ESTIMATING THE LUNG DEPOSITION OF PARTICULATE POLYCYCLIC AROMATIC HYDROCARBONS ASSOCIATED WITH MULTIMODAL URBAN AEROSOLS

Abstract

Recent epidemiological evidence of associations between increased mortality and ambient particle concentrations suggests that ultrafine particles (smaller than 1 mum diameter) and their constituents are potentially significant contributors because of their enhanced toxicity and their potential for greater lung penetration. Measurements of size distributions of potentially carcinogenic polycyclic aromatic hydrocarbons (PAH) in urban aerosols show their predominance in ultrafine and fine modes centered about 70 and 700 nm. In this article we present estimates of the regional lung deposition of PAH associated with a typical multimodal urban aerosol and examine the effect of varying size distribution characteristics on lung deposition. Size-dependent deposition fractions, from a deterministic calculation using deposition probabilities in a lung anatomical model, have been combined with the fraction of aerosol or PAH mass present in each particle size range and integrated over the entire size range to obtain the fraction of pollutant mass deposited in each lung region and the total lung. For typical urban aerosols the fraction of aerosol mass deposited ranges from 0.48 to 0.56 in the total lung, with 0.20 to 0.26 in the pulmonary lung and 0.28 to 0.30 in the tracheobronchial lung. For associated PAH the fractional mass deposited ranges from 0.27 to 0.32 in the total lung, with a large fraction (0.23-0.28) in the pulmonary lung and 0.04 to 0.05 in the tracheobronchial lung. The fraction deposited is relatively invariant with varying size distribution characteristics like mass median diameter and mass fraction in each mode, making it possible to generalize the results to most locations. Exposure in a polluted urban air basin like the Los Angeles basin, with average PM-10 concentrations from 47 to 87 mug m-3 and particulate PAH concentrations from 0.1 to 3 ng m-3, would result in average daily exposures to PM-10 and PAH, respectively, of 402 mug h m-3 and of 9.0 ng h m-3. Resulting average daily doses, calculated from the lung deposition estimates, are 294 mug day-1 and 3.3 ng day-1, respectively, of aerosol and PAH mass deposited in the total lung with 142 mug day-1 and 2.9 ng day-1, respectively, in the pulmonary lung and compare with doses from sidestream environmental tobacco smoke (ETS) exposure in smoking areas. While it is difficult to interpret the toxicological implications of this dose, these calculations indicate high pulmonary deposition and the risk of delivery of carcinogenic PAH in ambient aerosols directly to the lung interstitium. This reiterates the need for detailed study of ultrafine atmospheric aerosols and their toxic constituents in various microenvironments in which population exposures occur.