CONSIDERATIONS FOR THE RESPIRATORY-TRACT DOSIMETRY OF INHALED NITRIC-ACID VAPOR

Nitric acid (HNO3) vapor is a component of ambient photochemical pollu tion. Because of its high water solubility and reactivity, it would be anticipated to undergo significant removal within the upper respirato ry tract (URT). However, recent studies have shown that inhaled vapor can alter bronchial responsiveness and other functions of the lower re spiratory tract (LRT). The penetration of HNO3 into the lungs was asse ssed using a physical system that mimicked the residence time of inhal ed air in the URT of the animal used in this laboratory for toxicologi c studies, namely, the rabbit. The system allowed for mixing of precis ely metered concentrations of ammonia (NH3), which is present in the U RT, under conditions of controlled relative humidity. The size of part icles produced when HNO3 was introduced into humid atmospheres was als o determined. In an NH3-free atmosphere maintained at 77% relative hum idity, ultrafine particles (0.003 mu m) were formed When NH3 was added , the particle size increased to 0.15 mu m. While there was greater pr oduction of particles when NH3 was present at various humidity levels, even in the absence of NH3 almost 50% of HNO3 vapor formed particles at high humidity. The ability of other particles to act as vectors for adsorbed/absorbed HNO3 was also examined; such particles could then b e carried into the LRT. The results indicate that HNO3 vapor is likely transformed into particle Form al some point following inhalation. Fu rthermore, endogenous ammonia may react with inhaled HNO3 producing pa rticulate ammonium nitrate. Once formed these particles, and others pr esent in the respiratory-tract air, may serve as vectors for HNO3 deli very to the LRT.