What Is a PAH Chemical and How Does It Affect Health?

Polycyclic aromatic hydrocarbons, commonly called PAHs, are a family of over a hundred chemicals built from fused rings of carbon and hydrogen atoms. They form whenever organic material burns incompletely, which means they show up almost everywhere: in wildfire smoke, car exhaust, grilled meat, cigarette smoke, and industrial emissions. PAHs are considered some of the most widespread organic pollutants on Earth, and several of them are classified as probable or known carcinogens. The concern around these chemicals runs deeper than cancer alone, touching neurodevelopment in children, heart defects in fish embryos, and heritable changes to DNA activity that researchers are only beginning to understand.

What PAHs Actually Are

A PAH molecule is made of two or more benzene-like carbon rings fused together side by side. The simplest PAH, naphthalene, has just two rings and is the chemical responsible for the smell of mothballs. Add more rings and you get compounds like pyrene (four rings), benzo[a]pyrene (five rings), and coronene (seven rings). These larger molecules tend to have high melting and boiling points, meaning they exist as solids at room temperature. They also dissolve poorly in water but readily in fats and oils, which is why they accumulate in fatty tissues of living organisms rather than washing away in rain.¹ That fat-solubility is a key reason PAHs cause problems: once they enter a body, they tend to stick around and concentrate rather than being quickly flushed out.

Regulatory agencies generally focus on a shortlist of PAHs. The U.S. Environmental Protection Agency tracks 16 “priority” PAHs, selected because they are common in the environment, measurable with standard lab methods, and known or suspected to cause harm. Among those, benzo[a]pyrene (often shortened to BaP) gets the most attention because it is one of the most potent carcinogens in the group and serves as a benchmark against which other PAHs are compared.

Where PAHs Come From

Any time carbon-based material burns without enough oxygen for complete combustion, PAHs form. That covers an enormous range of activities. Coal and biomass burning are dominant sources in many parts of the world, along with vehicle exhaust and industrial processes.² Coke ovens, aluminum smelters, asphalt plants, and oil refineries all release PAHs as part of their normal operations. Tobacco smoke is a major individual-level source, and even something as mundane as burning a candle or a stick of incense generates measurable amounts.

Wildfires are an increasingly significant contributor. A global modeling study found that fires can account for up to about 42% of local PAH concentrations in fire-prone regions such as Australia, sub-Saharan Africa, Siberia, and Canada, although across the entire globe the wildfire share of total PAH levels sits closer to 5%.³ Even in cities that are not themselves on fire, wildfire smoke drifting in from hundreds of kilometers away can dominate pollutant levels. Researchers in Calgary found that a single wildfire smoke day deposited over 20% of the total PAH loading measured on urban surfaces during an entire three-month sampling window.⁴ With wildfire seasons growing longer and more intense in many regions, this source is getting harder to ignore.

Indoor burning of biomass for cooking and heating is one of the largest PAH sources globally, particularly in rural areas of developing countries. Burning wood, crop residues, or dung in traditional stoves releases large quantities of two- to four-ring PAHs, and the output gets worse when the fire is starved of oxygen or when the fuel has high moisture content.⁵ Real-world measurements from cooking stoves in rural China confirmed that indoor biomass burning is a major contributor to PAH emissions, with wide variability depending on the type of stove, fuel, and ventilation.⁶

How PAHs Travel and Persist

Once released into the air, lighter PAHs exist as gases and heavier ones cling to tiny soot particles. Both forms can travel long distances. A global atmospheric model estimated that the average lifetime of PAHs in the atmosphere is less than a day, which sounds short but is more than enough time for winds to carry them hundreds or even thousands of kilometers.⁷ Researchers have detected PAHs at monitoring stations in the high Arctic, far from any combustion source, confirming that these pollutants reach the most remote corners of the planet.

Sunlight is the main force breaking PAHs down in the atmosphere. Studies of Arctic aerosols showed that PAH concentrations drop sharply when the sun returns after the polar winter, with specific chemical ratios confirming active photochemical degradation during the polar spring.⁸ In water and soil, however, the heavier PAHs are much more stubborn. Sheltered from UV light and resistant to chemical breakdown, they can persist in sediments for decades.

PAHs in Your Food

For non-smokers who do not work in heavy industry, food is often the largest source of PAH exposure. Grilling, smoking, barbecuing, and charring meat are the classic culprits. PAHs form in grilled food through a few overlapping pathways: the high heat breaks down organic compounds on the food’s surface directly, the charcoal or wood produces PAH-laden smoke that settles on the food, and fat dripping onto hot coals flashes into smoke that carries PAHs back up onto the meat.⁹ The formation process is influenced by cooking temperature, duration, and the type of meat; fattier cuts tend to produce more PAHs because of the drip-and-flare cycle.¹⁰

You can reduce dietary PAH exposure without giving up grilling entirely. Preventing fat from dripping directly onto the heat source, using lower-temperature cooking methods, avoiding charring, and trimming visible char from meat all help. Gas grills generally produce less PAH contamination than charcoal, and wrapping food in foil creates a barrier between the meat and the smoke.

How PAHs Cause Cancer

PAHs themselves are not directly toxic to DNA. They become dangerous only after the body tries to break them down. When you inhale or ingest a PAH like benzo[a]pyrene, liver enzymes (primarily from the cytochrome P450 family) attempt to make the molecule water-soluble so it can be excreted. In doing so, they convert the PAH into an intermediate called a diol-epoxide, which is far more reactive than the original compound.¹¹ There are actually multiple enzyme pathways that process PAHs, each generating a different type of reactive product: diol-epoxides, radical cations, and o-quinones.¹² All three types can damage DNA.

The diol-epoxide of benzo[a]pyrene (called BPDE) is the best-studied of these reactive intermediates. It physically attaches to DNA, forming what is called a DNA adduct. In lab experiments, the relationship between adduct levels and mutations was linear all the way down to the lowest doses tested, with no apparent safe threshold.¹³ The practical implication is sobering: even small exposures carry some risk, because every BPDE adduct has a chance of causing a mutation when the cell copies its DNA.

BPDE has a particular affinity for certain spots on the genome. One well-documented target is codon 273 of the P53 gene, which is the single most commonly mutated gene in human cancers. When researchers placed a BPDE adduct at this spot in human cells, the resulting mutation rate was roughly 17–27%, and the dominant error was a specific letter swap (G to T) that disables P53’s ability to suppress tumors.¹⁴ This finding helps explain why lung tumors in smokers frequently carry exactly that mutation: the PAHs in tobacco smoke are essentially writing a known error into the body’s primary anti-cancer gene.

Health Effects Beyond Cancer

PAH exposure affects more than cancer risk. One of the more alarming areas of research involves prenatal brain development. A study following children in New York City found a strong dose-response relationship between PAH levels measured during pregnancy and reductions in white matter across the left hemisphere of the brain years later. Those reductions were in turn linked to slower processing speed on intelligence tests and more behavioral problems, including symptoms associated with ADHD and conduct disorder.¹⁵ Other reviews have linked prenatal PAH exposure to a broader range of neurodevelopmental outcomes including anxiety, depression, autism-spectrum features, and poorer school performance.¹⁶

These effects likely operate through a different mechanism than the DNA-adduct pathway responsible for cancer. One emerging explanation involves epigenetics: PAH exposure has been associated with changes in DNA methylation, histone modifications, and small RNA regulation.¹⁷ In a study of newborns, higher prenatal PAH exposure was associated with lower levels of global DNA methylation in cord blood.¹⁸ These epigenetic shifts do not change the DNA sequence itself but alter which genes are active and how strongly they are expressed, potentially setting the stage for developmental disruptions or disease susceptibility that unfolds over years.

Workers Who Face the Highest Exposure

Several occupations carry elevated PAH exposure, and international agencies have formally classified some of these workplaces as posing a known cancer risk. Coke production, coal tar distillation, aluminum smelting, and coal gasification are all on the list.¹⁹ Among these, coke oven workers have been studied most extensively. Research on European coke oven workers found that biomarkers of PAH exposure and DNA damage increased with proximity to the ovens: workers stationed on top of the ovens, where fumes are most concentrated, showed the highest levels of urinary 1-hydroxypyrene (a PAH metabolite) and carcinogenic DNA adducts in bladder cells.²⁰

The connection to lung cancer in coke oven workers has been studied across British plants. The evidence was suggestive but not straightforward: workers who spent five or more years on oven tops had a relative risk of about 1.8 for lung cancer, which was statistically significant, but there was no clear dose trend at lower exposure levels.²¹ This pattern is common in occupational studies: it is hard to disentangle the contribution of PAHs from smoking, other workplace exposures, and the general challenges of measuring cumulative exposure over decades.

PAHs in Water and Wildlife

Oil spills are the most dramatic source of PAH contamination in aquatic environments, but runoff from roads, atmospheric deposition, and industrial discharge are the routine, ongoing sources. Because PAHs are fat-soluble and poorly water-soluble, they tend to accumulate in sediments and in the bodies of organisms that live near or feed from the bottom.

Fish are particularly vulnerable during embryonic development. Research on zebrafish showed that even brief exposure to crude oil PAHs during the embryonic stage altered heart shape and reduced swimming performance nearly a year later in adulthood, pointing to lasting cardiac damage from a developmental window lasting just days.²² This cardiotoxicity pattern is not limited to a single oil type. Experiments using two geologically distinct crude oils produced nearly identical heart defects in developing zebrafish, including impaired ventricular contraction and abnormal looping of the heart chambers.²³ In Pacific herring, a species heavily affected by the Exxon Valdez spill, cardiac arrhythmia appeared at PAH tissue concentrations below 1 micromole per kilogram, suggesting the developing heart is an extremely sensitive target.²⁴

PAHs also move through marine food webs, though the pattern is not always the simple “concentrations increase up the food chain” story associated with some other pollutants. In a coral reef ecosystem in the South China Sea, researchers documented trophic amplification: PAH levels increased as they moved from prey to predator, aggravating the contamination burden on higher-level reef species.²⁵ But in a temperate bay ecosystem in China, the opposite was observed: PAH concentrations were highest in algae and bottom-dwelling invertebrates and actually declined in fish, a pattern called biodilution.²⁶ The difference probably comes down to whether organisms at higher levels can metabolize and excrete PAHs efficiently. Fish generally have more active detoxification enzymes than invertebrates, which helps explain why they sometimes carry lower concentrations despite eating contaminated prey.

Cleaning Up PAH-Contaminated Soil

Because heavier PAHs resist natural breakdown, contaminated industrial sites can remain polluted for decades. Traditional cleanup methods include digging out and incinerating contaminated soil, or using chemical oxidants to break PAHs apart in place. Both are expensive and disruptive. Bioremediation, using living organisms to degrade the contaminants, offers a cheaper and less invasive alternative.

The most effective bioremediation strategies combine plants and bacteria working together. In one study of heavily contaminated soil, a combination of the bacterium Kocuria sp. and ryegrass removed about 70% of PAHs, roughly double the removal seen in untreated control soil.²⁷ In another experiment, certain PAHs including naphthalene and benzo[a]anthracene dropped below detectable levels in soil treated with a grass-plus-bacteria combination.²⁸ The plant roots create a hospitable zone in the soil for PAH-degrading microbes, and the microbes in turn improve plant growth by reducing the toxicity of the surrounding soil. The approach works best for lighter two- and three-ring PAHs; the heaviest compounds resist microbial attack and degrade far more slowly.

Measuring PAH Exposure in People

Doctors and researchers cannot easily measure PAHs directly in the body because the compounds are metabolized quickly. Instead, they rely on urinary metabolites as stand-in markers. The most widely used is 1-hydroxypyrene, a breakdown product of the PAH pyrene. It is relatively abundant in urine, straightforward to measure, and correlates well with overall PAH exposure from both air and food.²⁹ This marker has been used to compare exposure across populations. For example, a study of three-year-old children in Ukraine found that those living near a steel mill and coke facility had higher urinary 1-hydroxypyrene levels than children in a cleaner urban environment.³⁰

Urinary 1-hydroxypyrene is useful but imperfect. Pyrene is not itself carcinogenic, so measuring its metabolite tells you how much total PAH came in but not how much of the dangerous cancer-causing metabolites actually formed inside the body. For that, researchers sometimes measure DNA adducts directly in blood cells or other tissues, which gives a picture of how much damage has already been done at the genetic level. Adduct measurements are more technically demanding and expensive, so they are used mostly in research and high-risk occupational monitoring rather than routine screening.

PAHs as Useful Chemicals

Despite the serious health and environmental concerns, PAHs are not exclusively a problem. Their electronic properties, specifically the way electrons can move freely across their ring systems, have made them interesting to materials scientists. PAH-based compounds have been explored as components in organic thin-film transistors, the flexible electronics found in some displays and sensors. Hetero-PAHs, which incorporate atoms like nitrogen or sulfur into the ring structure, have shown promising performance in early transistor prototypes.³¹ More broadly, PAHs have been investigated for applications in optoelectronics and advanced materials, where their chemical stability and tunable electronic behavior are advantages rather than liabilities.³² The same fused-ring structure that makes PAHs persistent pollutants in the environment also makes them robust building blocks for engineered materials, a reminder that the properties of a chemical are rarely all good or all bad.