Efficiency of sampling and analysis of asbestos fibers on filter media: implications for exposure assessment

Our plain-English summary of Efficiency of sampling and analysis of asbestos fibers on filter media: implications for exposure assessment, by D.A. Vallero, J.R. Kominsky, M.E. Beard, et al (Journal of Occupational and Environmental Hygiene, 2009).

The study

• Authors: D.A. Vallero, J.R. Kominsky, M.E. Beard, O.S. Crankshaw
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2009;6(1):62-72
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

To measure airborne asbestos and other fibers, an air sample must represent the actual number and size of fibers. Typically, mixed cellulose ester (MCE, 0.45 or 0.8 microm pore size) and, to a much lesser extent, capillary-pore polycarbonate (PC, 0.4 microm pore size) membrane filters are used to collect airborne asbestos for count measurement and fiber size analysis. In this research study, chrysotile asbestos (fibers both shorter and longer than 5 microm) were generated in an aerosol chamber and sampled by 25 mm diameter MCE filter media to compare the fiber retention efficiency of 0.45 microm pore size filters vs. 0.8 microm pore size filter media. In addition, the effect of plasma etching times on fiber densities was evaluated. This study demonstrated a significant difference in fiber retention efficiency between 0.45 microm and 0.8 microm pore size MCE filters for asbestos aerosols (structures longer than or equal to 0.5 microm length). The fiber retention efficiency of a 0.45 microm pore size MCE filter is statistically significantly higher than that of the 0.8 microm pore size MCE filter. However, for asbestos structures longer than 5 microm, there is no statistically significant difference between the fiber retention efficiencies of the 0.45 microm and 0.8 microm pore size MCE filters. The mean density of asbestos fibers (longer than or equal to 0.5 microm) increased with etching time. Doubling the etching time increased the asbestos filter loading in this study by an average of 13%. The amount of plasma etching time had no effect on the filter loading for fibers longer than 5 microm. Many asbestos exposure risk models attribute health effects to fibers longer than 5 microm. In these models, both the 0.45 microm and 0.8 microm pore size MCE filter can produce suitable estimates of the airborne asbestos concentrations. However, some models suggest a more significant role for asbestos fibers shorter than 5 microm. Exposure monitoring for these models should consider only the 0.45 microm pore size MCE filters as recommended by the U.S. Environmental Protection Agency Asbestos Hazard Emergency Response Act (AHERA) protocol and other methods.

In plain English

This was a laboratory experiment, not a study of people. The authors tested how well two air-sampling filter types hold asbestos fibers for electron-microscope counting. Chrysotile asbestos was blown into an aerosol chamber and collected on 72 mixed cellulose ester filters with 0.45 or 0.8 micrometer pores. For fibers 0.5 micrometers or longer, the 0.45 micrometer filters retained significantly more overall (p = 0.01), though only one of four batches differed significantly on its own. For fibers longer than 5 micrometers, the filter types did not differ significantly. In a separate test of high-loaded 0.45 micrometer filters etched for 2, 4, 8 or 16 minutes, longer plasma etching (a preparation step) raised the total count of fibers 0.5 micrometers or longer; doubling etching time added an average of 13%. Etching time had no statistically significant effect on fibers longer than 5 micrometers. The authors concluded that either filter may be used if only fibers longer than 5 micrometers are of interest, but 0.45 micrometer filters should be used for studies focused on shorter fibers. Only chrysotile was tested.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment comparing air-sampling filter pore sizes, and filter preparation (plasma etching) times, using chrysotile asbestos generated in an aerosol chamber and counted by transmission electron microscopy
• Who was studied: 72 mixed cellulose ester (MCE) air filters, 25 mm in diameter, with 0.45 µm or 0.8 µm pores, prepared by SRI International in four batches of 18. Separately, 0.45 µm filters loaded at the 'high' level were etched for 2, 4, 8 or 16 minutes, with 12 filters at each time.
• Exposure: Chrysotile asbestos fibers, both shorter and longer than 5 µm, loaded onto filters at a 'low' level (200–500 structures per square millimeter) and a 'high' level (more than 500 structures per square millimeter)
• Main results:
  • For fibers 0.5 µm or longer, the 0.45 µm pore size filters had a statistically significantly higher fiber retention efficiency than the 0.8 µm filters when the four batches were analyzed together (p = 0.01)
  • Mean fiber density was higher on the 0.45 µm filters in every batch, but taken batch by batch the difference was statistically significant only in Batch 2 (t-test p = 0.008; Wilcoxon p = 0.01)
  • For fibers longer than 5 µm, the 0.45 µm filters had higher mean densities in Batches 1 and 2 and the 0.8 µm filters had higher mean densities in Batches 3 and 4. The authors found no difference between the two filter types for these fibers.
  • On high-loaded 0.45 µm filters, the mean density of fibers 0.5 µm or longer rose with etching time: 1123 structures per square millimeter at 2 min, 1251 at 4 min, 1512 at 8 min and 1635 at 16 min
  • On these high-loaded 0.45 µm filters, doubling the etching time increased the measured total asbestos density (fibers 0.5 µm or longer) by an average of 13%
  • For fibers longer than 5 µm on 0.45 µm filters, there was no statistically significant relationship between etching time and fiber density
• The authors' conclusion: The authors concluded that studies focused on fibers shorter than 5 µm should use 0.45 µm pore size filters, while studies interested only in fibers longer than 5 µm may use either pore size. They also concluded that the 8-minute etching time in ISO 10312:1995 is adequate for fibers longer than 5 µm.
• Limitations the authors state:
  • The sample sizes for individual batch comparisons were too small to give statistically significant results, except for Batch 2
  • Differences between batches loaded at the same nominal level were larger than the differences between the two filter types
  • The results apply to transmission electron microscopy (TEM), not directly to the NIOSH phase contrast microscopy (PCM) method
  • Only chrysotile was tested; the authors said amphiboles should be tested to see whether the findings apply to them
  • Only asbestos fibers were used, with no other dust or particle matrix
  • The experiment did not identify the etching time beyond which fiber counts stop increasing
  • Polycarbonate filters were not tested

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Asbestos Exposure, AHERA.

Related studies

• Sample preparation method for visualization of nanoparticulate captured on mixed cellulose ester filter media by enhanced darkfield microscopy and hyperspectral imaging — N.M. Neu-Baker, A.C. Eastlake, et al. (2019)
• Comparison of direct and indirect methods of measuring airborne chrysotile fibre concentration — C. Eypert-Blaison, S. Veissiere, et al. (2010)
• An examination of MCE filter morphology and implications on preparation and analysis of air samples for asbestos — M. Getman, J. Webber, et al. (2017)
• Counting rules for estimating concentrations of long asbestos fibers — K.S. Crump, D.W. Berman (2011)
• Performance of membrane filters used for TEM analysis of asbestos — J.S. Webber, A.G. Czuhanich, et al. (2007)
• The influence of external aerodymanic factors on the measurement of the airborne concentration of asbestos fibres by the membrane filter method — A.M. Johnston, A.D. Jones, et al. (1982)

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