Cohort mortality study of North American industrial sand workers. III. Estimation of past and present exposures to respirable crystalline silica

Background: Lung cancer and silicosis mortality were examined longitudinally and by a case-referent analysis in a cohort of workers selected from the North American industrial sand industry. Date of hire in the case-referent sub-cohort extended as far back as the second decade of the twentieth century. Objective: The aim of this study component was to develop estimates of average and cumulative exposure to respirable crystalline silica for the 342 selected cases and referents. Methods: Process and dust control histories were developed for each plant, and quantitative exposure data obtained from each of them and from a trade organization. An algorithm was developed to convert historical exposures reported in particle count concentrations to modern measures of mass concentration of respirable crystalline silica. Personal exposures were adjusted for use of protective equipment based on frequency of use and type of protection. Findings: Between 1974 and 1998, a total of 14 249 exposure measurements had been taken using a cyclone and membrane filter and gave an overall geometric mean of 42 mug/m(3). The only exposure data identified earlier were based on approximately 500 samples collected across the industry between 1947 and 1955 using the Greenburg-Smith impinger, with analysis by microscopy. These data were converted to modern measures using a factor of 1 mppcf = 276 mug/m(3) respirable dust and then adjusting for percentage silica. In general, the highest exposures occurred in bagging and bulk-loading operations and the lowest in wet processing of sand. Conclusions: There has been a substantial decline in exposure levels in this industry over time. The decline was rapid between the 1940s and 1970s and current exposures are, on average, less than 50 mug/m(3). The use of personal protective equipment was judged to have had little impact on exposure before the 1970s. (C) 2001 British Occupational Hygiene society. Published by Elsevier Science Ltd. All rights reserved.