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PAHs are a class of diverse organic compounds containing two or more fused aromatic rings of carbon and hydrogen atoms. They are ubiquitous pollutants formed from the combustion of fossil fuels and are always found as a mixture of individual compounds. The relative concentrations of PAHs in air, water, and food are usually the same, although this can change depending on certain sources of pollution. The largest amount of PAHs enters the environment via the atmosphere from incomplete combustion processes, such as processing of coal and crude oil (e.g. refining, coal gasification, coking), industrial use of coal and mineral oil products (aluminium production, iron and steel production, foundries), heating (power plants and residential heating using wood, coal, and mineral oil), fires (e.g. forest, straw, agriculture, cooking), incineration of refuse, vehicle traffic, tobacco smoking, and volcanic activities (for quantitative data on the release of PAHs in the environment. PAHs are emitted mainly into the atmosphere and have been detected long distances from their source. Because of their low vapour pressures, compounds with five or more aromatic rings will exist mainly adsorbed to airborne particulate matter, such as fly ash and soot.
The main source of PAH contamination in drinking-water is usually not the raw water sources but the coating of the drinking-water distribution pipes. At least in the past, coal tar was a common coating material for water pipes, used to give effective protection against corrosion. After the passage of drinking-water through those pipes or after repair work, significantly increased PAH levels have been detected in the water.
PAHs have been detected in fresh vegetables, fruits, and cereals as a result of the deposition of airborne PAHs, particularly near industrial sources or in areas with high traffic. PAHs have also been found in mussels, snails, and fish from contaminated waters. PAHs are also present at elevated levels in some vegetable oils and margarine, probably formed during processing. PAHs are also formed during some methods of food preparation, such as char-broiling, grilling, roasting, frying, or baking. The highest levels were detected in smoked and grilled meat and fish samples.
Effects on Humans
A high lung cancer mortality in Xuan Wei, China, has been linked to PAH exposure from unvented coal combustion. PAHs present in tobacco smoke (mainstream and sidestream) are implicated as contributing to lung and other cancers. Evaluation of some studies shows, however, that it is plausible that the increased risk of lung cancer can be attributed in part to PAHs.
1. Polynuclear aromatic hydrocarbons in Drinking-water / WHO/SDE/WSH/03.04/59