A polycyclic aromatic hydrocarbon (PAH) is any compound made up of several rings connected together. These compounds are often created when organic materials burn incompletely, such as from car exhaust, smoke from cigarettes, smoke from burning trash, cooking meat or grains, or when plants burn at lower temperatures, like during wildfires. The simplest example is naphthalene, which has two rings, and other examples include anthracene and phenanthrene, which each have three rings. PAHs do not have an electric charge, are not attracted to water, and have a flat shape. Many PAHs are colorless. They are also found in natural resources like coal and petroleum. Contact with PAHs can cause cancer, problems during pregnancy, and issues with the heart or blood vessels.

Scientists sometimes study PAHs because they may be used as starting materials for creating materials needed by the earliest forms of life.

Nomenclature and structure

The terms "polyaromatic hydrocarbon" or "polynuclear aromatic hydrocarbon" (abbreviated as PNA) are also used to describe this concept.

By definition, polycyclic aromatic hydrocarbons contain multiple aromatic rings, which means benzene cannot be classified as a PAH. Organizations like the US EPA and CDC consider naphthalene to be the simplest PAH. Most sources exclude compounds that have atoms other than carbon in the rings or have additional chemical groups attached.

A polyaromatic hydrocarbon may have rings of different sizes, some of which are not aromatic. Those with only six-membered rings are called "alternant."

Examples show how these compounds can vary in structure.

Most PAHs, such as naphthalene, anthracene, and coronene, are flat. This shape occurs because the σ-bonds formed by combining sp hybrid orbitals of adjacent carbon atoms lie in the same plane as the carbon atoms. These flat molecules are achiral, meaning the molecule’s plane acts as a mirror of symmetry.

In rare cases, PAHs are not flat. Sometimes, the molecule’s shape forces non-planarity due to the rigidity of carbon-carbon bonds. For example, unlike coronene, corannulene forms a bowl-like shape to reduce bond stress. The two possible shapes (concave and convex) differ by a relatively low energy barrier (about 11 kcal/mol).

In theory, there are 51 structural isomers of coronene that have six fused benzene rings arranged in a cycle, with two edge carbon atoms shared between rings. All these isomers must be non-planar and have significantly higher bonding energy (calculated to be at least 130 kcal/mol) than coronene. As of 2002, none of these isomers had been created.

Some PAHs may appear flat when only the carbon structure is considered, but their hydrogen atoms may cause distortion due to repulsion or crowding. For example, benzo[c]phenanthrene, which has four rings fused in a "C" shape, has a slight helical twist because of repulsion between hydrogen atoms in the outer rings. This effect also causes distortion in picene.

Adding another benzene ring to form dibenzo[c,g]phenanthrene creates crowding between hydrogen atoms at the ends. Adding two more rings in the same direction produces heptahelicene, where the outer rings overlap. These non-planar forms are chiral, and their mirror-image forms can be separated.

Benzenoid hydrocarbons are defined as condensed polycyclic unsaturated, fully-conjugated hydrocarbons that are mostly flat, with all rings having six carbon atoms. Full conjugation means all carbon atoms and carbon-carbon bonds must have the sp structure found in benzene. This group is mostly a subset of alternant PAHs but includes unstable or hypothetical compounds like triangulene or heptacene.

As of 2012, over 300 benzenoid hydrocarbons had been isolated and studied.

Bonding and aromaticity

The level of aromaticity in polycyclic aromatic hydrocarbons (PAHs) depends on the number of separate aromatic pi sextets, which are groups of six pi electrons arranged like benzene. Clar's rule states that the most important structure for describing a PAH's properties is the one with the greatest number of these separate sextets.

For example, phenanthrene has two possible Clar structures. One structure has one aromatic sextet in the middle ring, while the other has two sextets in the first and third rings. The structure with two sextets better represents the molecule's electronic properties. This means the outer rings are more aromatic and less reactive, while the central ring is less aromatic and more reactive. In contrast, anthracene has three possible structures, each with one sextet in a different ring. Because the sextets can be in any ring, aromaticity is spread more evenly throughout the molecule.

This difference in the number of sextets affects the ultraviolet–visible (UV–Vis) spectra of these molecules. More sextets lead to larger energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Phenanthrene absorbs light at 293 nm, while anthracene absorbs at 374 nm.

In chrysene, a four-ring PAH, there are three Clar structures, each with two sextets. One structure has sextets in the first and third rings, another in the second and fourth rings, and another in the first and fourth rings. When these structures are combined, the outer rings show greater aromaticity because they each have a sextet in two of the three structures, while the inner rings have a sextet in only one of the three structures.

Properties

PAHs do not mix well with water and dissolve easily in fats. Larger PAHs generally do not dissolve in water, though smaller PAHs may dissolve. Larger PAHs also do not dissolve well in organic solvents or fats. For example, perylene is a large PAH that has a strong color.

Polycyclic aromatic compounds often produce certain types of ions when treated with alkali metals. Larger PAHs can form ions with two negative charges. The ability of these compounds to gain or lose electrons depends on their size.

Biodegradation

Algae and some invertebrates, like protozoans, mollusks, and polychaetes, can only break down PAHs to a small extent. Some of these organisms accumulate higher amounts of PAHs in their bodies than others. How PAHs are broken down can differ a lot between different invertebrate species. Most vertebrates break down and remove PAHs quickly. PAH levels in tissues do not increase as you move up the food chain.

PAHs change slowly into many different breakdown products. Microbes breaking down PAHs is a major way these chemicals change in the environment. Earthworms and other soil-eating invertebrates may help break down PAHs faster by either breaking them down directly or by creating better conditions for microbes to work. When PAHs break down without living organisms in the air and top water layers, they can form new chemicals. Some of these new chemicals may be more harmful, dissolve better in water, and move more easily than the original PAHs.

Distribution in the environment

Most PAHs do not dissolve in water, which limits how far they can move in the environment. However, they can stick to fine-grained sediments that are rich in organic material. The ability of PAHs to dissolve in water decreases as their molecular size increases.

PAHs with two rings and, to a lesser extent, those with three rings can dissolve in water. This makes them easier for living organisms to take in and break down. PAHs with two to four rings can also evaporate into the air, where they are mostly found as gases. However, the state of four-ring PAHs in the air may depend on temperature. In contrast, PAHs with five or more rings have very low solubility in water and do not evaporate easily. These compounds are mostly found in solid form, attached to particles in the air, soil, or sediment. When in solid form, these PAHs are harder for living things to take in or break down, which makes them stay in the environment longer.

People are exposed to PAHs differently around the world. This depends on things like smoking rates, the types of fuel used for cooking, and pollution controls at power plants, factories, and vehicles. Countries with stricter pollution rules, cleaner cooking methods (like gas or electricity instead of coal or biofuels), and laws that stop public smoking usually have lower PAH exposure. In contrast, developing and undeveloped countries often have higher PAH exposure. Studies have shown that surgical smoke contains PAHs.

Burning solid fuels like coal and biofuels at home for cooking and heating is a major source of PAH emissions globally. In developing countries, this leads to high exposure to indoor air pollution containing PAHs, especially for women and children who spend more time at home.

Vehicles like cars and trucks can release PAHs into the air as part of outdoor pollution. Major roads are common sources of PAHs, which can spread through the air or settle nearby. Catalytic converters can reduce PAH emissions from gasoline vehicles by about 25 times.

People can also be exposed to PAHs through jobs that involve using fossil fuels, wood-burning, or working with materials like carbon electrodes or diesel exhaust. Industrial activities that produce or spread PAHs include manufacturing aluminum, iron, and steel; coal gasification; tar distillation; shale oil extraction; production of coke, creosote, carbon black, and calcium carbide; road paving and asphalt manufacturing; rubber tire production; use of metal working fluids; and operations at coal or natural gas power plants.

In developed countries, people who smoke tobacco or are exposed to secondhand smoke are among the groups most affected by PAHs. Tobacco smoke contributes to about 90% of indoor PAH levels in homes where people smoke. For the general population in developed countries, diet is the main source of PAH exposure, especially from eating grilled meat or plant foods like leafy vegetables that may have PAHs deposited on them during growth. Exposure can also occur from drinking alcohol aged in charred barrels, flavored with peat smoke, or made with roasted grains. PAHs are usually found in very low amounts in drinking water.

PAHs spread from urban and suburban areas through road runoff, sewage, and air movement. Particulate air pollution can then settle in the environment. Soil and river sediment near industrial sites like creosote manufacturing facilities can have high PAH levels. Oil spills, creosote, coal mining dust, and other fossil fuel sources can also spread PAHs in the environment.

PAHs with two or three rings can spread widely when dissolved in water or as gases in the air. PAHs with larger molecular sizes tend to spread locally or regionally by sticking to particles in air or water until the particles settle. PAHs strongly attach to organic carbon, so highly organic sediments in rivers, lakes, and oceans can hold large amounts of PAHs.

A study by the British Geological Survey found PAH levels in urban soils at 76 locations in Greater London. The study showed that parent PAH levels ranged from 4 to 67 mg/kg (dry soil weight) with an average of 18 mg/kg, while total PAH levels (including different forms) ranged from 6 to 88 mg/kg. Fluoranthene and pyrene were the most common PAHs. Benzo[a]pyrene, the most toxic parent PAH, had an average background level of 6.9 mg/kg in London soils. London soils had more stable four- to six-ring PAHs, which are linked to sources like coal and oil burning and traffic pollution. However, the PAHs in London soils had also been changed by processes like evaporation and breakdown by microbes.

In the UK, burning moorland vegetation generates PAHs that mix into peat. Burning heather initially produces more two- and three-ring PAHs than four- to six-ring PAHs in surface sediments. Over time, lower molecular weight PAHs decrease due to natural decay and sunlight. Statistical methods like principal component analysis helped researchers connect PAH sources (burnt moorland) to their movement (sediment in streams) and where they settle (reservoir beds).

PAH levels in river and estuary sediments depend on factors like distance from industrial or city areas, wind direction, and how far they are from major roads. Tides also affect PAH levels in estuaries by mixing cleaner ocean water with freshwater. Pollutant levels in estuaries often drop near the river mouth. Understanding PAHs in estuary sediments is important for protecting fish like mussels and preserving habitats because PAHs can harm organisms that eat sediment or drift in water. In the UK, surface sediments in rivers and estuaries usually have lower PAH levels than buried sediments 10–60 cm below the surface, showing reduced industrial activity and better environmental laws. PAH levels in UK estuaries range from about 19 to 16,163 µg/kg (dry sediment weight) in the River Clyde and 626 to 3,766 µg/kg in the River Mersey. Sediments with higher organic carbon content tend to hold more PAHs because organic matter strongly attracts them. A similar pattern

Human health

Cancer is a major health risk from exposure to PAHs. Exposure to PAHs has also been linked to heart and blood vessel diseases and problems with fetal development.

PAHs are connected to cancers of the skin, lungs, bladder, liver, and stomach in well-known studies on animals. Specific PAH compounds are listed in the "Regulation and Oversight" section as possible or probable human carcinogens.

Historically, PAHs helped scientists understand how environmental chemicals can harm health, including how they cause cancer. In 1775, a surgeon named Percivall Pott noticed that chimney sweepers had high rates of scrotal cancer and thought the cause was exposure to soot from their work. A century later, Richard von Volkmann found that workers in Germany’s coal tar industry had more skin cancer. By the early 1900s, scientists widely accepted that soot and coal tar caused cancer. In 1915, Yamigawa and Ichicawa first created cancer in rabbits by applying coal tar to their ears.

In 1922, Ernest Kennaway found that the cancer-causing part of coal tar was a compound made only of carbon and hydrogen. This compound was later linked to a fluorescent pattern similar to benz[a]anthracene, a PAH that causes tumors. Cook, Hewett, and Hieger later matched the fluorescent pattern of benzo[a]pyrene, a PAH, to the cancer-causing part of coal tar. This was the first time a specific compound from an environmental mixture was shown to be carcinogenic.

In the 1930s and later, scientists in Japan, the UK, and the US, including Richard Doll, found that workers exposed to PAH-rich environments, such as those in coke ovens and coal processing, had higher rates of lung cancer death.

The structure of a PAH affects whether it causes cancer. Some PAHs are genotoxic, meaning they change DNA and start cancer. Others are not genotoxic but may help cancer grow or spread.

PAHs that cause cancer are first changed by enzymes into metabolites that react with DNA, leading to mutations. If these mutations occur in genes that control cell growth, cancer can develop. Mutagenic PAHs, like benzo[a]pyrene, often have four or more rings and a "bay region," which makes them more reactive. These PAHs form metabolites such as diol epoxides, quinones, and radical PAH cations. These metabolites can attach to DNA, creating bulky structures called DNA adducts. Stable adducts may cause DNA errors, while unstable adducts remove parts of DNA. If these errors are not fixed, they can turn normal genes into cancer-causing genes. Quinones can also create harmful molecules that damage DNA.

Enzymes in the cytochrome family (CYP1A1, CYP1A2, CYP1B1) process PAHs into diol epoxides. Exposure to PAHs can increase these enzymes’ activity, making them convert PAHs into mutagenic metabolites faster. PAHs bind to the aryl hydrocarbon receptor (AhR), which activates these enzymes. Sometimes, these enzymes may protect against PAH harm, but this is not fully understood.

Low-molecular-weight PAHs, with two to four rings, are more likely to help cancer grow after it starts. In this stage, a cell with a cancer-causing mutation is no longer controlled by nearby cells and begins to multiply. Low-molecular-weight PAHs with bay regions can disrupt gap junctions, which are channels that help cells communicate, and affect proteins that control cell growth. These PAHs do not need to be changed by enzymes first. They are common in the environment and may increase cancer risk during the growth phase.

Exposure to PAHs in adults has been linked to heart and blood vessel disease. PAHs are part of the many harmful chemicals in tobacco smoke and polluted air, which may contribute to these diseases.

In lab studies, animals exposed to certain PAHs developed more artery plaques (atherogenesis). PAHs may cause this by activating the CYP1B1 enzyme in blood vessel cells. This enzyme processes PAHs into quinones that attach to DNA, causing mutations. These mutations may lead to uncontrolled growth of blood vessel cells or their movement into arteries, steps in plaque formation. Quinones also create harmful molecules that may alter genes involved in plaque development.

Oxidative stress from PAH exposure can cause inflammation, which is linked to atherosclerosis and heart disease. Signs of PAH exposure in humans are associated with signs of inflammation, suggesting that oxidative stress may be a way PAHs cause heart disease.

Studies in Europe, the US, and China have linked PAH exposure during pregnancy, through air pollution or parents’ work, to poor fetal growth, weaker immune systems, and lower intelligence in children.

Regulation and oversight

Some government groups, such as the European Union, NIOSH, and the United States Environmental Protection Agency (EPA), set limits on the amounts of PAHs in air, water, and soil. The European Commission has set limits on the amounts of 8 cancer-causing PAHs in products that touch the skin or mouth.

The US EPA, the US Agency for Toxic Substances and Disease Registry (ATSDR), and the European Food Safety Authority (EFSA) have selected certain polycyclic aromatic hydrocarbons as priorities because they can cause cancer or harm DNA, and because they can be measured. These include:

Detection and optical properties

A database exists to help track polycyclic aromatic hydrocarbons (PAHs) in space. Scientists often use tools like gas chromatography-mass spectrometry, liquid chromatography with ultraviolet-visible or fluorescence methods, or quick test strips to detect PAHs in materials. The structures of PAHs have been studied using infrared spectroscopy.

PAHs have unique UV absorbance patterns. These patterns often have many absorbance bands, and each ring structure has a different pattern. This helps scientists identify different types of PAHs. Most PAHs also glow when light excites them, emitting specific wavelengths of light. The way their electrons are arranged causes this glow and other properties, such as semi-conducting behavior in larger PAHs.

PAHs may be common in space. They may have formed about two billion years after the Big Bang and are linked to new stars and planets. More than 20% of the universe’s carbon may be connected to PAHs. Scientists think PAHs could have been part of the earliest forms of life. Light from the Red Rectangle Nebula shows signs of anthracene and pyrene. This idea was debated because it suggests that as nebulae near the end of their lives, carbon and hydrogen in their centers may be carried by stellar winds. As these materials cool, they might bond and form large particles. Adolf Witt and his team suggested that PAHs, which may have helped create life on Earth, likely form in nebulae.

When PAHs are exposed to conditions in space, they can change through processes like hydrogenation, oxygenation, and hydroxylation. These changes lead to more complex organic compounds, which are steps toward forming amino acids and nucleotides, the building blocks of proteins and DNA. These changes may explain why PAHs are not often found in cold, dense regions of space or in the outer layers of protoplanetary disks.

Scientists are studying how simple organic compounds can form complex PAHs at low temperatures. These chemical processes may explain why PAHs are found in the cold atmosphere of Saturn’s moon Titan. They may also be important in the "PAH world hypothesis," which suggests these compounds could have helped create the building blocks of life as we know it.