SO2-particulate interactions: recent observations.

The toxicity of sulfur oxides is discussed briefly. The effects of sulfur dioxide (SO2) are largely confined to the upper airways except during exercise or if the gas is taken up by a carrier aerosol. SO2 may be adsorbed as a monomolecular layer on dry particles, such as elemental carbon, or dissolved in aqueous droplets. Hydrated SO2 forms bisulfite and sulfite ions, which are rapidly oxidized (detoxified) by sulfite oxidase, an enzyme, to form sulfate. SO2 in carrier aerosols (dry or aqueous solutions) may be oxidized to sulfuric acid. The mixture of SO2 (1 ppm) and a droplet of sodium chloride (1 mg/m3) has been shown to be synergistic in guinea pigs. In healthy adults, the same gas-aerosol mixture caused no functional lung changes at rest (two separate studies), but did cause significant changes following moderate exercise. Shortness of breath and wheezing were experiences by about half of the subjects.