Air Monitoring, Sampling, and Clearance
This domain tests personal and area monitoring, sampling methods, aggressive sampling, and clearance criteria. Expect calculations, interpretation of results, and questions about who may perform clearance.
Monitoring types
Personal breathing-zone samples estimate worker exposure and determine respirator selection. Area samples assess containment performance. Phase-contrast microscopy (PCM), commonly NIOSH 7400, counts fibers and is used for routine clearance; transmission electron microscopy (TEM) identifies fiber types and can detect thinner fibers. Pumps must be calibrated before and after sampling, and chain-of-custody records must be maintained.
Clearance
Clearance usually requires a visual inspection showing no debris or dust, followed by aggressive air sampling that disturbs settled dust. A common criterion is fewer than 0.01 fibers per cubic centimeter by PCM. In schools, AHERA clearance uses 0.01 f/cm³ by PCM or 70 structures/mm² by TEM. Clearance should be performed by an independent inspector or project monitor, not by the abatement contractor alone.
Worked example
Five clearance samples read 0.004, 0.006, 0.008, 0.012, and 0.005 f/cm³. The average is 0.007 f/cm³, which is below 0.01. However, if the criterion requires each sample to be below 0.01, the 0.012 sample fails. The area must be recleaned, HEPA-vacuumed, and resampled before clearance. Averaging does not cure a single exceedance under a per-sample standard.
Traps
Do not sample before surfaces are dry. Do not skip aggressive sampling. Do not accept uncalibrated pumps or missing chain-of-custody. Do not let the contractor self-certify clearance where independence is required.