The pulmonary effects of chronic exposure to sulfur-dioxide (7446095) were studied in 948 male smelter workers, mean age 41.3 years, employed at a copper smelter at Garfield, Utah. The comparisons were 262 males, mean age 37.0 years, employed at the shop for an open pit mine 15 miles from the smelter. The mean number of cigarettes smoked daily by the smelter workers and comparisons was 9.9 and 8.0, respectively. The subjects completed a respiratory symptom questionnaire and spirometric testing was performed. Industrial hygiene monitoring indicated that the smelter workers were exposed to time weighted average sulfur-dioxide concentrations of 0.4 to 3 parts per million (ppm). Exposure to particulate sulfites at the smelter was also significantly elevated. The smelter workers reported higher prevalences of cough, phlegm production, wheeze, dyspnea, and chest tightness than the comparisons. The increased prevalences of cough, phlegm production, and dyspnea were statistically significant. Both smoking and nonsmoking smelter workers took more time off from work because of illness than the comparisons. Heavy cigarette smokers tended to report more days of illness and hospitalization than nonsmokers. One second forced expiratory volumes (FEV1) and forced vital capacity (FVC) were significantly lower in the smelter workers than in the comparisons. The deficits in FEV1 and FVC generally increased with increasing amounts of smoking and time spent in the smelter but not in the mine shop. Among smoking smelter workers, the FEV1 and FVC deficits were greater than in smoking comparisons. When the FEV1 and FVC deficits associated with sulfur-dioxide or smoking alone were compared with those associated with both sulfur- dioxide and smoking, the effects of cigarette smoke and sulfur- dioxide exposure appeared to be additive. The authors conclude that chronic exposure to 0.4 to 3ppm sulfur-dioxide is associated with significant deficits in FVC and FEV1. A corresponding increase in some symptoms of respiratory disease, especially chronic bronchitis, was also observed. The present occupational standard for sulfur- dioxide 2ppm, which is primarily based on acute effects, should be reconsidered.