The Four New Groupings of ATH Substances - Substance-Specific Context

Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) have been declared CEPA toxic and are listed as a group on Schedule I of CEPA 1999. Many individual PAHs meet criteria for persistence, bioaccumulation and toxicity, as defined by various international bodies. PAHs as a group have been assigned Track 2 status under the Toxic Substances Management Policy (TSMP) because many sources are natural rather than resulting from human activity. They are listed on the UN/ECE POP Protocol as a toxic with the potential for long-range transport through the atmosphere.

PAHs arise from natural and anthropogenic sources. They are a group of over 100 different chemicals that are formed during the incomplete burning of coal, oil and gas, wood, garbage, or other organic substances. They are usually found as a mixture containing many compounds, in materials such as soot or creosote. Some PAHs such as benzo [a] pyrene have been classified as probably carcinogenic to humans by IARC, while others such as benzo [j] fluoranthene are classified as possible human carcinogens. PAHs are introduced to the environment either in the gaseous phase or bound to particles, most commonly soot. In ambient air, 25% of PAHs have been found adhered to particles and 75% in the gaseous phase; the exception is near aluminum smelters where the ratio has been found to be 50-50. PAHs are also usually found concentrated near specific point sources and their concentration tends to decrease rapidly with distance from their source.

Anthropogenic Sources

The potential sources for the releases of PAHs can be listed as:

• wood waste and bark-fired industrial boilers;
• natural gas fired industrial boilers;
• coal-fired industrial boilers;
• oil and waste oil-fired industrial boilers;
• primary aluminum producers;
• power plants;
• electric-arc furnaces;
• petroleum catalytic cracking;
• asphalt roofing manufacture;
• hot-mix asphalt plants;
• ferrous foundries;
• coke ovens;
• pulp mills;
• carbon black manufacture;
• wood preservation; and
• municipal incinerators.

PAHs are "naturally" present in crude oil and they do not disappear in refining.

The major Canadian source to the atmosphere is aluminum smelters. The second largest source is residential wood stoves, which individually only emit a small amount of PAHs, but collectively contribute significantly to the overall anthropogenic release.

Natural Sources

Forest fires, which release about 2,000 tonnes of PAHs per year, are the single most important natural source of PAHs in Canada. However, because 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. Atmospheric deposition from sources outside of Canada is another significant route of entry.