Sources of Exposure to Polycyclic Aromatic Hydrocarbons

5. The term polycyclic aromatic hydrocarbon refers to members of a large group of organic compounds widely distributed in the atmosphere, whose molecular structures contain two or more aromatic rings fused together. The best known PAH is benzo[a]pyrene, B[a]P, which contains five rings. Figure 1 illustrates the molecular structure and chemical formula of B[a]P.

6. Because of their low vapour pressures, some PAHs are present at ambient temperatures in air, both as gases and associated with particles. The lighter PAHs, such as phenanthrene, are found almost exclusively in the gas phase; the heavier PAHs, such as B[a]P, are almost totally adsorbed onto particles. PAHs are relatively insoluble in water but dissolve in fats and oils.

7. PAHs are formed naturally in the environment, during such processes as thermal geological reactions and natural fires. Human activities are more significant sources to the environment, as PAHs are formed in all processes involving incomplete combustion of carbon-based fuels, and are therefore emitted during burning of common fuels, such as coal, oil, wood and gas. Tobacco smoke is an important source in indoor air.

8. The main sources of both total PAHs and B[a]P for 1995 and 1996 in the United Kingdom are shown in Table 1. It can be seen that certain industrial processes, such as coke production and anode baking for aluminium production, are major sources of B[a]P which would most strongly influence the B[a]P concentration in the vicinity of the plants. Domestic fuel combustion also produces significant quantities of emissions. Road traffic emissions are a dominant source of total PAH and B[a]P in towns and cities. This is borne out by a study in Birmingham, which associated 88% of the concentration of B[a]P in air with road traffic emissions (Harrison et al., 1996). Considerable uncertainties surround the derivation of the estimates shown in Table 1. Owing to these uncertainties, it is not appropriate to assign proportions to each emissions source. It is likely that estimates of the contribution from road transport, which are derived from a small number of unrepresentative vehicles, may well underestimate the real contribution.

9. The current inventory of atmospheric emissions gives only one snapshot in time, and elucidation of trends for individual PAHs is based upon measurements in the atmosphere or for non-volatile PAHs, in archived samples, such as vegetation, which have been polluted predominantly by atmospheric deposition. The available data indicate a substantial decline in emissions of PAHs from the first half of the 20th century when emissions are believed to have been dominated by the combustion of coal (Waller, 1952).

10. In terms of the exposures of individuals to both total PAHs and B[a]P in air, the most important source is incomplete smoky combustion, with the internal combustion engine as an important source in towns and cities. PAH concentrations in indoor air are likely to be determined both by outdoor levels and indoor sources, such as cigarette smoke and wood fires (WHO, 1987; WHO, in press; Gold et al., 1990). Those PAHs which are adsorbed onto fine particles will remain in the air relatively longer, at least for several days, than those PAHs which are present mainly in the gas-phase. The particles contaminate rain and, together with dry deposition of particles and vapour, this leads to the contamination of soil, crops and plants which may then be responsible for low concentrations of PAHs in food and water. More substantial amounts of PAHs, including B[a]P, may be found in food, as a consequence of various methods of cooking, preservation and storage. As PAHs are formed in smouldering as well as flaming combustion, "burnt" food contains PAHs, as do smoked foods.

Table 1. Estimated United Kingdom emissions (particulate and gas phase) of both total polycyclic aromatic hydrocarbons and benzo[a]pyrene from selected emission sources (1995 and 1996), tonnes per year. (Source: National Environmental Technology Centre)

Notes

1 Where total means the following selected 16 PAHs are naphthalene, acenaphthylene, acenaphthene, fluorene, anthracene, phenanthrene, fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz[ah]anthracene, indenol[123cd]pyrene and benzo[ghi]perylene.

2 Excludes PAH emissions, which are not currently estimated, for bitumen and asphalt use, accidental fires, bonfires and other non-agricultural burning, domestic and industrial oil and gas combustion, foundries, non-ferrous metal except aluminium, off-road vehicles, waste oil combustion, petroleum processes, glass ceramic and other mineral processes.

3 Comprises emissions from Soderberg processes and pre-baked anode process.

4 B[a]P emissions from anode baking are predicted to decrease to below 2 tonnes in 1999.

5 The estimates of the contribution from diesel vehicles may be too low as they are based on data relating only to new vehicles.

6 Domestic and industrial creosote use.

7 Comprises electrical supply industry, waste incineration and iron and steel works.