The South Karelia Air Pollution Study. The effects of malodorous sulfur compounds from pulp mills on respiratory and other symptoms. The paper mills in South Karelia, the southeast part of Finland, are responsible for releasing a substantial amount of malodorous sulfur compounds such as hydrogen sulfide (H2S), methyl mercaptan (CH3SH), and methyl sulfides [(CH3)2S and (CH3)2S2], into ambient air. In the most polluted residential area the annual mean concentrations of hydrogen sulfide and methyl mercaptan are estimated to be 8 and 2 to 5 micrograms/m3 and the highest daily average concentration 100 and 50 micrograms/m3. The annual mean and highest daily concentrations of sulfur dioxide (SO2) are very low. We studied the effects of malodorous sulfur compounds on eye, nasal and respiratory symptoms, and headache in adults. A cross-sectional self-administered questionnaire was distributed in February 1987 and responded to by 488 adults living in a severely (n = 198), a moderately (n = 204), and a nonpolluted community (n = 86). This included questions about occurrence of the symptoms of interest during the previous 4 wk and 12 months and individual, behavioral, and other environmental determinants of the symptoms. The response rate was 83%. The odds ratios (OR) for symptoms experienced often or constantly in severely versus nonpolluted and moderately versus nonpolluted communities were estimated in logistic regression analysis controlling potential confounders. The odds ratios for eye (moderate exposure OR 11.70, Cl95% 2.33 to 58.65; severe exposure OR 11.78, Cl95% 2.35 to 59.09) and nasal symptoms (OR 2.01, Cl95% 0.97 to 4.15; OR 2.19, Cl95% 1.06 to 4.55) and cough (OR 1.89, Cl95% 0.61 to 5.86; OR 3.06, Cl95% 1.02 to 9.29) during the previous 12 months were increased, with a dose-response pattern. 