Occupational Hygiene and Exposure Assessment
Occupational hygiene follows a recognition-evaluation-control sequence. Candidates must understand exposure routes (inhalation, dermal, ingestion, injection), the difference between occupational exposure limits expressed as time-weighted averages over eight hours versus short-term or ceiling limits, and why ceiling limits must never be exceeded. Know the sampling strategy concepts: representative personal sampling, area sampling, sample duration, and the importance of matching sampling time to the averaging period. Understand that analytical detection limits and sampling uncertainty affect interpretation, and that mixtures and skin absorption complicate limit comparisons.
Traps: comparing a 15-minute sample directly to an 8-hour TWA; assuming area samples represent personal exposure; ignoring dermal uptake for substances with skin notations; and treating a single compliant sample as proof of control across all shifts or seasons.
Rule of thumb: sample the person, for the full averaging period, during the task with the highest expected exposure, and repeat to capture variability. Worked example: a solvent has an 8-hour TWA limit of 50 ppm. A worker is sampled for 6 hours at 60 ppm and spends 2 hours away from the process. The 8-hour TWA is (60 × 6 + 0 × 2) / 8 = 45 ppm, which is below the limit. Note that a short-term limit may still be exceeded during the task.