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During the last decade, a considerable amount of information has been gathered on the organic composition of aerosols, mostly sampled by Hi-Vol filtration, and analysed by gas chromatography or gas chromatography mass spectrometry. From our research, it appears that care should be taken in the use of such data for the evaluation of health hazards, resulting from air pollution. The following facts must be considered.(1) The conventional Hi-Vol filtration sampling technique can be subject to several sampling artifacts, such as volatilization from and chemical reaction on the filter. Indeed, a series of compounds of intermediate volatility has been shown to evaporate from particles on the filter, when exposed to large air volumes. Low molecular weight compounds are predominant in the gas phase, and high molecular weight ones in the particulate phase.Furthermore, some pollutants on collected particles can react with molecular or radical species, present in the atmosphere: e.g. the carcinogen benzo(a)pyrene can be degraded very rapidly by traces of ozone to highly polar compounds. (2) Besides pollutants of anthropogenic origin, natural sources may also contribute to the aerosol burden. Thus, the presence of the higher odd carbon numbered n-alkanes reflects the contribution from vegetation (plant waxes). The possible synergistic activity of these compounds in combination with carcinogens, justifies the monitoring of these contaminants in aerosols. (3) Since the retention of particles in the respiratory tract strongly depends on particle size, measurements of total pollutant concentrations have to be supplemented with particle size distribution data. The analysis of several size fractionated samples, taken at suburban as well as at remote rural and sea shore sites, allows the evaluation of the dilution of particulate organics and of the differences in particle size distributions in the background areas. Most pollutants are almost exclusively distributed over the respirable particle size range (below 3 pm) and any observed distribution shifts remain within this range. In conclusion, a realistic estimate is presentedtof the fractions deposited in and extracted by the tissues of the respiratory tract. These are directly responsible for the health hazards, associated with breathing aerosol particles. It follows that a nearly constant fraction of a given pollutant, enters the bloodstream, irrespective of the sampling site, and thus health hazards remain closely related to the total Concentration levels.

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