Changes to Asbestos Analysis
Most analysis of asbestos air samples follows NIOSH (National Institute for Occupational Safety and Health) method 7400, which is a fiber counting method and follows the OSHA regulations for personnel exposure monitoring. NIOSH is currently revising the method in hopes of improving accuracy and precision in fiber counting. No changes have yet been confirmed. The current method calls for phase-contrast microscopy (PCM) at a magnification of 400x. With PCM, a portion of the mixed cellulose ester (MCE) filter is removed and mounted on a slide. The filter is collapsed by acetone vapor and triacetin resin to make the filter transparent. In short, PCM makes the transparent fibers visible because they have a different refractive index (they bend light differently) than the filter media.
PCM cannot distinguish between asbestos fibers and non-asbestos fibers. The microscopist counts all fibers in a field (portion of the filter) that have a minimum length of 5 microns, a minimum diameter of 0.25 um and a length to width ratio of at least 3:1. A graticule in the microscope eyepiece has gradations to assist with the accurate sizing of the fiber. Asbestos fibers often occur in bundles of fibrils and the whole bundle is counted as one fiber. According to the method, microscopists must count least 20 fields and stop counting when they have counted 100 fibers, or when they have reached 100 fields, whatever comes first. The average number of fibers per graticule field is used to determine the number of fibers on the filter.
NIOSH 7400 requires that microscopists examine an in-house reference slide (reference samples with a range of fiber loadings). This is not a standardized reference slide however. The counts from the reference slide must be within established limits before the microscopist can analyze field samples. In addition, OSHA requires all laboratories to participate in a round robin testing program with at least two other laboratories using reference samples. The American Industrial Hygiene Association Proficiency Analytical Testing Program (PAT) is a means of independently verifying laboratories’ ability to provide quality data. PAT testing is conducted quarterly.
The revised NIOSH method will introduce a verification process involving reference slides with relocatable fields. Essentially taking the AIHA PAT program a step further by having reference slides that will enable microscopists to look at the same area of a slide as another microscopist. This should ensure greater precision and accuracy. Studies have suggested that the use of standardized reference slides can improve the precision of counting and provide an effective training aid for analysts. This should also reduce the likelihood of bias between analysts with respect to fiber length and partial fibers within a field. Martin Harker, chief of NIOSH Exposure Assessment Branch states that giving analysts a slide where they are told what they are going to see and asking them to look and examine will encourage the analysts to take more time, leading to better and more consistent results.
Another change to the method will allow the laboratory the option of using dimethylformamide solution and a synthetic resin called Euparal. Euparal produces a long-lasting mount that allows long-term archiving of slides. Acetone/triacetin can still be used, but this is just an alternative method for the laboratory to preserve the sample.
We suspect that these changes may result in requiring more time by the analyst to read samples. Therefore the number of samples that can be analyzed by a microscopist over a period of time may decrease, which may also increase costs. It is unknown how the quality control requirements might affect microscopists who analyze samples on a job site, such as what may occur during a large abatement project. However, the likely improvement in precision and accuracy appear to outweigh the disadvantages.