Considerations for the Respiratory-Tract Dosimetry of Inhaled Nitric Acid Vapor

Abstract

Nitric acid (HNO₃) vapor is a component of ambient photochemical pollution. Because of its high water solubility and reactivity, it would be anticipated to undergo significant removal within the upper respiratory tract (URT). However, recent studies have shown that inhaled vapor can alter bronchial responsiveness and other functions of the lower respiratory tract (LRT). The penetration of HNO₃ into the lungs was assessed using a physical system that mimicked the residence time of inhaled air in the URT of the animal used in this laboratory for toxicologic studies, namely, the rabbit. The system allowed for mixing of precisely metered concentrations of ammonia (NH₃), which is present in the URT, under conditions of controlled relative humidity. The size of particles produced when HNO₃, was introduced into humid atmospheres was also determined. In an NH₃,-free atmosphere maintained at 77% relative humidity, ultrafine particles (0.003 μm) were formed. When NH₃, was added, the particle size increased to 0.15 μm. While there was greater production 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 be carried into the LRT. The results indicate that HNO₃ vapor is likely transformed into particle form at some point following inhalation. Furthermore, endogenous ammonia may react with inhaled HNO₃ producing particulate ammonium nitrate. Once formed, these particles, and others present in the respiratory-tract air, may serve as vectors for HNO₃ delivery to the LRT.