Introduction

1. The first account of cancer associated with exposure to coal combustion products was published in 1775 by Sir Percival Pott, a surgeon at St Bartholomew's Hospital in London, who reported the development of tumours of the scrotum among chimney sweeps' apprentices. A century later, similar tumours were described in workers producing paraffin from oil shale in Scotland, and in the early years of the 20th century it was shown that chronic skin contamination by oils and tars led to skin cancer in other occupational groups. At the same time, repeated application of coal tar to the skin of experimental animals was reported to induce similar cancers. The various materials studied were not mechanical irritants and it seemed likely that their carcinogenic properties were intrinsically chemical in nature. Analytical and biological investigations of fractionated coal tars were conducted at the Institute of Cancer Research in London and benzo[a]pyrene, the first carcinogenic polycyclic aromatic hydrocarbon (PAH) to be extracted from coal tar, was described in 1933 (IARC, 1985). Many PAHs have now been characterised and several are carcinogenic when tested as pure compounds in laboratory animals (IARC, 1983). More epidemiological information in the 1950s and 1960s showed that workers chronically exposed to fumes containing combustion products from coke and coal gas manufacture were at an increased risk of lung cancer, and similar findings have been reported more recently among workers in aluminium smelters (Doll et al., 1965; IARC, 1984; Armstrong et al., 1994). In all these instances it is believed that certain PAHs are the principal carcinogens.

2. Polycyclic aromatic hydrocarbons are a large group of chemical compounds with a similar structure comprising two or more joined aromatic rings. Different PAHs vary both in their chemical characteristics and in their environmental sources and they are found in the environment both as gases and associated with particles (NETCEN, 1998). They may be altered after absorption into the body into substances that are able to damage the genetic material in cells and initiate the development of cancer, although individual PAHs differ in their capacity to damage cells in this way.

3. Environmental exposure of humans to single PAHs does not occur because these chemicals are always encountered as complex mixtures containing many different PAH constituents. A few studies have identified associations between individual PAHs and human cancer but such substances, however potent their own carcinogenic effects, serve as markers for exposure to the entire PAH mixture. The fact that PAH exposure is always to a mixture, which is not of constant composition, makes the evaluation of health consequences and attribution to specific components difficult.

4. In this report the Panel discuss the main sources of airborne PAHs, their properties, the methods by which they are measured and monitored in ambient air, and the concentrations found currently in the United Kingdom. We discuss the evidence associating their concentration in the air in workplaces with cancer in exposed workers and how this knowledge may be applied in determining risk to the general population from ambient exposures. We consider the use of benzo[a]pyrene as a marker for PAH exposure and conclude by recommending an air quality standard on this basis.