Polycyclic Aromatic Hydrocarbons (PAHs) and Cancer

Polycyclic aromatic hydrocarbon is a class name, not a single substance. A PAH is an organic compound made of carbon and hydrogen, built from two or more fused aromatic rings. Most PAHs with biological activity have two to six rings.

Hundreds exist. The National Toxicology Program does not list the whole class. It lists 15 named PAHs as reasonably anticipated to be human carcinogens, each in its own entry:

Eleven were added in the Second Annual Report on Carcinogens, in 1981. Four of them are benz[a]anthracene, benzo[b]fluoranthene, benzo[j]fluoranthene, and benzo[a]pyrene. Three more are dibenz[a,h]acridine, dibenz[a,j]acridine, and dibenz[a,h]anthracene. The last four are 7H-dibenzo[c,g]carbazole, dibenzo[a,h]pyrene, dibenzo[a,i]pyrene, and indeno[1,2,3-cd]pyrene.

Four more were added in the Fifth Annual Report, in 1989. They are benzo[k]fluoranthene, dibenzo[a,e]pyrene, dibenzo[a,l]pyrene, and 5-methylchrysene.

Three of the fifteen carry a nitrogen atom inside a ring. That makes them heterocyclic PAHs. The best-known member of the group is benzo[a]pyrene, CAS number 50-32-8. It forms pale-yellow needle-shaped crystals. For years it served as the stand-in marker for the whole family.

PAHs form during incomplete combustion. Burn coal, oil, gas, wood, or garbage without enough oxygen and you make them. Heating organic material until it breaks down does the same. Tobacco and charbroiled meat are both examples.

Sources of PAHs in the air you breathe include:

• Forest fires and volcanoes.
• Industrial emissions.
• Home and commercial heating with wood, coal, or other biomass. Oil and gas heating produce far less.
• Motor vehicle exhaust, especially diesel.
• Indoor cooking and tobacco smoke.

Mean concentrations of individual PAHs in urban outdoor air usually run 1 to 30 nanograms per cubic meter. Winter levels are typically at least ten times higher than summer levels, because home heating is the swing factor. Areas near heavy traffic run higher too.

For the general population, though, the largest single source is not air. It is food.

Daily PAH intake from food is estimated at a few nanograms to a few micrograms per person. The Joint FAO/WHO Expert Committee on Food Additives and Contaminants set two reference figures. A typical mean daily intake of benzo[a]pyrene is 4 nanograms per kilogram of body weight. A high-end daily intake of total PAHs is 10 nanograms per kilogram.

Dietary sources include:

• Barbecued, grilled, broiled, and smoke-cured meats.
• Roasted, baked, and fried foods made at high temperature.
• Breads, cereals, and grains, at least partly from gas or flame drying of grain.
• Vegetables grown in contaminated soil, or coated by PAH particles settling out of the air.

The highest levels among common foods turn up in three places. Grilled or barbecued steak. Chicken cooked with skin and bones. Hamburgers. In each case the levels rise when the meat is well done or very well done. An Italian dietary study found total PAH levels around 4 nanograms per gram in fried beef. It found benzo[a]pyrene up to about 4 nanograms per gram in well-done grilled meat.

One detail changes how you cook. PAHs form on or near the surface of food, not in the interior. Food cooked to the same doneness without flame exposure does not show significant PAH levels. The flame is the variable, not the temperature alone. Our page on chemicals in high-temperature cooked meat covers the kitchen changes that have been measured.

Preservation method matters too. Foods smoked in traditional kilns averaged 1.2 micrograms per kilogram of benzo[a]pyrene and 9 micrograms per kilogram of total PAHs. The same foods treated in a modern kiln averaged 0.1 and 4.5.

There is a reassuring piece of biology here. Livestock accumulate PAHs mainly from contaminated feed. Species near the top of the food chain, including humans, do not build them up, because we metabolize them faster.

PAHs share a common route to harm. The body converts them to oxides and dihydrodiols. Those are oxidized again into diol epoxides. Both the oxides and the diol epoxides are reactive, and that is the form that attacks DNA.

Whether a given PAH gets that far depends on how it behaves physically. Vapor pressure matters. So does how well it sticks to solid particles. So does how it dissolves in liquids. So does how it splits between fat and water inside tissue. Those traits vary widely across the class. That is why 15 compounds are listed and hundreds are not.

Exposure can be tracked in a person. 1-Hydroxypyrene, a breakdown product of pyrene found in urine, has been proposed as a biomarker of human PAH exposure.

This is the honest limit of the science, and it is unusual enough to state clearly.

No population studies exist on exposure to the individual listed PAHs. In the real world PAHs never appear alone. They arrive inside complex mixtures. Each one travels with the others, and with other carcinogens. Human cancer data exist only for those mixtures.

That is why the listings rest on animal evidence. All 15 are listed on sufficient evidence of cancer in laboratory animals. None rests on a human study of the single compound.

IARC re-evaluated PAHs in 2005. It classified some high-exposure jobs as causing cancer in humans. Coal gasification and coke production were two. Even so, it could not define the role of any single PAH inside those exposures. For what the labels mean in general, see what a carcinogen is.

Several environmental laws name these compounds. Most use the group term polycyclic organic matter.

• Clean Air Act. Polycyclic organic matter is a listed hazardous air pollutant, a listed mobile source air toxic, and one of 33 pollutants named in the Urban Air Toxics Strategy as the greatest threat to public health in urban areas.
• Safe Drinking Water Act. The maximum contaminant level for benzo[a]pyrene is 0.0002 milligrams per liter. EPA sets the health goal for it at zero.
• Clean Water Act. Polynuclear aromatic hydrocarbons are listed as toxic pollutants under the effluent guidelines.
• CERCLA. Reportable quantities range from 1 pound to 5,000 pounds depending on the compound.
• Community Right-To-Know Act. All 15 listed PAHs are subject to Toxics Release Inventory reporting.
• Resource Conservation and Recovery Act. Several waste codes are based wholly or partly on specific PAHs, including U018, U022, U063, U064, and U137.
• FDA. Bottled water may hold no more than 0.0002 milligrams per liter of benzo[a]pyrene. Limits on PAH levels in color additives are set in 21 CFR parts 74 and 178.

The reporting data show what regulation did. Industrial PAH releases peaked in 2000 at more than 4.9 million pounds, mostly to landfills and air. Releases have stayed at a lower, fairly stable level since 2002. In 2008, 1,192 facilities released just over 1.2 million pounds.

PAHs are everywhere, so they cannot be avoided entirely. But the biggest personal sources are known, and they can be cut. For most people those sources are tobacco smoke, flame-cooked and smoked food, and diesel exhaust. Not smoking removes the largest one outright. See tobacco and cancer for the rest of what smoke carries.

• National Toxicology Program, Report on Carcinogens, 15th Edition: Polycyclic Aromatic Hydrocarbons: 15 Listings, accessed August 6, 2026
• U.S. Environmental Protection Agency, National Primary Drinking Water Regulations, accessed August 6, 2026