Asbestos results interpretation

In adequate animal studies, these effects should be relevant to humans. The exposure situations should be similar and transferable to the situations at the workplace. Nevertheless, animal studies have clear limitations, which have to be considered. Usually, the concentrations used in the animal studies are higher than the concentrations at the workplace, usually by a factor of 10 up to over 1000. On the other hand, it has to be borne in mind, especially in the interpretation of aerosol results, that the upper respiratory tract of rodents is different from that of humans. As obligatory nose breathers their nasal filtration systems are considerably better. For example, the fibers from asbestos did not reach the alveolar region in inhalation studies. Similar findings were seen in the case of wood particles. 

If it is assumed that both the deposition of fibers on the filter and the selection of fields for counting are random, then there will be a variability in the results that can be described by a Poisson distribution. For fiber counts of 50 this leads to 95% confidence intervals in the results of + 15 (30%) and a coefficient of variation of 15%, although for smaller fiber counts the variability will be much larger. In addition to this variability there is an unavoidable degree of nonuniformity of the asbestos deposit on the filter. There is also subjective variation between microscopists in interpretation of fiber structures and in their ability to detect and identify fibers. Overall counting performances caimot then be expected to produce a coefficient of variation better than 25% for counts of 50 fibers. 

Interpretative errors can occur as a result of sample contamination. Possible contamination of paraffin blocks by asbestos fibers has been reported by some laboratories (Lee et al. 1995 Dodson et al. 1998). Therefore, asbestos fiber counts based on paraffin blocks should be interpreted with caution. Moreover, in a recent report, electron microscopy mineral analysis of lung tissue identified an unusual contamination by large amounts of fibrous sepiolite, a clay present in absorbent granules inadvertently used to provide the sample (Attanoos et al. 2003). 

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