“Benchmarks” are quantitative measures that aid in the determination of whether a particular asbestos exposure could cause disease. The epidemiologic literature was reviewed and analyzed to develop two types of benchmarks:
- The lowest average cumulative concentration reported in the peer-reviewed literature for cohorts occupationally exposed to asbestos
- Cumulative concentrations that correspond to a relative risk of disease of 2 (the doubling dose), which has been used to gauge whether the exposure in question was more likely than not to have caused the disease when compared to background.
In an important paper investigating dose-response relationships for asbestos, Hodgson and Darnton (2000) presented a list of average cumulative exposures experienced by occupationally exposed cohorts. The lowest average cumulative exposure of all cohorts was 13 f/ml-yr. This value was rounded up to establish a benchmark of 15 f/ml-yr, a value denoting the approximate lowest average exposure at which asbestos-related malignancies (both mesothelioma and lung cancer) have been observed in occupationally exposed cohorts.
The second type of benchmark represents the exposure at which the risk of asbestos-related disease has been shown to double the background risk of disease based on dose-response models derived from occupationally exposed cohort studies. A relative risk of 2 (RR = 2) determines that exposure above which the probability that the disease occurred because of exposure to asbestos exceeds the risk that would have occurred in its absence (i.e., the etiologic fraction is >50%).
For lung cancer, calculating the dose at which the risk doubles is relatively simple given that it is represented by a linear relative risk model [i.e., K
L = (RR - 1)/Cumulative Exposure; where K
L is the potency of the fiber] that has been developed and used by EPA (1986), Berman and Crump (2003), and Hodgson and Darnton (2000). The potency values (K
L; and for mesothelioma K
M) differ among the three models because of differences in the cohort studies relied upon and how fiber dimension is measured. Using potency values from these documents, the following doses at which the risk of lung cancer doubles were calculated:
|
KL
|
Cumulative dose at which risk doubles*
|
Comments
|
|
0.01
|
100
|
KL value adopted by EPA (1986) which did not distinguish fiber types; applicable to mixed fibers
|
|
0.0036
|
278
|
KL in McDonald et al. (2004); applicable to Libby miners
|
The risk of mesothelioma due to exposure can be estimated using an absolute risk model first developed by Peto (1982). EPA (1986) and Berman and Crump (2003) adapt this model which assumes incidence of mesothelioma is linear with exposure intensity (f/ml) but non-linear with respect to time (years). Hodgson and Darnton (2000) derive their own model which also results in a non-linear relationship between cumulative exposure and risk of mesothelioma. Using the results of these models and taking 3.6x10
-4 as the background lifetime risk of developing mesothelioma as estimated by Price and Ware (2004), the following doses at which the risk of mesothelioma doubles were computed:
|
KM
|
Cumulative dose at which risk doubles*
|
Comments
|
|
10-8
|
3.2
|
KM value adopted by EPA (1986) which did not distinguish fiber types; applicable to mixed fibers
|
|
0.36 x 10-8
|
8.9
|
KM derived from potency estimate for lung cancer, KL, in McDonald et al. (2004) under the assumption that KL/KM = 106 (EPA 1986); applicable to Libby miners
0.04 x 10-8
|
|
0.04 x 10-8
|
79
|
KM value from Berman and Crump (2003) for chrysotile fibers; only fibers greater than 10 microns in length and narrower than 0.4 microns in diameter are counted.
|
*Based on risk of developing mesothelioma by age 75, assuming exposure begins at age 20 and lasts for 45 years.
References:
- Berman D.W. and Crump K.S., 2003. Final Draft: Technical Support Document for a Protocol to Assess Asbestos-Related Risk. Prepared for: Office of Solid Waste and Emergency Response. U.S. E.P.A., Washington, D.C. (EPA #9345.4-06), October 2003.
- EPA. 1986. Airborne asbestos health assessment update. EPA/600/8-84/003F. US Environmental Protection Agency (USEPA), Office of Health and Environmental Assessment. June 1986.
- Hodgson, J.T. and A. Darnton. 2000. The quantitative risks of mesothelioma and lung cancer in relation to asbestos exposure. Ann. Occup. Hyg. 44(8):545–601
- McDonald, JC, J Harris, and B Armstrong. 2004. Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana. Occup Environ Med 61:363-366.
- Peto, J, H Seidman, and IJ Selikoff. 1982. Mesothelioma Mortality In Asbestos Workers: Implications For Models Of Carcinogenesis And Risk Assessment. Br J Cancer 45(1):124-135.
- Price, B and A Ware. 2004. Mesothelioma trends in the United States: An update based on surveillance, epidemiology, and End Results Program data for 1973 through 2003. Am J Epidemiol 159(2):107-112. January 15, 2004