A dose-response for asbestos
The article by Clin et al. (1) provides additional information for a dose-response relationship with asbestos and cancer. Information where a response curve changes effect as observed from background is critical in establishing a safe exposure limit (threshold -exposure/concentration- dose). Some investigators have reported this threshold is around 25 fiber/ml-years (2); although for some members of an exposed group this may be lower (3). However, a cumulative no-effect value does not provide information applicable for practical every day use when monitoring worker exposure.
Recent studies (4,5,6) have suggested levels of exposure where there was no excess increase in cancer, notably lung cancer and mesothelioma. This is especially noted for non-smokers where risk does not exceed unity (lung cancer), even for high asbestos exposure, while there is a significant increased risk for smokers and former smokers (6,7). Thus, it appears smoking is the primary contributor and confounder of disease for historical and current asbestos workers (6). Compounding this issue is the high number of smokers performing asbestos abatement work (approximately 60% or greater depending on the population observed even at the present time) (8,9). The high rate of smoking makes it difficult to estimate a threshold and subsequently establish an exposure value from this type of information. Certainly, for today's asbestos workers smoking constitutes their greatest risk (6,9,10). Recent studies with direct (4,5) and indirect (e.g. Job Exposure Matrix and Job Specific Modules) (6,11) exposure information do allow an estimate of a threshold exposure dose. Although, exposure can greatly vary over time (4), it appears even with mixed fiber exposure (chrysotile and amphibole) (5) a threshold of 1.0 f/ml-year is applicable, but at the upper range. If chrysotile alone is used in this estimate, it appears a higher value would be warranted (5). Taken together, with a safety margin to include a lower reported value in causation of asbestosis (about 23 f//ml - years) (11), a threshold of 0.8 f/ml-year would appear applicable (20 f/ml - years). From this, using a work time period of 25 years, which is not likely for asbestos workers in the United States, due to physical demands of the occupation and actual length of the "industry", allows suggestion for a threshold of 0.8 f/ml-Time Weighted Average. When present exposure levels are included in this evaluation (12), it is unlikely workers in developed nations will exceed a cumulative exposure of 20 or 25 fiber/ml-years. Comparison of this proposed threshold with data reported by Clin et al., for lung cancer and mesothelioma, with 25 years or less of exposure (table 1 in Clin et al.), provides support for this threshold value. However, any value will be controversial, especially in establishing a threshold.
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