Respiratory effects of exposure to ozone plus sulfur dioxide in Southern Californians and Eastern Canadians.

Volunteer subjects were exposed in an environmentally controlled chamber in Los Angeles to SO2 and O3, separately and in combination, to study the enhanced toxicity of the O3 + SO2 mixture observed in a previous study in Montreal. Toxic responses to O3 + SO2 were less severe than observed previously, and only slightly more severe than with O3 alone. Sulfur-containing aerosols formed by chemical reactions within the chamber air may have contributed to the O3 + SO2 responses in the Montreal study. Adaptation of Los Angeles residents to ambient O3 may also contribute to response variability.