An asbestos clearance test is performed after asbestos removal or cleanup to help determine whether the work area has been adequately cleaned and is ready to be released from containment or reoccupied.
In simple terms, asbestos clearance testing helps answer an important question:
After the asbestos work is finished, is the area clean enough to safely return to normal use?
Clearance generally begins with a visual inspection of the work area. Before air testing is performed, the asbestos removal contractor should have completed the removal work and thoroughly cleaned the containment area.
The inspector looks for visible dust, debris, or remaining asbestos-containing material that should have been removed. If the work area is still visibly dirty, additional cleaning may be necessary before air samples are collected.
This is important because air testing should not be used as a substitute for proper cleaning and a thorough visual inspection.
Once the area passes the visual inspection, air samples may be collected to evaluate the amount of airborne fibers or asbestos structures remaining in the work area.
Air samples are collected using a small sampling pump connected to a specialized filter cassette. The pump pulls a measured amount of air through the filter over a specified period of time.
Think of the filter as an extremely fine net. As air passes through it, microscopic fibers and particles are captured on the filter.
The cassette is then sent to a laboratory, where the filter is examined under a microscope.
There are two primary methods commonly used for asbestos air testing:
PCM — Phase Contrast Microscopy
and
TEM — Transmission Electron Microscopy
Although both methods analyze material collected from the air, they provide different types of information.
Phase Contrast Microscopy (PCM) is a commonly used and relatively economical method for asbestos air monitoring and certain clearance applications.
Under PCM, the laboratory examines the filter under a microscope and counts fibers that meet specific size and shape requirements.
The important limitation is that PCM generally cannot tell whether a counted fiber is actually asbestos.
For example, the microscope may detect a fiber from another material that has similar dimensions. PCM essentially tells us how many qualifying fibers are present in the air sample rather than confirming that every counted fiber is asbestos.
A simple way to think about PCM is:
PCM counts fibers, but it generally cannot identify exactly what those fibers are.
For PCM clearance testing, 0.01 fibers per cubic centimeter of air (f/cc) is a commonly used clearance criterion. When PCM is the appropriate clearance method, a result at or below 0.01 f/cc is commonly used as an indication that airborne fiber concentrations are sufficiently low for clearance.
The abbreviation f/cc means fibers per cubic centimeter of air.
A laboratory result of:
0.008 f/cc
would be below a 0.01 f/cc clearance criterion.
A result of:
0.015 f/cc
would be above a 0.01 f/cc criterion.
However, because PCM cannot normally distinguish asbestos fibers from other qualifying fibers, a higher PCM result does not necessarily mean that every fiber counted was asbestos.
This limitation is one of the reasons TEM analysis may be used when more specific information is required.
Transmission Electron Microscopy (TEM) is a much more powerful and sensitive analytical method.
Unlike PCM, TEM can examine extremely small particles and can specifically identify asbestos structures based on their physical and chemical characteristics.
A simple way to understand the difference is:
PCM asks: "How many qualifying fibers are present?"
TEM asks: "Are these actually asbestos fibers or structures?"
This makes TEM particularly valuable when a more sensitive or asbestos-specific clearance evaluation is necessary.
TEM can also detect asbestos structures that may be too small to be seen using PCM.
TEM may be required by regulation or project specifications, or selected when a more sensitive asbestos-specific analysis is desired.
For example, TEM is particularly important in certain AHERA-regulated school asbestos projects.
TEM may also be appropriate when there is a need to distinguish asbestos from other fibers in the air or when project requirements call for a higher level of analytical sensitivity.
The tradeoff is that TEM analysis is generally more expensive and more involved than PCM analysis.
For many projects, PCM provides a practical and cost-effective air-monitoring method. For projects requiring asbestos-specific identification or greater analytical sensitivity, TEM may be the more appropriate method.
PCM:
TEM:
TEM clearance is evaluated differently from PCM clearance.
For certain projects regulated under the federal Asbestos Hazard Emergency Response Act (AHERA), including applicable asbestos response actions in schools, 70 asbestos structures per square millimeter (70 structures/mm²) is an important TEM clearance criterion.
AHERA procedures can also involve comparing samples collected inside the work area with outdoor air samples. Therefore, TEM clearance should not simply be treated as another version of the 0.01 f/cc PCM criterion.
The appropriate clearance standard depends on the type of project and the regulations or specifications governing the asbestos work.
Another important distinction is the difference between a clearance criterion and a worker exposure limit.
These numbers are sometimes confused because both may be expressed using fibers per cubic centimeter of air.
For example, the OSHA and Cal/OSHA asbestos permissible exposure limit is 0.1 f/cc as an 8-hour time-weighted average for occupational exposure.
That does not mean that 0.1 f/cc should be used as a post-abatement clearance level.
A simple way to remember the distinction is:
0.01 f/cc — commonly used PCM clearance criterion
0.1 f/cc — OSHA/Cal/OSHA 8-hour occupational exposure limit
They serve different purposes and should not be used interchangeably.
Clearance testing should generally occur after the asbestos contractor has finished the removal work and completed detailed cleaning of the containment area.
The area should not still contain obvious dust, debris, or material that was supposed to be removed.
A typical process may therefore look like:
Asbestos Removal → Detailed Cleaning → Visual Inspection → Air Sampling → Laboratory Analysis → Clearance Determination
If the work area does not pass the visual inspection, additional cleaning may be necessary before clearance air samples are collected.
If air samples do not meet the applicable clearance criteria, the area may require additional cleaning followed by another round of air sampling.
Neither method is automatically "better" for every project.
The correct method depends on the purpose of the testing, the type and size of the asbestos project, applicable regulations, project specifications, and the level of analytical sensitivity required.
PCM is generally less expensive and provides useful information about airborne fiber concentrations, but it cannot normally confirm that the fibers being counted are asbestos.
TEM is more sensitive and can specifically identify asbestos structures, but it is generally more expensive and may require different sampling and interpretation procedures.
For homeowners and property managers, the most important point is that the clearance method and clearance criterion should be selected before the samples are collected based on the requirements of the specific project.
An asbestos clearance is more than simply collecting an air sample.
A proper clearance evaluation generally begins by confirming that the asbestos removal work has been completed and that the containment area is visually clean and free of remaining dust and debris. Air samples can then be collected and analyzed using the appropriate method.
PCM is a practical and commonly used method that measures airborne fiber concentrations but generally cannot determine whether individual fibers are asbestos.
TEM is a more sensitive analytical method that can specifically identify asbestos structures and may be required or preferred for certain projects.
When PCM is the appropriate clearance method, 0.01 f/cc is a commonly used clearance criterion. Certain AHERA-regulated TEM clearances use 70 structures/mm² as an important criterion, although the complete AHERA clearance procedure can also involve comparison with outdoor samples.
Ultimately, asbestos clearance requirements should be determined based on the specific project and applicable federal, California, local air-district, and project-specific requirements rather than applying a single clearance number to every asbestos project.