Air Monitoring, Sampling Methods, and Clearance Criteria
Methods and purposes
Phase contrast microscopy, NIOSH Method 7400, counts respirable fibers but cannot identify whether they are asbestos. It is used for personal exposure monitoring against the OSHA permissible exposure limit. Transmission electron microscopy identifies fiber type and is used for clearance and for many bulk confirmations. Polarized light microscopy is used for bulk samples to estimate asbestos content and fiber type. Personal samples are collected in the breathing zone; area samples support engineering evaluation but do not replace personal monitoring. Calibration, sample volume, filter loading, and chain of custody all affect validity.
Clearance
Clearance is a two-part process: a thorough visual inspection for dust and debris, then air sampling. Aggressive sampling methods disturb settled dust to simulate activity. School projects under AHERA commonly use a 0.01 f/cc transmission electron microscopy clearance benchmark, but the project specification and Maryland requirements govern. Clearance is not established by visual inspection alone, and it is not established by a sample collected before final cleaning.
Worked example
A phase contrast sample is collected over 400 liters. The analyst counts 100 fibers over 100 fields. With a filter area of 385 square millimeters, a field area of 0.00785 square millimeters, and 400,000 cubic centimeters of air, the concentration is 100 times 385 divided by 100 times 0.00785 times 400,000, or about 0.12 f/cc. That exceeds the 0.1 f/cc permissible exposure limit, so the result triggers exposure control and medical surveillance review. The calculation also shows why sample volume and microscope field area must be recorded.
Traps that cost marks
- Using phase contrast microscopy to identify asbestos fiber type.
- Treating an area sample as a personal exposure result.
- Calling clearance after visual inspection without air sampling.
- Sampling before final cleaning or without aggressive methods where required.