how should the off-work shooting be handled?
Read the worked answer
Under the common regulatory definition, a standard threshold shift is an average change of 10 dB or more at 2, 3 and 4 kHz in either ear. In the left ear the average is (5 + 20 + 15)/3 = 13.3 dB, so the shift meets that definition and must be recorded and followed up under the applicable scheme, including retest and audiological referral. The right ear average of 5 dB does not.
The attribution question is separate and cannot be settled by the numbers. A 3 kHz notch is equally consistent with occupational noise, with gunfire, and with both acting together; a right-handed shooter is more exposed on the left, which matches this pattern. Age also contributes. As the engineer you do not diagnose causation; you report the shift, refer for audiological assessment, and manage the exposure. What you can say with confidence is that the measured exposure is sufficient to cause hearing loss on its own, so the shift is at minimum consistent with occupational exposure and cannot be dismissed as hobby-related.
The statement that plugs are removed to hear alarms is a substantive finding, not a discipline problem. A hearing conservation programme whose protectors conflict with safety-critical communication is failing at its last line of defence, and the fix is engineering, not exhortation.
3. Recommend the control strategy, including hearing protector selection.
Read the worked answer
Work down the hierarchy. At source: enclose the guillotine with an acoustic hood or partial enclosure that still permits feeding, damp the sheet-metal panels that are radiating, and fit a silencer to the compressed-air exhaust, which is the likely impulse source. In the room: add absorption to cut the reverberant field, which is why the office time and the machine time differ by only 13 dB. Organisationally: rotate staff off the guillotine and schedule the loudest work when fewest people are present.
PPE comes last but is still needed now. The worker's objection is valid: an earplug that masks alarms creates a new hazard. The answer is level-dependent or electronic earmuffs that attenuate the press but pass speech and warning signals, or a visual/flashing alarm at the guillotine, ideally both. On attenuation: a label SNR of 31 dB is a laboratory number, not a workplace number. A common conservative approach halves it (about 15 dB assumed), which would bring 91 dB(A) to roughly 76 dB(A) - apparently adequate, but that calculation is irrelevant while the protector is out of the ear. Dual protection is justified only if peak levels exceed the peak action level (commonly 135 dB(C)), which should be measured before it is specified.
The audit answer is that the shop is overexposed, has a recordable shift, and has a PPE programme that conflicts with safety communication. Fixing only the third will not fix the first.