Urban air pollution by polycyclic aromatic hydrocarbons: levels and sources of variability.

The atmospheric levels of polycyclic aromatic hydrocarbons (PAHs), measured since the mid-1970s from approximately 60 towns world-wide, are reviewed. For each investigation, a concise description of sampling conditions and analytical methods is also given. The strongly carcinogenic benzo[a]pyrene was typically found in the range of 1-20 ng/m3 in Europe, and around 1 ng/m3 in the USA. For other PAHs, individual concentrations were generally in the range of 1-50 ng/m3 in Europe, 0.1-1 in North and South America and in Australia, 1-10 in Japan, and 10-100 in two towns in India and New Zealand. Data on the size distribution of PAH-containing particles are also presented, showing that PAH content is adsorbed mostly on respirable particles. The ratios of winter to summer levels, and of urban to rural levels, were found in the range of 2-10 in both cases. PAH levels have progressively decreased in the last few decades, most probably due to the introduction of anti-smoke regulations and the decrease of coal burning for heating. Measurements from different investigations can only be compared with great prudence, owing to sampling factors which can affect results. Sources of urban PAH contamination and of the variability in levels are discussed.