Synergistic interaction of ozone and respirable aerosols on rat lungs. I. Importance of aerosol acidity.

A synergistic interaction, as defined by biochemical and morphological criteria, between ozone (or NO2) and respirable aerosols of ammonium sulfate or sulfuric acid has been described previously. Experiments in the present paper show that it is the acidity, not the sulfate content, of the aerosol that is responsible for such synergy; neutral aerosols of Na2SO4 or NaCl do not elicit synergistic effects when combined with ozone. Aerosol size (and, therefore, site of deposition in the lung) is also an important determinant of synergy with ozone; 0.5 micron mass median aerodynamic diameter (MMAD) aerosols are effective whereas 0.02 micron MMAD aerosols are not. The synergistic interaction between ozone and acidic aerosols could be demonstrated by biochemical and toxicological criteria in addition to those we have previously reported, for example increases in whole lung protein content and free (acid-soluble) proline content of lungs. A synergistic interaction has been demonstrated at concentrations of 0.64 ppm (1.3 mg/m3) of ozone and 1 mg/m3 of acid aerosol in this study. We conclude that acidity of an aerosol determines whether or not it interacts synergistically with ozone, and that an aerosol size that impacts maximally upon the alveolar duct region of the lung is most active with ozone.