Effects of repeated exposure to nitric acid vapor and ozone on respiratory tract clearance in the rat

This study assessed the effects of repeated exposure to environmentally realistic levels of two urban air pollutants nitric acid vapor (HNO3) and ozone (O3) - on tracer particle clearance from the rat respiratory tract. The measurement of particle clearance efficiency is important because failure of this function could contribute to a buildup of foreign matter in the respiratory tract. Separate groups of rats (30 rats per group) were exposed nose-only for 4 h/day, 3 days/wk (a surrogate for an air pollution episode) for 40 wk to 4 atmospheres: purified air; 0.15 ppm O3; 50 μg/m3 HNO3; and 0.15 ppm O3 + 50 μg/m3 HNO3. At 4 wk prior to the end of the exposure, the rats inhaled radiolabeled tracer particles nose-only for 30 min. A clearance measurement protocol was then followed to estimate the rates of early and late clearance. Early (presumably upper respiratory tract) clearance was monitored during the 48 h following tracer deposition by the analysis of radioactivity excreted in the feces, while late (presumably deep lung) clearance was characterized by a combination of chest counting and 30 day postexposure sacrifice radioactivity analyses. The exposure to O3, both alone and in combination with HNO3, produced a more than 30% (statistically significant) delay during the initial phase of upper respiratory tract clearance, and all of the pollutant exposures may have stimulated deep lung clearance. The directions of these effects were the same as those observed previously when rats were acutely exposed to O3- containing atmospheres