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 environmentall y realistic levels of two urban air pollutants-nitric acid vapor (HNO3 ) and ozone (O-3)-on tracer particle clearance from the rat respirator y tract The measurement of particle clearance efficiency is important because failure of this function could contribute to a buildup of fore ign matter in the respiratory tract Separate groups of rats (30 rats p er group) were exposed nose-only for 4 h/day, 3 days/wk (a surrogate f or an air pollution episode) for 40 wk to 4 atmospheres: purified air; 0.15 ppm O-3; 50 mu g/m(3) HNO3; and 0.15 ppm O-3 + 50 mu g/m(3) HNO3 . At 4 wk prior to the end of the exposure, the rats inhaled radiolabe led tracer particles nose-only for 30 min. A clearance measurement pro tocol was then followed to estimate the rates of early and late cleara nce. Early (presumably upper respiratory tract) clearance was monitore d during the 48 h following tracer deposition by the analysis of radio activity excreted in the feces, while late (presumably deep lung) clea rance was characterized by a combination of chest counting and 30 day postexposure sacrifice radioactivity analyses. The exposure to O-3, bo th alone and in combination with HNO3, produced a more than 30% (stati stically significant) delay during the initial phase of upper respirat ory tract clearance, and all of the pollutant exposures may have stimu lated deep lung clearance. The directions of these effects were the sa me as those observed previously when rats were acutely exposed to O-3- containing atmospheres.