Toxicological implications of the organic fraction of aerosols: a chemist's view

Abstract

In the first part of this review, the analytical methodology used to determine the particle size distributions of organic pollutants in particulate matter is described. Several problems affecting the precision of this procedure are discussed: the calibration of commercial Hi-Vol cascade impactors, physical and chemical sampling artifacts, recovery of organics from particles, and analytical methods for quantitation. In combination with the ICRP model for the retention of particles in the human respiratory tract, deposited concentrations of organic pollutants can be calculated, which are of great toxicological significance. The application of this model to the polydisperse aerosol fractions collected by cascade impaction was validated using particle size distributions from the literature, which have been measured in very small particle size intervals. Results for suburban and rural aerosol samples are briefly discussed. Reliable estimates of the average daily intake (ADI) of organic pollutants by respiration will require the experimental determination of the absorption efficiency. As a first approximation for organic pollutants, the ADI can be set at 4 C ng, where C is the total concentration of the compound in the aerosol. The second part of this review is concerned with the chemical reactivity of organic aerosol constituents in the atmosphere as well as on sampling substrates. The use of the Ames Salmonella microbiological assay has shown significant direct mutagenic activity of the frameshift type in aerosol extracts from different sources, which cannot be attributed to known carcinogenic polycyclic aromatic hydrocarbons (PAH). Thus, it was realized that aerosol particles might contain secondary pollutants as the result of gas-particle interactions in emission stacks, plumes or during atmospheric transport. Laboratory experiments were performed to evaluate the chemical reactivity of PAH towards several gaseous pollutants (SO 2, NO 2, O 3) under simulated atmospheric conditions both on filter substrates and on PAH-enriched aerosol samples. The results were most encouraging: the carcinogen benzo[ a]pyrene, for example, could be degraded rapidly by ozone to a series of polar compounds; the direct mutagenicity of the reaction mixture was found to be caused by the presence of a benzo[ a]pyrene-epoxide, in small amounts. However, it is expected that the efficiency of chemical transformation of PAH in aerosols will largely depend on several factors: particle size and specific surface area will affect the conversion of PAH in a heterogeneous system; the close interaction between aerosol matrix and the adsorbed organics can induce a different chemical behaviour of PAH from that observed in model systems.