This study challenges the assertion of low relative risk of
chrysotile in the causation of
mesothelioma. Data are provided on the time period use of various types of
asbestos in the United States and in insulation materials. The focus of the study is on
mesothelioma among railroad machinists employed in the
steam locomotive era who were exposed to
chrysotile. Within a cohort of machinists alive January 1, 1945, a sub-cohort method was applied to all successive machinists hired in each of the respective years (1920-1929) followed through 1986. The total cohort was 181 and the number of deaths 156, with 14
mesotheliomas identified among 41
cancer deaths. The findings demonstrated an extremely high relative risk for machinists exposed to
chrysotile for the induction of
mesothelioma in the individual year of hire cohorts. A similar observation was noted for other crafts hired in the same time period. One
mesothelioma occurred for every 13 machinists hired in the succeeding years (1920-1929) and constituted 34% of all
cancer deaths. It is concluded that
chrysotile is far more hazardous in the induction of
mesotheliomas and that the
asbestos cancer risk in the
steam locomotive eras was much higher than had been previously estimated.