- Response Actions; §763.90 (i)
- Appendix A (to Subpart E) – Interim Transmission Electron Microscopy Analytical Methods – Mandatory and NonMandatory – and Mandatory Section to Determine Completion of Response Actions
AHERA allows final clearance air sampling to be done by phase contrast microscopy (PCM) methodology for projects less than or equal to 160 square feet (SF) or 260 linear feet (LF) by the National Institute for Occupational Safety and Health (NIOSH) 7400 methodology (Issue 3: 14 June 2019 is the current issue). For projects greater than 160 SF or 260 LF clearance shall be done by the AHERA transmission electron microscopy (TEM) method (requirements at 763.90 (i) (4) and Appendix A). Since this article’s purpose is to discuss why we should not be using the PCM method we will focus our discussion on this method specifically. An important point to remember is that the method was designed for personal sampling of workers in environments with actual asbestos exposures. AHERA adapted the method for clearance requiring that each sample must be less than or equal to a limit of quantitation (LOQ) for PCM of 0.01 fibers per cubic centimeter (f/cc).
Over the years, it has become abundantly clear that the PCM method should not be used for clearance sampling. The top five reasons it should not be used for clearance sampling are:
Size of the Fibers Analyzed
The rules for the PCM method specifically require the microscopist to count only fibers that are greater than 5 microns long and width greater than or equal to 0.25 microns. All other fiber lengths and widths are not counted or may not even be seen. At the Professional Abatement Contractors of New York’s (PACNY’s) 2022 Environmental Conference Lee Poye, Vice President Emeritus Eurofins Built Environment, discussed his presentation “Asbestos in Human Tissue and the Environment – Does Size Matter?”
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| Lee Poye Presenting at PACNY 2022 |
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Is the Work Area Actually Clean?
See our Asbestos Floor Tile Debate Results Post and our article in Healthy Buildings. Our major points were:
- When using the AHERA method for clearance, what was the typical size of the fibers found? The answers we got were 58.8% less than 5 microns; 29.4% of both sizes were equal amounts; and 11.8% greater than 5 microns.
- Have you ever encountered during asbestos flooring removal when utilizing both the NIOSH 7400 (PCM) & the AHERA (TEM) methods of analyses, that the NIOSH 7400 passed while the AHERA method failed? The answers we got were 52.6% yes, 36.8% no, and 10.5% never used both.
- AHERA TEM method counts for total asbestos structures per cubic centimeter averaged 22 times greater than the PCM fiber counts on the same filters.
- AHERA TEM asbestos concentrations obtained during mastic removal with a commercial mastic remover averaged 11 times higher than those measured when removal used amended water.
Is 0.01 fibers/cubic centimeter (f/cc) Safe?
Based on this information a better clearance level would be 0.001 f/cc.
- Many believe or have been misled to believe that PCM sampling is the same as TEM sampling in terms of sampling volume. This is not the case. A PCM sample volume meeting AHERA clearance requirements are not at 1200 liters. To do so is outside of the NIOSH 7400 method requirements for this purpose. In the NIOSH 7400 method, the issue regarding “relatively clean” environments” is addressed on page 4, number 4, note number 1 which states “In relatively clean atmospheres, where targeted fiber concentrations are much less than 0.1 f/cc, use larger sample volumes (3000 to 10,000 liters) to achieve quantifiable loadings.” Even though the formula calculates that 3,850 liters of air should be collected, many people use note 1 to collect 3,000 liters of air for clearance. Either way clearance samples should be collected using no less than 3,000 liters of air as the minimum allowed for the NIOSH 7400 method requirements and AHERA compliance.
The NIOSH 7400 method, utilizes the formula above to determine the amount of time needed to achieve the fiber density, E, for optimum filter loading. So, the minimum density the method allows is 100 fibers per square millimeter (mm2). The Ac is the collection area for a 25-mm cassette which is 385 mm2. The Q is the sampling flow rate in LPM, so let’s say that is 16 (the maximum flow rate allowed by the method). The t is the time we are collecting the sample 1200 liters of air divided by 16 liters per minute gives us 75 minutes. So if you plug these numbers into the formula you get 0.032 fibers/cubic centimeter is the number you would actually clear at. For NYC requirements, the flow rate (Q) is a maximum of 15 liters per minute with a volume of 1800 liters for a time of 120 minutes. Plugging these numbers in means 0.021 f/cc is the actual clearance in NYC.
The Cost of Clearance Sampling
We know what you are going to say TEM samples cost way more than PCM samples. We agree they do, but not compared to when AHERA first came out. When AHERA first came out there were hardly any laboratories that did TEM analysis and those that did the samples cost around $500 per sample. The difference between PCM analysis costs and TEM analysis costs has come way down. A recent quote we received from a reputable laboratory for PCM analysis with a 3-hour turnaround was $12.50 per sample while TEM AHERA analysis with a 4-hour turnaround was $150 per sample. The price difference is smaller than it once was and considering the above points we would say PCM is not worth the money nor the paper it is printed on.
