PURPOSE: The aims of this study were to investigate the relations of occupational exposures in tile industry to lung function and to evaluate potential interaction between smoking and tile dust exposure containing silica. METHODS: A cross-sectional study of 232 workers (response rate 100%) in a tile factory and 76 office workers (response rate 73%) from four factories in Thailand was conducted in 2006-2007. Participants answered a questionnaire and performed spirometry. RESULTS: Factory workers had lower spirometric functions than office workers, especially those with high dust exposure. There was a dose-response relation between duration of dust exposure and FEV1 and FVC, the adjusted effect of =21 years of exposure on FEV1 being -240 ml (-100 to -380) and on FVC -300 ml (-140 to -460). The adverse effect of dust on lung function was larger in current smokers suggesting synergism between smoking and tile dust exposure. CONCLUSIONS: This study provides evidence that long-term exposure to dust in tile industry is related to lung function reduction. There was a suggestion of synergistic effect between dust exposure and smoking. Tile factories should consider measures to reduce dust exposure and arrange spirometry surveillance for workers with such exposure. Smoking cessation should be promoted to prevent harmful effects of occupational tile dust exposure.