Industrial Hygiene and Occupational Health
The classic sequence
Anticipation → recognition → evaluation → control. Recognition means knowing what agents are present (chemical, physical, biological, ergonomic, psychosocial). Evaluation means sampling: personal breathing-zone sampling to judge compliance, area sampling to characterize sources and engineering controls. Calibrate pumps before and after sampling, record flow rates and durations, and use accredited labs. Compare results to the PEL as an 8-hour time-weighted average, supplemented by STEL (15-minute) and ceiling limits where they exist.
Noise
OSHA's action level is 85 dBA as an 8-hour TWA, triggering hearing conservation - audiometric testing, hearing protection availability, training, and evaluation. The PEL is 90 dBA TWA, and the exchange rate is 5 dB: each 5 dBA increase halves allowable exposure time. Hearing protectors are evaluated by the NRR, derated in practice.
Controls
Substitution, isolation/enclosure, local exhaust ventilation, general dilution ventilation, administrative rotation, then respiratory protection. Respirators require a written program, medical evaluation, fit testing, and training. Assigned Protection Factors express the expected workplace protection of a respirator class.
Health surveillance
Audiometry, spirometry, blood lead and other biological monitoring, and medical surveillance for specific standards (lead, benzene, asbestos, cadmium). Biological monitoring measures dose absorbed, not ambient concentration.
Traps
- Dividing a short high exposure wrongly when computing an 8-hour TWA.
- Using the 3 dB doubling rule on an OSHA noise problem (OSHA uses 5 dB).
- Confusing STEL (15-minute average) with ceiling (never to be exceeded).
- Treating an area sample as proof of personal compliance.
Worked example
A worker spends 4 hours at 95 dBA and 4 hours in a quiet area. TWA = 95 + 16.61 × log10(4/8) = 95 − 5 = 90 dBA, at the PEL. At 95 dBA for 2 hours, TWA = 95 − 8.3 = 86.7 dBA - above the action level, so hearing conservation applies.
Rule of thumb
Sample where the worker breathes, for as long as the worker is exposed.