Prior to 1980, essentially no health related effects had been observed for short-term ( < 1 hr) exposures to sulfur dioxide (SO2) levels similar to those found in the ambient environment (= or < 1 ppm). In 1980 and 81, the results from several studies indicated that asthmatics' airways were substantially more responsive to SO2 than those of nonasthmatic individuals and that, when combined with moderate exercise, significant bronchoconstriction resulted from exposures to as low as 0.5 ppm SO2. Since then, a multitude of reports regarding short-term exposure of asthmatics to low SO2 levels have appeared in the literature. From these studies, reflex bronchoconstriction, mast cell degranulation and other, yet unidentified, mechanisms have been implicated in the induction of response. A broad range of responsiveness to SO2 exists within the asthmatic population. Precise knowledge of factors involved in this variable responsiveness is lacking; differences in nonspecific airway sensitivity and severity of disease have been suggested.