Annals of Occupational Hygiene Advance Access published online on October 7, 2009
Annals of Occupational Hygiene, doi:10.1093/annhyg/mep066
Comparison of Direct and Indirect Methods of Measuring Airborne Chrysotile Fibre Concentration
Institut National de Recherche et de Sécurité, Centre de Lorraine, Département Métrologie des Polluants, Laboratoire d'Analyses Inorganiques et de Caractérisation des Aérosols, Rue du Morvan, CS 60027, 54519 Vandoeuvre les Nancy Cedex, France
* Author to whom correspondence should be addressed. Tel: +33-3-83-50-87-19; fax: +33-3-83-50-20-60; e-mail: celine.eypert-blaison{at}inrs.fr
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 µm 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.
asbestos • cellulose ester (CE) filter • direct preparation • filters • indirect preparation • polycarbonate (PC) filter • transmission electron microscopy (TEM)
Received March 31, 2009; in final form July 20, 2009