Exposure Assessment
Core concepts
Exposure assessment begins with basic characterization: gather process, agent, and workforce information, then group workers into similar exposure groups (SEGs) - workers whose exposures should be alike because tasks, materials, and controls are alike. The deliverable is an exposure profile for each SEG, not a perfect number for one individual.
Sampling strategy is where questions concentrate. Distinguish worst-case/troubleshooting sampling from compliance sampling from statistical characterization. Representative sampling spreads across workers, shifts, and days to capture between-worker and within-worker variability, and full-period consecutive sampling is preferred over a grab sample when comparing to an 8-hour TWA. Match the metric to the limit: 8-hour TWA to the TWA PEL, 15-minute results to STELs, instantaneous peaks to ceiling limits. Keep exposure (external concentration at the boundary) separate from dose (amount absorbed).
Traps
- Reporting a non-detect as zero; it is "<LOD," and the LOD depends on flow rate, sample duration, and analytical sensitivity.
- Comparing a 15-minute result directly to an 8-hour TWA.
- Treating one worst-case sample as proof of compliance.
- Applying total-dust limits to respirable sampling (or vice versa).
Worked example
A worker is sampled at 40 ppm for 6 hours and 10 ppm for 2 hours. TWA = (40×6 + 10×2) / 8 = 260/8 = 32.5 ppm. Compare that to the 8-hour PEL - never to the 2-hour value.
Rule of thumb
Roughly 6–10 representative samples per SEG support a 95th-percentile estimate; if every result sits below about 10% of the OEL, a qualitative assessment with periodic confirmation is often defensible.