Lc. Chen et Rb. Schlesinger, CONSIDERATIONS FOR THE RESPIRATORY-TRACT DOSIMETRY OF INHALED NITRIC-ACID VAPOR, Inhalation toxicology, 8(7), 1996, pp. 639-654
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.