PAHs and other toxic compounds; dioxins and furans
Polycyclic aromatic hydrocarbons are a large group of chemical compounds consisting of two or more benzene rings joined together. Examples include napthalene (2 rings), phenanthrene/anthracene (3 rings), pyrene (4 rings) and one of the most studied, benzo[a]pyrene, has 5 rings. In ambient air they are found either as gases or associated with particles, produced from the incomplete combustion of carbon based fuels such as coal, oil, wood, gas, and petroleum fuels. Fossil-fueled power stations, industrial processes (e.g. aluminium production), domestic burning of fossil fuels and traffic, particularly in urban areas, are the main sources of PAHs in outdoor air. Cigarette smoke is an important source of PAHs in indoor air. Levels of individual PAHs are monitored in outdoor air, but they are always present with other PAHs as part of a complex mixture. Therefore, people are always exposed to a number of different PAHs, rather than single PAHs on their own.
Some PAHs have been shown to be toxic and/or carcinogenic (cancer-causing)in animals and this information is used to determine the risk to humans. For example, benozo[a]pyrene, benz[a]anthracene, dibenz[ah]anthracene are classified by the IARC (International Agency for Research on Cancer) as ‘probably carcinogenic to humans’. As PAHs can be inhaled into the lungs as gases or attached to particles, there is the risk of lung cancer. For example, increased incidence of lung cancer has been observed in men working in gas and coke production and aluminium refining who were exposed to high concentrations of PAH fumes over long periods of time.
Levels of PAHs in outdoor air are much lower than those encountered in occupational settings and there is no convincing evidence, as yet, to suggest that PAHs in outdoor air are a significant cause of lung cancer in the general population. Estimating the health effects due to PAHs in outdoor air is made difficult by cigarette smoking, which is a source of PAHs in itself, and which can obscure contributions from outdoor sources in population studies.