Update of Potency Factors for Asbestos-Related Lung Cancer and Mesothelioma

The most recent update of the U.S. Environmental Protection Agency (EPA) health assessment document for asbestos (61. Nicholson W. J. Airborne asbestos health assessment update. U.S. Environmental Protection Agency, Washington, DC 1986, Report 600/884003F

View all references, referred to as “the EPA 1986 update”) is now 20 years old. That document contains estimates of “potency factors” for asbestos in causing lung cancer (K_L's) and mesothelioma (K_M's) derived by fitting mathematical models to data from studies of occupational cohorts. The present paper provides a parallel analysis that incorporates data from studies published since the EPA 1986 update.

The EPA lung cancer model assumes that the relative risk varies linearly with cumulative exposure lagged 10 years. This implies that the relative risk remains constant after 10 years from last exposure. The EPA mesothelioma model predicts that the mortality rate from mesothelioma increases linearly with the intensity of exposure and, for a given intensity, increases indefinitely after exposure ceases, approximately as the square of time since first exposure lagged 10 years. These assumptions were evaluated using raw data from cohorts where exposures were principally to chrysotile (South Carolina textile workers, 33. Hein M. J., Stayner L. T., Leyman E., Dement J. M. Follow-up study of chrysotile textile workers: Cohort mortality and exposure-response. Occup. Environ. Med. 2007; 64: 616–625

View all references; mesothelioma only data from Quebec miners and millers, 46. Liddell F. D.K., McDonald A.D., McDonald J.C. The 1891–1920 birth cohort of Quebec chrysotile miners and millers: Development from 1904 and mortality to 1992. Ann. Occup. Hyg. 1997; 41: 13–36

View all references) and crocidolite (Wittenoom Gorge, Australia miners and millers, 9. Berry G., de Klerk N.H., Reid A., Ambrosini G. L., Fritichi L., Olsen N. J., Merler E, Musk A. W. Malignant pleural and peritoneal mesotheliomas in former miners and millers of crocidolite at Wittenoom, Western Australia. Occup. Environ. Med. 2004; 61: 1–3

View all references) and using published data from a cohort exposed to amosite (Paterson, NJ, insulation manufacturers, 70. Seidman H., Selikoff I. J., Gelb S. K. Mortality experience of amosite asbestos factory workers: Dose-response relationships 5 to 40 years after onset of short-term work exposure. Am. J. Ind. Med. 1986; 10(5/6)479–514

View all references).

Although the linear EPA model generally provided a good description of exposure response for lung cancer, in some cases it did so only by estimating a large background risk relative to the comparison population. Some of these relative risks seem too large to be due to differences in smoking rates and are probably due at least in part to errors in exposure estimates. There was some equivocal evidence that the relative risk decreased with increasing time since last exposure in the Wittenoom cohort, but none either in the South Carolina cohort up to 50 years from last exposure or in the New Jersey cohort up to 35 years from last exposure.

The mesothelioma model provided good descriptions of the observed patterns of mortality after exposure ends, with no evidence that risk increases with long times since last exposure at rates that vary from that predicted by the model (i.e., with the square of time). In particular, the model adequately described the mortality rate in Quebec chrysotile miners and millers up through >50 years from last exposure. There was statistically significant evidence in both the Wittenoom and Quebec cohorts that the exposure intensity-response is supralinear¹ rather than linear. The best-fitting models predicted that the mortality rate varies as [intensity]^0.47 for Wittenoom and as [intensity]^0.19 for Quebec and, in both cases, the exponent was significantly less than 1 (p< .0001).

Using the EPA models, K_L's and K_M's were estimated from the three sets of raw data and also from published data covering a broader range of environments than those originally addressed in the EPA 1986 update. Uncertainty in these estimates was quantified using “uncertainty bounds” that reflect both statistical and nonstatistical uncertainties. Lung cancer potency factors (K_L's) were developed from 20 studies from 18 locations, compared to 13 locations covered in the EPA 1986 update. Mesothelioma potency factors (K_M's) were developed for 12 locations compared to four locations in the EPA 1986 update. Although the 4 locations used to calculate K_M in the EPA 1986 update include one location with exposures to amosite and three with exposures to mixed fiber types, the 14 K_M's derived in the present analysis also include 6 locations in which exposures were predominantly to chrysotile and 1 where exposures were only to crocidolite.

The K_M's showed evidence of a trend, with lowest K_M's obtained from cohorts exposed predominantly to chrysotile and highest K_M's from cohorts exposed only to amphibole asbestos, with K_M's from cohorts exposed to mixed fiber types being intermediate between the K_M's obtained from chrysotile and amphibole environments. Despite the considerable uncertainty in the K_M estimates, the K_M from the Quebec mines and mills was clearly smaller than those from several cohorts exposed to amphibole asbestos or a mixture of amphibole asbestos and chrysotile.

For lung cancer, although there is some evidence of larger K_L's from amphibole asbestos exposure, there is a good deal of dispersion in the data, and one of the largest K_L's is from the South Carolina textile mill where exposures were almost exclusively to chrysotile. This K_L is clearly inconsistent with the K_L obtained from the cohort of Quebec chrysotile miners and millers.

The K_L's and K_M's derived herein are defined in terms of concentrations of airborne fibers measured by phase-contrast microscopy (PCM), which only counts all structures longer than 5 μm, thicker than about 0.25 μ m, and with an aspect ratio ≥3:1. Moreover, PCM does not distinguish between asbestos and nonasbestos particles. One possible reason for the discrepancies between the K_L's and K_M's from different studies is that the category of structures included in PCM counts does not correspond closely to biological activity. In the accompanying article (6. Berman D. W., Crump K. S. A meta-analysis of asbestos–related cancer risk that address fiber size and mineral type. Crit. Rev. Toxicol. 2008; 38(S1)49–73
[Taylor & Francis Online]
View all references) the K_L's and K_M's and related uncertainty bounds obtained in this article are paired with fiber size distributions from the literature obtained using transmission electron microscopy (TEM). The resulting database is used to define K_L's and K_M's that depend on both the size (e.g., length and width) and mineralogical type (e.g., chrysotile or crocidolite) of an asbestos structure. An analysis is conducted to determine how well different K_L and K_M definitions are able to reconcile the discrepancies observed herein among values obtained from different environments.

Key words 

• Amphibole,
• asbestos,
• chrysotile,
• lung cancer,
• mesothelioma,
• risk assessment

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