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Chrysotile and lung cancer: time-related effects and pooled analysis.

Loomis-D; Elliot-L; Dement-J; Richardson-D; Lu-M
Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-007803, 2011 Apr; :1-13
Background: Although all forms of asbestos are currently classified as human carcinogens, there is ongoing controversy about the risks associated with the chrysotile form. The purpose of this project was to strengthen quantitative risk assessments for chrysotile exposure. The project aimed to: conduct pooled analyses of the association of lung cancer with exposure to chrysotile asbestos in previously-enumerated cohorts of asbestos textile workers in North Carolina and South Carolina; evaluate exposure-time-response relationships between chrysotile asbestos and lung cancer mortality with several alternative models, and conduct analyses applying biologically-based models of carcinogenesis. Methods: Men and women who worked .30 days in production departments in any of 4 asbestos textile plants in North Carolina or South Carolina and were employed between 1-01-1950 and 31-12-1973 in NC or 1 -01-1940 and 31-12-1965 in SC were eligible for inclusion. This cohort was followed for mortality through the end of 2003. Cumulative exposure to asbestos fibers was estimated from over 9000 historical dust samples using data from our previous research. Fiber numbers and dimensions were determined by transmission electron microscope (TEM) analysis of 160 filters collected from the 1960s to the 1980s. Department-specific exposures were estimated using regression models and adjustment factors. The mortality of the study population relative to national and state populations was estimated using the NIOSH modified lifetable program to estimate standardized mortality ratios (SMRs). Associations of lung cancer mortality with cumulative fiber exposures were evaluating using Poisson regression to estimate mortality rate ratios (RRs) adjusted for age, sex, race and calendar year. Results: The study included 6,136 workers, contributing 218,631 person-years of observation and 3,356 deaths. Mortality from all causes and all cancers was significantly higher than expected. The SMR for lung cancer (SMR = 1.93, 95% CI 1.73-2.15). The relative rate for lung cancer was 1.11 (95% CI 1.06-1.16) at 100 fiber-years/ml compared with 0 fiber-years/ml. Stratification showed different effects in SC (RR = 1.65, 95% CI 1.42-1.92) than in NC (RR = 1.12, 95% CI 1.06-1.19). Exposure to fibers throughout the range of length and diameter was significantly associated with increased risk of lung cancer. The best fitting models were those for fibers >5 um long and <0.25 um in diameter. The greatest magnitude of association with lung cancer was seen for fibers 5-10 um long and <0.25 um in diameter (RR approx. 1.04 per intra-quartile range, p < 0.001). When indicators of mean fiber length and diameter were modeled simultaneously, length was positively associated with lung cancer while diameter was negatively associated. Conclusions: The findings of this study add to the evidence that the chrysotile form of asbestos is carcinogenic to humans. Our findings also support the hypothesis that long, thin fibers are more carcinogenic than shorter, wider fibers, although exposures to fibers of all sizes were associated with lung cancer. Heterogeneity in risk observed in previous, separate studies of the NC and SC cohorts does not appear to be a result of the use of different analytical methods or inclusion criteria.
Asbestos-products; Humans; Men; Women; Carcinogens; Exposure-levels; Risk-factors; Lung-cancer; Lung; Pulmonary-disorders; Pulmonary-system; Pulmonary-system-disorders; Respiratory-system-disorders; Textile-workers; Mortality-rates; Morbidity-rates; Fibrous-dusts; Fibrous-bodies
Dana Loomis, University of Nevada, Reno, 16444 N. Virginia St. Reno NV 89559
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Final Grant Report
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National Institute for Occupational Safety and Health
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University of Nevada, Reno