Air Monitoring and Clearance Procedures
Monitoring types
Personal sampling estimates worker exposure; area sampling verifies containment and clearance. Phase-contrast microscopy (PCM, NIOSH 7400) counts fibers and is the usual clearance method; transmission electron microscopy (TEM) identifies fibers and is used when PCM is inadequate or for AHERA-type clearance.
Clearance rules
- Clearance generally requires visual inspection (no visible debris) and air samples below the project’s clearance threshold, commonly 0.01 f/cc by PCM; TEM clearance often uses 70 structures/mm².
- Use aggressive sampling: fans or blowers to disturb air before and during sampling.
- Do not turn off air scrubbers until final clearance samples are collected, unless the protocol says otherwise.
- Calibrate pumps before and after sampling; record flow rates, times, and chain of custody.
Worked example
An enclosure is cleaned and sampled by PCM. Results are 0.006, 0.008, and 0.012 f/cc. The 0.012 exceeds the 0.01 f/cc clearance criterion, so the area fails. The supervisor must re-clean, HEPA-vacuum and wet-wipe, then re-sample before removing the enclosure.
Traps
Do not sample before final cleaning; do not accept a single sample for a large enclosure; do not confuse a personal sample with a clearance sample. Do not remove containment before clearance results are accepted.