Human Exposure and Risk Assessment for Naturally Occurring Asbestos (Part 2)
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There is not clear evidence of lower chrysotile toxicity in relationship to lung cancer and asbestosis, a debilitating scarring of the lung tissues. Evidence, such as that reviewed by Hardy in 1995, suggests that iron also plays a part in asbestos toxicity. Iron is present in all asbestos minerals. Iron ions on the surface of asbestos fibers may be catalytic sites for free radical and reactive oxygen species (ROS) generation, resulting in the initiation or promotion of cancer.
EPA’s carcinogenicity assessment dates back to 1986. The lung cancer model in that assessment (Nicholson, 1986) assumes a linear function of cumulative asbestos exposure in units of fibers-years/ml as measured with phase contrast microscopy, and can be expressed as follows: IL IE(1+KL*f*d), where: IL lung cancer incidence observed or projected in an exposed population IE lung cancer expected in the absence of exposure KL= proportionality constant measure of the carcinogenic potency of exposure f intensity of exposure (fibers/ml) d duration of exposure (years) The model assumes equal potency for all six regulated asbestos types and all asbestos fibers greater than 5 μm in length. The 1986 assessment document does point out that fiber size distribution varies with asbestos type and mineral processing, and accepts that length and width are important variables in fiber carcinogenicity in animal studies. Stanton et al (1981) developed the “Stanton Hypothesis,” which suggested that long thin fibers were the most toxic. Later studies, such as those reviewed by Dodson et al, suggested that all fiber sizes may contribute, to some extent, to asbestos toxicity. One source of uncertainty in asbestos exposure estimates is the uncertainty of conversions between analytical measurements performed with PCM and measurements performed with transmission electron microscopy (TEM).
Asbestos unit risk is based on fiber counts made with PCM because PCM is typically the method used for measurements in the occupational environment. Unfortunately, PCM is not fiber specific. All fibers are counted, regardless of identity. PCM also does not have the resolution necessary to image smaller fibers, generally resolving fibers longer than 5 μm and greater than 0. 4 μm in diameter. Transmission electron microscopy (TEM) resolves much shorter and thinner fibers and allows for identification of fibers based on chemical composition and selected area electron diffraction (SAED) of the mineral’s crystal structure. The correlation between PCM and TEM is highly uncertain. Asbestos measurement techniques and the level of understanding of asbestos toxicity have improved substantially since EPA’s 1986 assessment document.
A proposed updated methodology for conducting asbestos risk assessments (Berman and Krump, 2003) is under review at this time. The proposed methodology, which distinguishes between asbestos types and fiber sizes in assessing risk, is a topic of debate. The report on EPA’s peer consultation workshop to discuss the proposed methodology (Eastern Research Group, 2003) documents several discussion topics. Issues under discussion include fiber diameter and length (what size cut-off points to use in considering fibers), the use of different carcinogenic potency factors for different asbestos fiber types for lung cancer versus mesothelioma, how to address mineral cleavage fragments of equal dimension and biopersistence as fibers, the potency of unregulated asbestos minerals, statistical analysis methods, consideration of the synergistic impact of cigarette smoking, and localized exposures to naturally occurring asbestos such as that in California. The potential for health risks associated with exposure to asbestos minerals continues to be a public concern. Much of the epidemiological asbestos data studied over the past several decades has focused on occupational exposure. Since asbestos is a generic term used to identify a group of naturally-occurring minerals, however, there are areas of the United States in which geological deposits of asbestos minerals pose a potential environmental exposure risk. Unlike occupational asbestos exposures, which can be controlled with personal protective equipment and specialized work practices, exposure to naturally occurring asbestos may not be easily controlled and may impact susceptible subpopulations. Given the asbestos toxicity questions that remain and the vigorous research debate, it is obvious that asbestos is still a relevant exposure and risk assessment topic.
References
Berman, D. W. and Krump, K. (2003). “Technical Support Document for a Protocol to Assess Asbestos-Related Risk – Final Draft. ” Report No. EPA 935. 4-06600/8-84/003F, Prepared for U. S. EPA Office of Solid Waste and Emergency Response, Washington, DC.
Bernarde, M. (1990). Asbestos The Hazardous Fiber. CRC Press: Florida.
Dodson, R. Atkinson, M. and Levinson, J. (2003). “Asbestos Fiber Length as Related to Potential Pathogenicity: A Critical Review,” American Journal of Industrial Medicine, 44: 291-297.
Eastern Research Group, Inc. (2003). “Report on the Peer Consultation Workshop to Discuss a Proposed Protocol to Assess Asbestos-Related Risk. ” Contract No. 68-C-98-148, Prepared by Eastern Research Group, Inc. for U. S. EPA Office of Solid Waste and Emergency Response, Washington, DC.
Fubini, B. and Fenoglio, I. (2007). “Toxic Potential of Mineral Dusts,” Elements, 3: 407-414. Hardy, J. and
Aust, A. (1995). “Iron in Asbestos Chemistry and Carcinogenicity,” Chemical Reviews, 95(1): 97-118.
Ladd, K. (2005). “El Dorado Hills Naturally Occurring Asbestos Multimedia Exposure Assessment, Preliminary Assessment and Site Inspection Report Interim Final. ” Contract No. 68-W-01-012, Prepared by Ecology and Environment, Inc. Superfund Technical Assessment and Response Team (START) for U. S. EPA Region IX. Nicholson, W. J. (1986). “Airborne Asbestos Health Assessment Update. ” Report No. EPA/600/8-84/003F, Prepared for U. S. EPA Environmental Criteria and Assessment Office, Research Triangle Park, NC.
Stanton M. F. Layard M. Tegeris E. Miller E. May M. Morgan E. and Smith A. (1981). “Relation of Particle Dimension to Carcinogenicity in Amphibole Asbestoses and Other Fibrous Minerals. ” Journal of the National Cancer Institute, 67: 965-975.
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