Fundamentals of Asbestos Air Clearance Testing

Asbestos Air Clearance testing


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.


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.

Jordan Thomas

Jordan Thomas

Jordan Thomas is a Project Manager that performs indoor air quality assessments with a specialty in asbestos and lead. Mr. Thomas holds a Bachelors of Arts degree in Earth Science from DePauw University. Jordan is an ACAC Council-Certified Indoor Environmentalist (CIE), Licensed Lead and Asbestos Inspector, Licensed Air Sampling Professional, and HAZWOPER certified. He also holds an asbestos microscopist certificate from the McCrone Research Institute. Prior to working at Indoor Science, Jordan worked as an Industrial Hygienist at Environmental Analysis, Inc and as an Asbestos/Lead Analyst at Metro Technology Laboratory. In his words… “While not in the field, I’m a Nu-Jazz and movie enthusiast.”

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12 thoughts on “Fundamentals of Asbestos Air Clearance Testing

    We had an abatement contractor remove asbestos insulation from heating pipes in our basement. He took an air sample after completion, but the lab said the volume was too low (only 150 liters). I told them to test anyway, and the PCM result was .078 fibers/cc. Given the low volume, is the result even useful? Would it make sense to bring a testing company in now to do another test? The abatement work was done three days ago, and we have stayed out of the basement since then. Thank you.

    Hello John,

    The volume recorded is very low. Typically clearance volume is around 1,200 liters. It is always recommended to have a third party company with no financial ties to the contractor to conduct clearance testing. I would recommend conducting air sampling or dust sampling by a licensed professional.

    Hi Jordan!
    I am looking for regulatory confirmation or guidance regarding PCM clearance sampling for a school setting (AHERA) and the use of aggressive methods. (two questions)
    1. The PCM clearance option is only for available when dealing with a project that is below de minimis (in my state, that is defined as less than or equal to 64 sq. ft. or 60 ln. ft). If the abatement is below de minimis, and performed using glove bags, how can I use aggressive sampling for clearance? It is my understanding that the AHERA requirement for aggressive sampling is to be used clearance after asbestos removal had been conducted under containment. If a glove bags are used, and the amount is under de minimis, then where is it stated that the clearance has to be “aggressive”?
    Thank you for your time.

    Hello Patti,

    If the units for the 0.0041 results are fibers per cubic centimeter (F/cc), then this would be below the EPA clearance criteria of 0.01 F/cc. This number would be acceptable for clearance.


    What are your thoughts on a TEM sample of 855str/mm2 when the air was not stirred up? Should more aggressive sampling be done to determine how much actual exposure someone had over a 2400 hour period?

    Hello William,

    The AHERA limit is 70 str/mm2 and these levels are far above that threshold. I would recommend air sampling again only after the area has been abated again by a licensed asbestos professional. This would possibly include wet wiping and HEPA vacuuming surfaces along with HEPA air scrubbing. After all of these activities I would recommend conducting clearance testing.

    As far as how much exposure one would have received, it’s hard to speculate the amount of exposure unless sampling was done while the person was occupying the space. However, that result shows highly elevated levels of airborne asbestos. If the sample was done with aggressive sampling then one could assume a higher result would have been reported as asbestos tends to settle into dust with time.

    You are missing a few important details that come from AHERA: the need to conduct clearance sampling using aggressive methods for both TEM and PCM – not just TEM as you mention, and the fact that TEM air sampling should actually performed at a flow rate less than 10 LPM – not the 10-15 LPM that you mention.