Atmospheric polycyclic aromatic hydrocarbons: Size distribution, estimation of their risk and their depositions to the human respiratory tract

Polycyclic aromatic hydrocarbons (PAHs) in the atmosphere may be one of the causes of lung cancer. However, in many countries including Japan, only benzo[a]pyrene is regulated, though more toxic PAHs than benzo[a]pyrene have been reported. Therefore, it is necessary to characterize the risk of atmospheric PAHs and to find major contributing compounds that can act as suitable indicators for estimating and monitoring the risk of PAHs as a whole. The primary objectives of this study are to investigate the risk of 22 combined atmospheric PAHs and to find major contributing compounds that can act as adequate indicators for estimating the risk. The average lifetime cancer risk for all 22 PAHs in winter was 1.7×10^-4 in the Yokohama residential area. Although benzo[a]pyrene has been considered to be a useful indicator in many countries, the present study also showed that benzo[a]pyrene contributed only ca. 40% of the total cancer risk for all 22 PAHs, as calculated by unit risk. Six PAHs (dibenzo[a,h]anthracene, benzo[b]fluoranthene, indeno[1,2,3-cd]perylene, dibenzo[a,h]pyrene, dibenzo[a,e]pyrene, and benzo[a]pyrene) contributed 93% of the overall risk. Therefore, these six PAHs, including the high molecular ones, would be better indicators of the risk than benzo[a]pyrene alone. The secondary objective of this study is to reveal the PAHs that are major contributors to the deposition that contribute to the total deposition of PAHs in the human respiratory tract. The results revealed that the same six PAHs identified using unit risk evaluation were large contributors in all five parts of the respiratory tracts in adult males in spite of deposition process. It was concluded that the reason for this was that these six PAHs had the same size distributions, of which the 0.43-1.1 μm fraction was the most largest, about 57-70% of the whole concentration.