Executive Summary
The hazard, exposure, and risks associated with asbestos fibers have been explored and debated for many years. Human evidence suggests an association between exposure to asbestos and asbestosis, lung cancer, and mesothelioma, although the lack of consistent information on fiber type, size, and exposure concentrations and duration limit our ability to establish causal relationships between exposure and disease in some cases. While uncertainties remain in our ability to consistently and accurately quantify asbestos risk to humans, progress has been made in characterizing those key factors, namely hazard and exposure, that are critical to an assessment of health risk.
Because asbestos is a natural material, there will always be some background or ambient exposure to humans. Although mining and commercial applications have diminished in some parts of the world, asbestos continues to have commercial applications, and hence, there remains exposure potential from these sources. Chrysotile and amphibole asbestos are the types most commonly used and hence studied experimentally, and it has become increasingly clear that they differ with respect to toxicity and disease potential. This has been demonstrated in animal models, which appear to be reflective of the human situation as well.
Progress on a number of fronts has led to general scientific consensus on the following: (1) amphibole fibers (which tend to be relatively long and thin) are a more potent risk factor for the development of mesothelioma and, to a lesser degree, lung cancer than are chrysotile fibers (which tend to be relatively short and wide); (2) longer, thin fibers are more pathogenic and there appear to be fiber size thresholds below which asbestos fibers do not pose any threat; and (3) those animal studies in which high exposure concentrations resulted in lung overloading are not considered relevant to humans.
Analysis of the epidemiological literature supports some common patterns including:
(1) for occupational and industrial exposures, the weight of evidence does not consistently support causal relationships between asbestos exposure and onset of pulmonary disease, some studies showing associative relationships but others showing no relationship between exposure and disease onset; and (2) chrysotile alone, uncontaminated by other fiber types, particularly amphiboles, does not appear to be a risk factor for mesothelioma, as once thought.
Advances in risk assessment methodology and analytical techniques, together with reevaluation of historical data, reveal that the current Environmental Protection Agency (EPA) approach for risk assessment of asbestosis is not in step with current scientific consensus, particularly for chrysotile fibers. In recent years, new knowledge about how asbestos risk can be more accurately and quantitatively determined has been generated. There is thus a scientific basis for adoption of these methods by regulatory agencies, including the EPA. While occupational exposures to asbestos remain and should be vigilantly monitored, there appears to be no compelling scientific evidence that ambient exposure to chrysotile asbestos poses a significant health risk.
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