Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs) are a group of chemical compounds that are primarily formed from the incomplete combustion of carbon-containing materials such as wood, coal, oil, and gas. These compounds are considered reasonably anticipated human carcinogens and have been associated with various types of cancers, including lung, colon, skin, and bladder cancer. PAHs can enter the human body through inhalation, ingestion, or skin contact, with exposure sources including vehicle exhaust, tobacco smoke, industrial emissions, and certain cooking methods, particularly grilling.
Historically, the carcinogenic nature of PAHs has been recognized since the 18th century, with significant research confirming their link to cancer risk in both animals and humans. Notable studies have shown that repeated exposure to PAHs can lead to genetic changes that induce cancer. Specific populations at heightened risk include industrial workers in settings such as coking plants and petroleum refineries, as well as smokers and individuals exposed to tobacco smoke.
Regulatory bodies like the Environmental Protection Agency and Occupational Safety and Health Administration establish guidelines to limit PAH exposure in the environment and workplace. In recent years, ongoing research has continued to explore the connections between PAHs, cancer risk, and environmental factors, including the effects of climate change on exposure levels, particularly from smoke produced by wildfires.
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Polycyclic aromatic hydrocarbons
ROC STATUS: Reasonably anticipated human carcinogens since 1981
ALSO KNOWN AS: Polynuclear aromatic hydrocarbons, polyaromatic hydrocarbons, PAHs
RELATED CANCERS: Lung cancer, colon cancer, skin cancer, and bladder cancer
DEFINITION: Polycyclic aromatic hydrocarbons (PAHs) refer to a group of chemicals formed from burning wood, coal, oil, gas, and other carbon-containing substances.
Exposure routes: Inhalation, ingestion, skin contact
Where found: In the air (from motor vehicle exhaust, burning wood and refuse, tobacco smoke, industrial emissions, smoke from fires), contaminated water and food, meat cooked by specific high-temperature methods (such as grilling), and coal tar products used to treat skin conditions
At risk: Industrial workers in coking plants; coal tar, aluminum, iron, steel, and asphalt production plants; and petroleum refineries; as well as road construction workers, smokers, and those exposed to tobacco smoke
Etiology and symptoms of associated cancers: Polycyclic aromatic hydrocarbons are procarcinogens in that they are converted by the human body's detoxification system to substances that can cause cancer. Such substances bind to deoxyribonucleic acid (DNA), which carries a person’s genetic blueprint. Studies suggest that this binding induces changes in DNA that lead to cancer.
Some symptoms of associated cancers include shortness of breath, coughing, weight loss (lung cancer); blood in the urine (bladder cancer); weight loss, blood in stools, change in bowel habits (colon cancer); and changes in a wart or mole, or a skin growth that may exhibit redness (skin cancer).
History: Polycyclic aromatic hydrocarbons are naturally present in the environment (such as in coal, peat, and crude oil) and are also formed artificially by various burning processes. The involvement of substances containing polycyclic aromatic hydrocarbons in causing cancer was shown in 1775 when Percival Pott, a British surgeon, described scrotal cancer in chimney sweeps who had been exposed to coal soot. Coal tar was subsequently (1915) found to induce tumors when repeatedly applied to rabbits’ ears. In 1933, a specific polycyclic aromatic hydrocarbon isolated from coal tar, benzo(a)pyrene, was shown to cause skin cancer in mice. Animal studies have since shown that repeated administration of this and other polycyclic aromatic hydrocarbons to animals through the skin, the air, or diet causes various types of cancer, including breast, skin, stomach, lung, and bladder cancer. Human studies have shown an association between lung, bladder, colon, and skin cancer in industry workers exposed to high levels of polycyclic aromatic hydrocarbons.
In the 2020s, studies continued to find myriad associations between developing cancer and exposure to polycyclic aromatic hydrocarbons. For example, 2024 studies in Canada and Iran found associations between ambient exposure to polycyclic aromatic hydrocarbons and an increase in premenopausal breast cancer in women, as well as an increase in the severity of the breast cancer diagnosed. Studies also confirmed the increased risk of developing cancers from polycyclic aromatic hydrocarbons found in the air of urban areas. Polycyclic aromatic hydrocarbons found in alcohol were also investigated as a source of cancer risk. Finally, exposure to smoke from wildfires, an increasing phenomenon with the advent of global climate change, was a source of exposure to cancer-causing polycyclic aromatic hydrocarbons.
Several agencies regulate polycyclic aromatic hydrocarbon emissions. The Occupational Safety and Health Administration (OSHA) regulates industrial workers' exposure to polycyclic aromatic hydrocarbons. The Environmental Protection Agency regulates the amount of polycyclic aromatic hydrocarbons released into surface waters (Clean Water Act) and the air (Clean Air Act) and present in drinking water (Safe Drinking Water Act). The Food and Drug Administration regulates the amount of polycyclic aromatic hydrocarbons in bottled drinking water.
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