SILICA, SILICOSIS, AND LUNG CANCER IN DIATOMITE WORKERS

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Millions of workers world-wide are or have been exposed to crystalline silica. Recently, the International Agency for Research on Cancer classified crystalline silica as a probable human lung carcinogen. However, nearly all of the epidemiologic studies on this topic have suffered from a lack of detailed exposure data needed for dose-response estimation, and few have had data on chest radiograph readings which would permit clarification of a potential precursor role of fibrosis on lung carcinogenesis. The proposed study will add 6 years of follow-up to a previously conducted historical cohort mortality study of 2,961 diatomaceous earth (diatomite) workers exposed to crystalline silica. The original study indicated a lung cancer mortality excess overall in the cohort during 192-87 (SMR=1.43), and strong dose-response relations with crystalline silica exposure (Relative Risk=2.74 in the highest exposure category). The research objectives of the proposed study are: 1)to quantify the dose-response association between crystalline silica and lung cancer; 2) by reading chest radiographs, to test the hypothesis that pulmonary fibrosis is a necessary precursor of silica-induced lung cancer; and 3) to evaluate dose-response relations for mortality risks for non-malignant respiratory diseases and other causes of death for which overall excesses are observed. Exposure and work history data, and information on smoking, will be obtained from available industry records. Mortality follow-up will be conducted using the National Death Index. Quantitative estimates of crystalline silica does information to work history records. Chest radiographs, which have been maintained for the majority of workers since the 1930s, will be read and classified by standard International Labour Office criteria for the pneumoconioses. This will permit stratification of workers into categories of "absent" "possible," and "present" silicosis. Comparisons of lung cancer risks among these groups will serve as a test of the fibrosis/carcinogenesis hypothesis. This investigation will generate significant new information needed for quantification of the dose-response relation between silica and lung cancer, which is essential for scientifically-based risk assessments and environmental standard setting. Moreover, the study findings will add further to an understanding of the pathogenetic role of pulmonary fibrosis in carcinogenesis.