The research presented in the toxicology session of the Symposium on the Health Effects of Acid Aerosols significantly advances our understanding of the health effects of acid aerosols and clearly illustrates the importance of animal inhalation toxicology to risk assessment. The description of the effects of acid on airway mucus buffering capacity and viscosity helps explain some of the mechanisms responsible for the effects of sulfuric acid on mucociliary clearance and pulmonary function observed in man and animals. Several of the papers illustrate that other pollutants interact with sulfuric acid (H2SO4), causing concern about exposure risks and helping in elucidating the effects observed in epidemiology studies that have not yet been duplicated in a laboratory. For example, H2SO4 absorbed on zinc oxide (ZnO) particles appears to be about a log more potent than H2SO4 alone in causing pulmonary function decrements. Low levels of H2SO4 and O3 were found to be synergistic in increasing collagen synthesis, implying a risk in development of lung fibrosis. More complex mixtures containing H2SO4 cause a variety of interactions, depending upon the end points examined and the chemistry of the mixture. Other reports indicate that dose rate and length of exposure issues are critical to toxicological outcomes.