Air Monitoring, Sampling, and Clearance
What this domain tests
This domain tests personal exposure monitoring, area sampling, PCM versus TEM, pump calibration, aggressive sampling, visual inspection, and clearance criteria. Questions often ask whether a numeric result alone can clear a project.
Core rules and concepts
- Personal monitoring uses a calibrated pump and filter cassette, commonly following NIOSH 7400. Sample at a representative flow rate, document the task and duration, and calculate the 8-hour TWA and 30-minute excursion limit.
- Phase contrast microscopy (PCM) counts fibers longer than 5 micrometers with an aspect ratio of at least 3:1. PCM cannot distinguish asbestos from other fibers, so it is used for exposure and many clearance measurements, not for definitive identification.
- TEM identifies asbestos fibers and is required for AHERA school clearance and useful for confirming ambiguous PCM results. Follow the specific AHERA protocol for sample number, aggressive sampling, and analysis.
- Clearance requires a visual inspection first. No visible debris or dust means the project is not ready, regardless of a low PCM number. Then perform aggressive sampling where required and meet the applicable criterion, such as 0.01 f/cc by PCM for a non-AHERA project or the AHERA TEM criterion for a school.
- Calibrate pumps before and after use, include field blanks, maintain chain of custody, and use an accredited laboratory.
Worked example
A clearance sample reports 0.008 f/cc by PCM, but an inspector sees dust in a corner. Can the project be cleared?
Reasoning: no. Clearance is a two-part process: visual inspection and air sampling. Visible dust fails the visual inspection. Clean the area with wet methods and HEPA vacuums, re-inspect, and then resample. If the project is an AHERA school, use the required TEM clearance protocol rather than relying on the PCM number alone.
Traps that cost marks
- Clearing on a low PCM result despite visible debris.
- Treating PCM as asbestos-specific.
- Skipping pump calibration or field blanks.
- Sampling before final cleaning and aggressive sampling.
- Using the wrong cassette or flow rate.
- Failing to document sample location, time, and chain of custody.