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Determination of asbestos in occupational air samples is performed by phase contrast optical microscopy (PCM). Any particle with an aspect ratio exceeding 3:1 is considered to be a fiber, and in the PCM method, all fibers longer than 5 µm are counted. Asbestos is measured in ambient air by electron microscopy, and all fibers that can be identified as asbestos are counted. The asbestos fibers are identified by using observations of morphology and a combination of selected area electron diffraction (SAED) and energy dispersive X-ray analysis (EDXA) measurements. The cost of absolute fiber identification, in the legal sense, is often prohibitive and, depending on previous knowledge of the situation, a combination of measurements acceptable as adequate identification may be agreed upon before the analysis is made.
The published information about the relative toxicity of fibers as a function of their dimensions indicates that the fiber definition used in PCM fiber counting excludes many fibers known to induce tumors in animals. For asbestos measurement in ambient air samples, no clear definition of an asbestos fiber exists, and all fibers within the toxic size ranges are examined. Those fibers that are compositionally and crystallographically consistent with asbestos are incorporated into the fiber count.
The relevance of fiber definition in both occupational and ambient air measurements is discussed. The author suggests that the best current information on biological effects should be used to define a new carcinogenicity index, thus bypassing the requirement for arbitrary fiber definitions.
health-related silicates, asbestos, fiber definition, occupational exposure, environmental exposure, transmission electron microscopy, phase contrast microscopy, fiber counting, fiber aspect ratio, asbestos identification
Manager, Electron Optics Centre, Ontario Research Foundation, Sheridan Park Research Community, Mississauga, Ontario