Growth and neutralization of sulfate aerosols in human airways

Evidence derived from in vivo and in vitro laboratory experiments, controlled human exposure studies, and epidemiological studies on mortality and morbidity point to a positive correlation between acid aerosol inhalation and lung impairment. The lung has two important lines of defense against acid aerosols: 1) neutralization by oral or nasal airway ammonia and 2) buffering by mucus lining of the airway. A mathematical model is developed to study the growth and endogenous ammonia neutralization of sulfate-containing aerosol particles in the human respiratory tract. It is shown that an accurate prediction of the luminal ammonia concentration and relative humidity in each generation is essential for quantifying the degree of neutralization of the acidic particles. The model predicts substantial growth and neutralization for small particles (<0.1 μm), whereas larger particles (>1.0 μm) experience negligible neutralization. The predicted neutralization of intermediate-sized particles depends on the parameter values used in the model. Water supersaturations that occur in the respiratory tract when ambient conditions are cool and humid cause rapid particle growth and consequently enhance neutralization. Thus the neutralization depends on the particle size as well as on ambient conditions.