Occupational Health and Industrial Hygiene
The industrial hygiene process
Every exposure question fits the classic sequence: anticipate, recognize, evaluate, control. Recognition means knowing the agent (chemical, physical, biological, ergonomic) and its route of entry - inhalation, absorption, ingestion, injection. Evaluation means measuring the exposure of the worker, not the room: personal breathing-zone samples, integrated sampling for laboratory analysis, or direct-reading instruments such as a photoionization detector or detector tubes.
Which limit applies
Do not treat exposure limits as interchangeable. The OSHA PEL is the legally enforceable limit, usually an 8-hour time-weighted average. The NIOSH REL is a health-based recommendation, often lower. The ACGIH TLV is a professional guideline revised annually. A STEL is a 15-minute average; a ceiling limit must never be exceeded; an IDLH atmosphere requires supplied air and outside support.
Noise and the hearing conservation trigger
The action level is 85 dBA as an 8-hour TWA - that is what triggers a hearing conservation program (monitoring, audiometric testing, hearing protection availability, training, recordkeeping). The PEL is 90 dBA TWA. OSHA uses a 5 dB exchange rate, so every 5 dB increase halves permissible exposure time: 90 dBA for 8 hours, 95 for 4, 100 for 2.
Controls
Apply the hierarchy: elimination, substitution, engineering controls (local exhaust ventilation, enclosure, process change), administrative controls (rotation, scheduling), then PPE as the last line.
Worked example
A worker is exposed to 120 ppm for 4 hours and 40 ppm for 4 hours. TWA = (120 × 4 + 40 × 4) ÷ 8 = 80 ppm.