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 longitudinall y 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 centu ry. Objective: The aim of this study component was to develop estimates of aver age and cumulative exposure to respirable crystalline silica for the 342 se lected 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 re ported in particle count concentrations to modern measures of mass concentr ation of respirable crystalline silica. Personal exposures were adjusted fo r use of protective equipment based on frequency of use and type of protect ion. Findings: Between 1974 and 1998, a total of 14 249 exposure measurements ha d been taken using a cyclone and membrane filter and gave an overall geomet ric mean of 42 mug/m(3). The only exposure data identified earlier were bas ed on approximately 500 samples collected across the industry between 1947 and 1955 using the Greenburg-Smith impinger, with analysis by microscopy. T hese 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 gene ral, 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 thi s 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 perso nal protective equipment was judged to have had little impact on exposure b efore the 1970s. (C) 2001 British Occupational Hygiene society. Published b y Elsevier Science Ltd. All rights reserved.