For asbestos air testing, there are two primary sampling methods: PCM (Phase Contrast Microscopy) and TEM (Transmission Electron Microscopy). Both methodologies analyze for fiber concentration through pumping the ambient air into filtered air cassettes. The process usually involves running a number of high volume air pumps depending on the size of the property to achieve a volume of 1,200 liters of air. In this blog post, I will discuss the benefits and drawbacks to each method of testing.
The most common methodology used for asbestos air testing is Phase Contrast Microscopy (PCM). This testing method involves using a PCM microscope to count the fibers present on the cassette filter in order to determine if the total fiber concentration is less than the EPA clearance level of 0.01 fibers/cc (fibers per cubic centimeter)[note]”Asbestos sampling protocol cannot be used for objective data – OSHA.” https://www.osha.gov/laws-regs/standardinterpretations/2015-10-07. Accessed 27 Mar. 2018.[/note]. Phase contrast microscopy enhances the contrast of transparent materials, while also filtering out background light. The air cassettes used for PCM contain a cellulose ester filter with a pore size of 0.8 um. Once the air sampling is complete, the sample is sent to an accredited laboratory. Alternatively, the samples can be analyzed on-site with a microscope for expediency, but these results will not be accredited. Whether in a lab or on-site, the filter is cut into a pie shape and prepped using a mix of vaporized acetone and triacetin. The sample is then placed on a PCM microscope where the fibers are counted up to 100 fields of view.
The second type of air testing is called TEM (Transmission Electron Microscopy). This test is often used in schools but can be performed in residential and commercial properties. This test uses an electron microscope, which is an expensive instrument often costing more than $100,000. These instruments scan and identify the number of asbestos fibers present to see if it passes the Illinois Department of Public Health clearance level of 70 structures per square millimeter. The TEM air cassette has a pore size 0.45 um filter and is composed of a cellulose ester similar to the PCM. However, each TEM cassette has multiple grids along the filters which are used during analysis. The sample is prepared with acetone and the TEM microscope analyzes a number of grids to determine the asbestos content.
TEM vs. PCM Comparison
While both of these methods are useful in ascertaining the concentration of asbestos fibers in a work area, both methods have their shortcomings. Although PCM is the standard used by the EPA and OSHA, it has its deficiencies. The primary problem with PCM is that it can be skewed high by non-asbestos fibers. PCM testing doesn’t specifically look for asbestos itself; only fibers around the size of asbestos and is only concerned with the total fiber concentration in the air. For example, if there was a release of fiberglass fibers into the air during air testing, a PCM could potentially fail even though there were no asbestos fibers in the air. Another drawback of PCM is that it cannot distinguish fibers less than 5 microns (µm) in length and 0.25 µm in width, which may lead to small fibers being undetected. Alternatively, TEM can view fibers less than 0.5 µm in length and 0.01 µm in width. TEM also can positively identify asbestos directly, distinguishing it from other fibers by scanning for its morphology and elemental composition. Recent research has shown that the concentration of Chrysotile asbestos fibers greater than 5 µm will be 4 times higher when analyzed by TEM compared to PCM[note]Céline Eypert-Blaison, Anita Romero-Hariot, Frédéric Clerc & Raymond Vincent (2017) Assessment of occupational exposure to asbestos fibers: Contribution of analytical transmission electron microscopy analysis and comparison with phase-contrast microscopy, Journal of Occupational and Environmental Hygiene, 15:3, 263-274, DOI: 10.1080/15459624.2017.1412583[/note].
While TEM is a superior analytical method, OSHA states that PCM is the best method to compare results to an index of exposure, such as a Permissible Exposure Limit (PEL)[note]”Use of Transmission Electron Microscopy (TEM) instead of … – OSHA.” https://www.osha.gov/laws-regs/standardinterpretations/2005-06-30. Accessed 27 Mar. 2018.[/note]. This is due to there being a lack of research showing health effects associated with time-weighted exposures with the TEM method. TEM is also much more expensive than a PCM sample. At the laboratory, TEM air samples can cost 5 times more than a PCM for a standard turnaround time. TEM is not as commonly used and there are no options for on-site TEM analysis.
Conclusions on Asbestos Air Testing
Overall, TEM air sampling is the superior method due its precision and accuracy. However, PCM has its value as far as being able to index exposure and its lower cost. While many companies only use PCM as their testing method, Indoor Science offers both methods of asbestos air testing.