Occupational hazards, toxicology and exposure assessment
Exposure is the assessment
Toxicology in occupational practice is mostly about dose at the target organ, not the label on the drum. Four routes matter - inhalation, dermal absorption, ingestion and (rarely) injection - and the dermal route is the one most often overlooked, particularly for pesticides, solvents, cutting oils and amines.
Toxicokinetics (absorption, distribution, metabolism, excretion) explains individual and species differences; metabolism can detoxify or bioactivate (e.g. styrene to styrene oxide). Latency separates acute effects (solvent narcosis, hydrogen sulphide) from chronic ones (mesothelioma, silicosis, bladder cancer from aromatic amines).
UK Workplace Exposure Limits are set for a long-term (8-hour time-weighted average) and, for some substances, a short-term (15-minute) reference period. WELs are not a boundary between safe and unsafe - they are benchmarks for good control, and exposure must be reduced as far as reasonably practicable. Watch for the 'Sen' notation (respiratory or skin sensitiser, effectively no safe threshold) and 'Sk' (skin absorption may be significant, so air monitoring alone is inadequate).
The control hierarchy runs: eliminate, substitute, engineering controls, safe systems of work, then PPE last. PPE depends on correct fit and user behaviour and is the least reliable layer.
Worked example. A worker is exposed to substance X at 40 ppm for 2 hours, 10 ppm for 4 hours, and is unexposed for the remaining 2 hours of the shift. The 8-hour TWA is (40×2 + 10×4 + 0×2)/8 = 15 ppm. Compare this with the 8-hour WEL, and separately check whether any 15-minute period breaches the short-term limit.
Rule of thumb: air sampling describes inhalation; biological monitoring describes total absorbed dose by all routes.