A new model for health effects of air pollutants: evidence for synergistic effects of mixtures of ozone and sulfuric acid aerosols on rat lungs.

Chronic respiratory disease-free rats were exposed to relatively low levels of ozone (0.4 to 0.5 ppm) and H2SO4 aerosols (11 to 3,000 microgram/m3) and to mixtures of these pollutants. We independently evaluated exposure effects on conducting airway metabolism as rate of secretion of mucus glycoproteins by tracheal explants and on lung homogenates (predominantly lung parenchyma) by various chemical assays. True synergism was observed in that the response to the mixture of gases exceeded the sum of effects observed with the same concentration of either gas alone. Wet to dry weight ratios; DNA, protein, and RNA content; and the activities various lysosomal hydrolases in the lung homogenate all increased upon exposure to O3-H2-SO4 mixtures concomitant with observed increases in the rate of secretion of mucus glycoproteins by tracheal explants from the same rats. Preliminary experiments suggest that all of these effects are reversible when rats are allowed to recover after exposure. We conclude that the lack of toxicity noted upon exposure of experimental animals to individual pollutant gases at near-ambient levels should be interpreted with caution, since there is now epidemiological, physiological, and biochemical evidence that mixtures of pollutant gases may show synergistic effects.