AbstractEpidemiological and animal toxicological studies have indicated that reactions between SO2(g) and metal containing aerosols result in the formation of respiratory irritants. It had initially been suggested by EPA that sulfate per se was responsible for the observed health effects. That now appears unlikely. These studies point out the importance of understanding in detail the chemical species formed by such interactions. In the present paper procedures which have been used to study the formation of aerosol inorganic S(IV) species are described, together with the results obtained from studies in the flue line, workroom, and plume of smelters.Both atmospheric and laboratory studies indicate that very stable complexes of S(IV) with Fe(III) or Cu(?) can form in aerosols. The data suggest that the concentration of these S(IV) complexes in primary particulate emissions from smelters will be about 10% of the sulfate concentration. In plumes the concentration of inorganic S(IV) varied from 10 to 80% of the sulfate concentration. The most important variable controlling the formation of these S(IV) complexes in a plume aerosol droplet is the aerosol acidity. The formation of aerosol inorganic S(IV) complexes in the plume is not related to the formation of sulfate.There have been no studies previously reported that would allow an unequivocal evaluation of the toxicological implications of the chemistry reviewed in this paper. If these various S(IV) species are responsible at the low concentrations reported here for the “synergistic” effects previously reported between SO2(g) and aerosols, then additional toxicological work would appear warran