Background: Besides a clear relationship to silicosis, crystalline silica-quartz-has been associated with lung cancer, nonmalignant renal disease, and auto-immune disease. To study diseases associated with crystalline silica further, NIOSH conducted a cohort mortality study of workers from 18 silica sand plants, which had quarry, crushing, and bagging operations to produce industrial sand. Twelve of these plants also had grinding mills to produce fine silica powder. The historical crystalline silica exposures of workers at these plants were estimated to facilitate exposure-response analyses in the epidemiologic study.
Methods: NIOSH obtained personal respirable dust measurement records from Mine Safety and Health Administration (MSHA) compliance inspections at all 18 plants and from the archives of seven plants which had collected samples. These samples had been analyzed for quartz content by x-ray diffraction. Although no personal samples were available before 1974, impinger dust measurements were reported for 19 silica sand plants in 1946; these data were converted and used to estimate exposures prior to 1974. Statistical modeling of the samples was used to estimate quartz exposure concentrations for workers in plant-job-year categories from the 1930s when mortality follow-up of the cohort began until 1988 when follow-up stopped.
Results: Between 1974 and 1996, there were 4,269 respirable dust samples collected at these 18 plants. The geometric mean quartz concentration was 25.9 microg/m(3) (GSD = 10.9) with a range from less than 1 to 11,700 microg/m(3). Samples below 1 microg/m(3) were given a value of 0.5 microg/m(3). Over one-third of the samples -37%) exceeded the MSHA permissible exposure limit value for quartz (PEL = 10 mg/m(3)/(%quartz + 2)) and half (51%) of the samples exceeded the NIOSH recommended exposure limit (REL=50 microg/m(3)). The samples were collected from workers performing 143 jobs within the 18 plants, but too few samples were collected from many of the jobs to make accurate estimates. Therefore, samples were combined into 10 categories of jobs performing similar tasks or located within the same plant area.
Conclusions: The quartz concentrations varied significantly by plant, job, and year. Quartz concentrations decreased over time, with measurements collected in the 1970s significantly greater than those collected later. The modeled exposure estimates improve upon duration of employment as an estimate of cumulative exposure and reduce exposure misclassification due to variation in quartz levels between plants, jobs, and over time. Am. J. Ind. Med. 38:389-398, 2000. Published 2000 Wiley-Liss, Inc.