Inhaled acids are associated with adverse health effects, a conclusion based largely on studies with particulate-associated acid sulfates. The acidic component of ambient air in some regions, such as California, contains nitric acid (HNO3) vapor, but there is a limited database concerning its biological effects. Furthermore, effects of HNO3 may be modulated by coexposure to other pollutants, such as ozone (O3). Rabbits were exposed for 4 h/day, 3 dayslwk for 4 wk to HNO3 vapor at 0, 50, 150, and 450 μg/m3 alone; 0.15 ppm O3 alone; or to a mixture of 50 μg/m3 HNO3 + 0.15 ppm O3. Exposure was followed by assays of biochemical markers in lavage fluid, pulmonary macrophage function, and in vitro bronchial responsivity to smooth muscle constrictor challenge. Nitric acid had no effect on viability or numbers of cells recovered, nor on lactate dehydrogenase or total protein in lavage. All acid concentrations reduced both basal levels and stimulated production of superoxide anion by macrophages, while the...