AbstractSubmicrometer H2SO4droplets can affect pulmonary functions in humans. The lowest concentration which has been reported to produce a statistically significant effect on pulmonary mechanics is l mg/m3. Effects on tracheobronchial mucociliary clearance of inert particles were observed in our laboratory at much lower concentrations. For ten nonsmoking humans exposed for one hour via nasal mask to 0.1 mg/m3of H2SO4, the mean bronchial clearance half‐time was reduced by 38% (p<0.02) while for l mg/m3, it was increased by 48% (p<0.03). Four donkeys exposed for l hour daily to 0.1 mg/m3six months developed clearance abnormalities which persisted for at least three months after the last exposure. The patterns of mucociliary clearance responses to H2SO4are similar to those seen previously in humans and donkeys following exposure to cigarette smoke, a known causal factor for chronic bronchiti