Impact of Regulations of Traffic Emissions on PAH Level in the Air

4 Conclusions

Comparison with air quality criteria

The Danish Environmental Protection Agency has not established standards for PAH and other PAC in ambient air. As many PAC are carcinogenic compounds the opinion has been that the levels should be as low as possible, and outdoor air levels of PAC are regulated by means of emission limits for the various sources. In the Netherlands a draft (annual average) tolerable level of 5 ng/m³ and an acceptable level of 0.5 ng/m³ for the content of benzo(a)pyrene (BaP) in the outdoor air has been given. The content of BaP in city air in Denmark appears to be between these two levels.

Air pollution and sources

The traffic emissions are the major sources for the presence of PAH, other PAC and mutagens in street air. Occasionally, an additional contribution from long-range transported polluted air and atmospheric processes during the transport may increase the mutagenicity levels significantly. PAH and oxy-PAH are present at the same concentration levels, while S-PAC and the basic N-PAC concentrations are about one order of magnitude lower than those of PAH. The mutagenicity of the basic extracts was only 3% of the mutagenicity of the total extracts at low levels of ozone but made up a higher proportion if the polluted air masses had been affected by chemical atmospheric processes. The sources for the oxy-PAH, benzanthrone, was emission and atmospheric chemistry. The concentration of benzo(a)pyrene turned out to be a poor indicator for the air pollution with carcinogenic and mutagenic components. Evidence was obtained for photolytic degradation of benzo(a)pyrene in summer-time. In addition, the increase in the levels of oxy-PAH and mutagenicity in the summer half-year compared to the winter half-year was in contrast to the corresponding decrease in benzo(a)pyrene. The summer increase in mutagenicity was caused by the impact of the long-range transport episode.

Impact of legislation

The increase in the number of petrol driven passenger cars with catalysts and the application of light diesel fuel for buses had caused a larger reduction in the street air levels of PAH and other mutagens than in the levels of nitrogen oxides, carbon monoxide and soot, despite the major argumentation for the legislation was to reduce the three latter components. Both emissions-reduction initiatives were estimated to contribute to the reduction in air levels of PAH and other mutagens. About 2/3 of the reduction appeared to be caused by the light diesel fuel and 1/3 by the replacement of older petrol passenger cars with new ones equipped with catalysts. The major argument for this is that the PAH composition of phenanthrene and methylphenanthrenes was the same in 1996 and in 1992 as well as the composition of benzo(ghi)perylene and coronene and other stable PAH in 1996 and in 1992-1993. The estimate is supported by a comparison of the reduction in CO and in PAH from 1992 to 1993 and 1996. The reduction in the mutagenicity levels appears to be larger than that for PAH.

Health risks

The most significant health risk of ambient air pollution with PAH and other mutagens is expected to be an excess lung cancer. It was estimated in a previous risk assessment (Nielsen et al., 1995c) that for residents in Copenhagen and other heavily urbanised areas the air pollution would cause, as a maximum, five extra lung cancer cases each year among one million individuals. The risk estimate was a composite estimate based on various methods, including benzo(a)pyrene and mutagenicity levels, respectively, as markers of risk. The risk would be lower for individuals living at country sites as judged from risk estimates based upon benzo(a)pyrene only. However, the risk ratio between country and city might be higher than the PAH ratio between country and city considering that atmospheric processes cause an increase in the levels of mutagenicity compared to the PAH levels.