Polycyclic Aromatic Hydrocarbons (PAH)

Polycyclic aromatic hydrocarbons (PAHs) are emitted into the Canadian environment from both natural and anthropogenic sources. Forest fires, which release approximately 2 000 tonnes of PAHs per year, are the single most important natural source of PAHs in Canada. However, since releases from that source are generally widely separated in time and space across the country, they do not result in continuous exposure in any specific area.

Anthropogenic sources are numerous and result in emissions of PAHs into all environmental compartments. The greatest anthropogenic sources of PAHs released to the atmosphere are aluminum smelters [925 tonnes/year (t/yr)], with most of these emissions being released from smelters that use the Horizontal Stud Söderberg process. Major sources of PAHs to the aquatic and soil environments include creosote-treated products (up to 2000 t/yr), spills of petroleum products (76 t/yr), metallurgical and coking plants (4 t/yr), and deposition of atmospheric PAHs (amount unknown).

With the exception of some of the lighter compounds, which volatilize from water or soil, PAHs are relatively non-volatile and of low solubility in water. In air, Soil, and water, PAHs are mostly adsorbed to particulate matter, on which they are transported and can be degraded slowly. Since degradation is very slow in sediments, this medium is the major environmental sink for PAHs.

Polycyclic aromatic hydrocarbons have been measured in water from railway and utility ditches at concentrations higher than those recognized to cause harmful effects to biota under laboratory conditions. Concentrations of PAHs in surface waters elsewhere in the country are orders of magnitude lower, and are below concentrations that cause harmful effects to aquatic biota.

At several sites in Canada, including the tidal flats of Muggagh Creek in the vicinity of the tar ponds in Sydney Harbour, Nova Scotia, and near a former oil gasification plant at Kettle Creek, Port Stanley, Ontario, populations of aquatic organisms have been adversely affected by contamination by PAHs. Moreover, PAHs in sediments from Hamilton Harbour, Ontario, have been associated with mortality of sensitive aquatic invertebrates under laboratory conditions.

Under laboratory conditions, neoplastic effects such as liver tumours in aquatic organisms have been associated with exposure to PAHs. Field evidence also supports this association. In Vancouver harbour, neoplastic liver lesions were observed in up to 75% of the English sole caught in areas where sediments are highly contaminated by PAHs.

Polycyclic aromatic hydrocarbons generally do not absorb light of wavelengths critical to global warming. Unlike substances associated with depletion of stratospheric ozone, they are non-halogenated compounds of low to moderate persistence in the atmosphere. Given these properties and the low steady-state concentrations of PAHs in the atmosphere, they are not considered to contribute significantly to stratospheric ozone depletion, global warming, or ground-level ozone formation.

Owing principally to the limitations of the available data base, it was necessary to focus the human health assessment primarily on a small number of PAHs and consider exposure from air only.

Based primarily on the results of carcinogenicity bioassays in which PAHs have been administered to experimental animals by inhalation (benzo[a]pyrene only) and dermal application, and on supporting data, the five PAHs considered in the human health assessment (benzo[a]pyrene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, and indeno[1,2,3-cd]pyrene) have been classified as "Probably Carcinogenic to Humans" (i.e., substances for which there is believed to be some chance of adverse effects at any level of exposure). For such substances, estimated exposure is compared to quantitative estimates of cancer potency to characterize risk and provide guidance for further action, i.e., analysis of options to reduce exposure. Based on consideration solely of the potential effects of PAHs on human health by two limited approaches, the priority for analysis of options to reduce exposure would be moderate to high.

Based on these considerations, it has been concluded that polycyclic aromatic hydrocarbons are entering the environment in a quantity or concentration or under conditions that may have harmful effects on the environment. Polycyclic aromatic hydrocarbons are not considered to constitute a danger to the environment on which human life depends. The PAHs benzo[a]pyrene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, and indeno[1,2,3-cd]pyrene may constitute a danger in Canada to human life or health.

View or download the full Priority Substances List Assessment Report on Polycyclic Aromatic Hydrocarbons (PAHs) (PDF).