Air Monitoring, Sampling, and Clearance
Air-monitoring questions test sampling strategy, fiber-counting math, and the clearance sequence.
Monitoring and clearance
- Personal samples characterize employee exposure; area samples support engineering controls.
- PCM (NIOSH 7400) counts fibers but cannot identify asbestos; TEM (NIOSH 7402) identifies and sizes fibers.
- Clearance requires a visual inspection, then aggressive air sampling that disturbs settled dust, then analysis.
- A common clearance criterion is 0.01 f/cc by PCM; AHERA-style TEM clearance uses 70 structures/mm².
- Include field blanks and chain-of-custody documentation.
Worked example
A sample has 35 fibers counted over 100 fields. Filter area is 385 mm², field area 0.00785 mm², and sampled volume is 480 L (480,000 cm³). C = (35 × 385) ÷ (100 × 0.00785 × 480,000) = 13,475 ÷ 376,800 = 0.036 f/cc. That exceeds 0.01 f/cc, so reclean and resample.
Traps
- Passing clearance before final cleaning.
- Using non-aggressive sampling for clearance.
- Treating PCM results as asbestos-specific.
- Forgetting blanks or calibration records.
- Sampling only the center of the enclosure.