Fundamentals of Asbestos Air Clearance Testing

Apr 24, 2018

Let’s say you found an asbestos-containing material and you had it professionally abated.  Now you’re left wondering if the contractors really removed the microscopic asbestos fibers or if these carcinogens are still floating in the air.  Asbestos air clearance testing refers to the process in which the work area is visually evaluated to ensure that abatement is complete, the work area has been properly cleaned, and finally if the air is suitable for re-occupancy. This process should be done by a third party company with no financial ties to the abatement company to ensure that the process is done correctly and without bias.


Asbestos clearance should always be conducted after asbestos removal, once the abatement contractor states that the work is complete. While asbestos air clearance testing is required for schools and public buildings, it may not be required for single family homes depending on the municipality and local regulations. Clearance testing should be conducting before the abatement containment is taken down to prevent the potential release of fibers in the air.

Visual Inspection

The first part of an asbestos clearance involves a visual inspection. The licensed asbestos consultant will look around the containment area for any residual debris or dust. The visual inspection is also used to see if the abatement contractor removed all of the material that was requested. If the work area fails the visual inspection, the area need not be air sampled, and additional cleaning will have to be conducted by the asbestos abatement contractor.

Air Clearance Testing

Once the work area passes visual inspection, the asbestos consultant can begin clearance air testing. The process starts when the consultant calibrates their air pumps to a desired flow rate, usually between 10 – 15 liters per minute to achieve a total volume of air of 1200 liters. The consultant then places an asbestos air cassette onto the pump to begin sampling.


For asbestos air clearance testing, there are two methods available to determine asbestos concentration in the air. The first and most common variety of clearance testing is called PCM (Phase Contrast Microscopy). Phase contrast microscopy is a method where the asbestos air cassette is analyzed by counting the fibers present on the cassette filter to determine if the levels are below the EPA clearance level of 0.01 fibers per cubic centimeter (f/cc). The drawback is that PCM only checks for fibers that are of a similar morphology as asbestos, but not asbestos in particular.


The second method, called TEM (Transmission Electron Microscopy), is mostly used for school abatement projects. This method involves aggressive air sampling which involves stirring up the air with a leaf blower and a box fan. This test evaluates if asbestos fibers are below 70 structures per square millimeter (s/mm2), which is the clearance criteria set by the Illinois Department of Public Health. While this testing method can identify asbestos fibers directly, it is usually more expensive than PCM air testing. The key advantage is that the air can be stirred up before the air sample, which hold the abatement contractor to a higher standard. To learn more about the difference between PCM and TEM air sampling, please check out the blog I recently wrote on the subject.

Lab Analysis

Once the sampling is complete, the air samples are submitted to an accredited laboratory to be analyzed. If the air samples pass clearance criteria, the asbestos consultant will submit a clearance letter to show that abatement was successful and the air is suitable for reoccupancy. The asbestos abatement contractor can then begin containment teardown activities. If the air test fails clearance criteria, the area will need to be re-cleaned by the abatement contractor. The area will need to be retested, and only if it passes can containment be torn down.


We at Indoor Science highly recommend having asbestos air clearance testing after asbestos removal, even if isn’t required by local or federal regulations for all building types.  The testing ensures that the area is safe for re-occupancy and provides documentation for a future purchaser of the property. Clearance testing is also useful to verify that all abatement activities have been completed and that the abatement contractor can be paid in full. We recommend hiring a third party company such as Indoor Science with no financial ties to the abatement firm conducting the work to ensure accurate results. If the abatement company suggests a company for the testing, politely decline and chose someone independent.