Filter Cassette Method for Analyzing Man-Made Vitreous Fibers Settled on Surfaces

A new method was developed to analyze the surface count of fibers in a variety of environments. The method entails sampling surfaces with the help of suction to a filter cassette holder containing a cellulose filter. The filters were collapsed using microwave digestion in dilute acid, and the fibers filtered to polycarbonate filters, gilded, and analyzed by scanning electron microscopy (SEM). The method was compared to traditional gel tape sampling as described in International Standards Organization (ISO) standard 16000-27, following analysis with phase contrast microscopy. The methods were compared in industrial environments and in office-type environments, with the concentration range studied spanning from 0.1 to 100,000 fibers/cm². The methods yielded similar results (p < 0.05) in concentrations from 100 to 10,000 cfu/cm², while the filter cassette method gave systematically higher results in high concentrations (>10,000 cfu/cm²) as well as in all office-type environments studied, where the fiber count ranged from 0.1 to 20 fibers/cm². Consequently, we recommend using the new method in working environments where the surface count is more than 100 fibers/cm², as well as in office-type environments where the fiber count is below 10 fibers/cm². It should be noted, however, that a similar limit of quantitation as with the gel tape method (0.1 fibers/cm²) requires sampling a minimum area of 100 × 100 cm² with the fiber cassette method. Using the filter cassette method will require new guide values to be formed for office-type environments, since the results are higher than with the gel tape method. Alternatively, if present guide values or limit values are to be used with the filter cassette method, conventions as to which fiber sizes to count should be set, since SEM analysis in any case will allow for including a larger size range than phase contrast microscopy (PM). We, however, recommend against such an approach, since fibers less than 1 µm in width may not be less harmful by inhalation than larger fibers.