Comparison of direct and indirect methods of measuring airborne chrysotile fibre concentration

Our plain-English summary of Comparison of direct and indirect methods of measuring airborne chrysotile fibre concentration, by C. Eypert-Blaison, S. Veissiere, O. Rastoix, et al (The Annals of Occupational Hygiene, 2010).

The study

• Authors: C. Eypert-Blaison, S. Veissiere, O. Rastoix, E. Kauffer
• Journal: The Annals of Occupational Hygiene
• Published: 2010;54(1):55-67
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Transmission electron microscopy observations most frequently form a basis for estimating asbestos fibre concentration in the environment and in buildings with asbestos-containing materials. Sampled fibres can be transferred to microscope grids by applying either a direct [ISO (1995) Draft International ISO/DIS 10312. Ambient air. Determination of asbestos fibres. Direct transfer transmission electron microscopy procedure. Geneva, Switzerland: International Standardization Organization] or an indirect [AFNOR (1996) Détermination de la concentration en fibres d'amiante par microscopie électronique à transmission-Méthode indirecte. Cedex, France: AFNOR, p. 42; ISO (1997) Draft International ISO/DIS 13794. Ambient air. Determination of asbestos fibres. Indirect-transfer transmission electron microscopy procedure. Geneva, Switzerland: International Standardization Organization] method. In the latter case, ISO Standard 13794 recommends filtering calcination residues either on a polycarbonate (PC) filter (PC indirect method) or on a cellulose ester (CE) membrane (CE indirect method). The PC indirect method requires that fibres deposited on a PC filter be covered by a carbon layer, whereas in the CE indirect method, the CE membrane has to be directly processed using a method described in ISO Standard 10312. The purpose of this study was to compare results obtained using, on the one hand, direct preparation methods and, on the other hand, PC indirect or CE indirect methods, for counting asbestos fibres deposited on filters as a result of liquid filtration or air sampling. In direct method-based preparation, we observed that an etching time of 6-14 min does not affect the measured densities, except for fibres <1 microm deposited by liquid filtration. Moreover, in all cases, the direct method gives higher densities than the PC indirect method because of possible fibre disappearance when using the carbon evaporator implemented in the PC indirect method. The CE membrane used for sample preparation in the CE indirect method is collapsed prior to passing it through the carbon evaporator, so the fibres are less likely to disappear at this stage. We then note that the resulting fibre densities for chrysotile-loaded filters prepared using the direct method are close to those obtained with filters prepared using the CE indirect method. Our study therefore shows that, under the implemented experimental conditions, the PC and CE indirect preparation methods described in ISO Standard 13794 are not equivalent.

In plain English

This laboratory study compared ways of preparing filters for counting asbestos fibres by transmission electron microscopy. The authors used proficiency-testing filters loaded with chrysotile only, by liquid filtration or air sampling. They compared a direct method with two indirect methods that refilter ashed residue onto a polycarbonate (PC) filter or a cellulose ester (CE) membrane. Etching times of 6 to 14 minutes had little effect on direct-method densities, except for fibres under 1 micrometre on liquid-filtration filters. The direct method gave higher densities than the PC indirect method, with average ratios of 1.75 for liquid-filtration filters and 1.51 for air-sampling filters. In a test on eight liquid-filtration filters, gold-coated halves that also passed through the evaporator vacuum, without carbon coating, averaged 80 fibres longer than 5 micrometres per square millimetre by scanning electron microscopy, against 101 for halves only gold-coated. Average direct to CE indirect ratios were 1.06 for liquid-filtration and 1.07 for air-sampling filters. The authors concluded that the two indirect methods in ISO Standard 13794 were not equivalent under their conditions, noting this may be specific to their evaporator.

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 methods-comparison study of filter preparation for transmission electron microscopy (TEM) fibre counting. It compared a direct method with two indirect methods. In the indirect methods the sampling filter is ashed and the residue is refiltered onto a polycarbonate (PC) filter or a cellulose ester (CE) membrane.
• Who was studied: Chrysotile-loaded test filters from the asbestos fibre counting proficiency testing schemes run by the authors' institute in France. The filters were produced either by liquid filtration (until 2003) or on a laboratory air sampling bench (since 2003). Sets of eight filters were used per comparison. No people were studied.
• Exposure: Chrysotile only, deposited on filters. Fibres were counted in three length classes: under 1 micrometre, 1 to 5 micrometres, and over 5 micrometres.
• Main results:
  • Etching times of 6 to 14 minutes had little influence on fibre densities measured with the direct method. A 12-minute etch was used for the rest of the study.
  • The exception was fibres shorter than 1 micrometre on filters prepared by liquid filtration, where the etching-time effect had p less than 0.05.
  • For liquid-filtration test filters, the direct method gave higher densities than the PC indirect method at all but one point. The average direct to PC indirect ratio was 1.75.
  • For air-sampling test filters, the average direct to PC indirect density ratio was 1.51.
  • In the evaporator test, eight liquid-filtration filters prepared by the PC indirect method were cut in half. One set of halves was put through the evaporator vacuum without any carbon layer being deposited.
  • Both sets of half-filters were then given a sputtered gold layer for SEM viewing. The second set received only the gold layer.
  • In that test, only fibres longer than 5 micrometres were counted, using a scanning electron microscope (SEM).
  • For fibres longer than 5 micrometres counted by SEM, the half-filters exposed to the evaporator vacuum averaged 80 fibres per square millimetre. The half-filters that were only gold-coated averaged 101. This was a difference of about 25% on average.
  • The authors reported that fibres do disappear in the evaporator.
  • Fibre distribution on the PC filter was non-uniform.
  • The CE indirect method gave results close to the direct method. For liquid-filtration filters, the average direct to CE indirect ratios were 1.22 (over 5 micrometres), 1.04 (1 to 5 micrometres) and 0.92 (under 1 micrometre), averaging 1.06.
  • For air-sampling filters, the average direct to CE indirect ratios were 1.18, 0.99 and 1.04 across the same three length classes, averaging 1.07.
  • The authors stated that the CE indirect method gives results close to those of the direct method.
• The authors' conclusion: The authors concluded that the two indirect preparation methods in ISO Standard 13794 (PC and CE) are not equivalent under the experimental conditions of this study. They attributed this to the PC indirect method losing some fibres when the carbon layer is deposited in the evaporator.
• Limitations the authors state:
  • The observed effect may be specific to the type of evaporator used in this study.
  • The indirect methods used differed from those described in ISO Standard 13794: they used pre-carboned PC filters and did not use ultrasound.
  • Results may have been biased by analysts' insufficient practice with the method they do not routinely apply.
  • The etching-time study involved fewer filters (16) than a comparable study (48), which may explain the differing findings for fibres under 5 micrometres.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Chrysotile, Transmission Electron Microscopy (TEM), Asbestos-Containing Material (ACM), Air Sampling.

Related studies

• Performance of membrane filters used for TEM analysis of asbestos — J.S. Webber, A.G. Czuhanich, et al. (2007)
• Comparison of a fibrous aerosol monitor (FAM) with the membrane filter method for measuring airborne asbestos concentrations — P.J. Iles, T. Shenton-Taylor (1986)
• 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)
• A comparison of parallel dust and fibre measurements of airborne chrysotile asbestos in a large mine and processing factories in the Russian Federation — E. Feletto, S.J. Schonfeld, et al. (2017)
• Counting rules for estimating concentrations of long asbestos fibers — K.S. Crump, D.W. Berman (2011)
• Use of direct versus indirect preparation data for assessing risk associated with airborne exposures at asbestos-contaminated sites — M.P. Goldade, W.P. O'Brien (2014)

Browse more research in The Annals of Occupational Hygiene, or return to medical research.