A Case-Control Study of Lung Cancer in Relation to Silica Exposure and Silicosis in a Rural Area in Japan

Abstract

PURPOSE: In southeast Okayama Prefecture, Japan, there have been reports of a high prevalence of silicosis among refractory brick production workers. Recently, a high mortality rate of lung cancer among the local residents has been observed. Therefore, a population based case-control study was conducted concerning the relationship between silica, silicosis, and lung cancer using multiple cancer controls. METHODS: Cases and controls were restricted to male subjects and information was obtained from death certificates from 1986 to 1993 in the area. Three categories of deceased control groups were selected: a series of deaths from liver cancer, colon cancer, and cancers of other organs, which was assumed not to be related to silica exposure. Age and smoking habits were adjusted by stratified analysis using the Mantel-Haenszel odds ratio estimates. Unconditional logistic regression analysis was also conducted to control potential confounding factors; such as age and smoking habits. RESULTS: The age-, smoking-adjusted odds ratios were 1.94 (0.94–4.43) for the colon cancer control group, 2.13 (1.19–3.85) for the other cancer control group related to silica exposure, and 2.94 (1.30–8.90) and 2.69 (1.43–5.37) related to silicosis, respectively. The direct weighted average using the estimates for colon and the other cancer controls was 2.06 (1.29–3.29) for silica exposure, and 2.77 (1.60–4.77) for silicosis. Histological or cytological types of lung cancer cases were obtained from 64.1% of the subjects (118/184). As for the histologic type of lung cancer, small cell carcinoma was higher among those who had been silica-exposed workers than the unexposed lung cancer cases and the data from the general Japanese population. On chest x-ray findings, elevated lung cancer mortality compared with cancers other than lung cancer was demonstrated among patients without large opacities. CONCLUSIONS: Silica exposure increased the lung cancer mortality in the area. A high lung cancer mortality rate in the area could be explained by silica exposure and silicosis prevalence in this area.