Growth and deposition of hygroscopic particulate matter in the human lungs

Abstract

Transport and fate of inhaled particulate matter in the human lungs is calculated for realistic physicochemical conditions by a new dosimetry model. The model solves a variant of the general dynamic equation for the size evolution of respirable particles within the human tracheobronchial airways, starting at the tracheal entrance. We focus on ambient anthropogenic aerosols, which are of concern in inhalation toxicology because of their potential irritant and toxic effects on humans. The aerosols considered are polydisperse with respect to size and heterodisperse with respect to thermodynamic state and chemical composition, having initially bimodal lognormal size distribution that evolves with time as a result of condensation-evaporation and deposition processes. The architecture of the human lung is described by Weibel's symmetric bronchial tree. Simulations reveal that, due to the rapid growth of submicron-sized particles, increased number and mass fractions of the particle population can be found in the intermediate size range 0.1 < φ < 1 μm. Since deposition by diffusion decreases with size increase, fine nonhygroscopic particles are less persistent in the inspired air than hygroscopic particles of comparable initial size distribution. In contrast, the enhanced deposition of hygroscopic particles, initially from the intermediate size range, increases their fraction in the biological dose delivered to the airways. The combined effect of growth and deposition tends to (i) decrease the size nonuniformity of persistent particles in the airways and form an aerosol that is characterized by smaller variance; and (ii) alter the deposition profile along the airways. The modulated local dose consists of excess intermediate-sized particles, some of which may carry toxic components causing a systemic response or inducing localized lesions that can affect observed health effects.