Polycyclic aromatic hydrocarbons (PAH) are a group of chemicals released from burning coal, oil, rubbish, tobacco and wood. They are also present in products made from fossil fuels, such as coal-tar pitch, creosote, and asphalt. PAH are found in the air, water, and soil, and can persist in the environment for months or years. The level of PAH in urban air may be 10 times greater than those found in rural areas. In the home, PAH are present in tobacco smoke, smoke from wood burning stoves and fireplaces, creosote-treated wood products, and some foods. Barbecuing, smoking, or cooking food over a fire can cause PAH to form in meat and other foods. In addition, manufactured PAH may be used in medicines and pesticides.
The main source of exposure is through the diet, particularly from smoking or grilling meat and other food or consuming the PAH deposited on plant foods, especially broad-leafed vegetables, during growth.  PAH are typically at low concentrations in drinking water. Other foods that may contain low levels of PAH include roasted coffee, roasted peanuts, refined vegetable oil, grains, vegetables, and fruits. A variety of cosmetics and shampoos are made with coal tar and therefore contain PAH.
Scientific evidence also suggests that natural gas extraction or fracking may be contributing to the amount of PAH in the air, although not at levels that would be expected to increase the risk of cancer. Vehicle exhaust emissions also contribute to the amount of PAH in the air.
Some PAH appear to have endocrine disrupting properties and have been linked to thyroid dysfunction and male infertility. They may also increase the risk of some cancers, especially skin, bladder, liver, stomach and lung as well as cardiovascular disease and poor foetal development.