Polycyclic aromatic hydrocarbons (PAH): What Are They?

by Joost Nusselder | Last Updated:  June 5, 2022

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PAHs are organic compounds that contain multiple rings of carbon atoms (polycyclic) and at least one aromatic ring (aryl).

They’re found in crude oil and natural gas, as well as in smoke from burning materials like wood, coal, and oil. PAHs can also be found in foods like meat, fish, and vegetables.

What are Polycyclic aromatic hydrocarbons

What are Polycyclic Aromatic Hydrocarbons?

What are they?

Polycyclic Aromatic Hydrocarbons (PAHs) are a group of compounds made up of multiple rings of carbon atoms. They’re also known as polyaromatic hydrocarbons or polynuclear aromatic hydrocarbons. PAHs don’t include benzene, but they do include naphthalene, which is the simplest PAH.

Different Types

PAHs come in all shapes and sizes, including:

  • Anthracene
  • Phenalene
  • Coronene
  • Ovalene

Most PAHs are planar, which means they’re flat. But some PAHs, like coronene, can be non-planar, which means they’re curved.


Some PAHs are chiral, which means they have two different forms that are mirror images of each other. An example of this is benzo[c]phenantrene, which has a slight helical distortion due to repulsion between the closest pair of hydrogen atoms in the two extremal rings.

Benzenoid Hydrocarbons

Benzenoid hydrocarbons are a subset of PAHs that are condensed, polycyclic, unsaturated, and fully-conjugated. That means all the carbon atoms and carbon-carbon bonds have the same structure as benzene. As of 2012, over 300 benzenoid hydrocarbons have been discovered.

What’s the Deal with Aromaticity and Bonding?

Clar’s Rule of Aromaticity

When it comes to PAHs (Polycyclic Aromatic Hydrocarbons), the aromaticity varies. Clar’s rule states that the most important resonance structure of a PAH is the one with the most disjoint aromatic pi sextets (aka benzene-like moieties).

Anthracene and Phenanthrene

Let’s take a look at two examples: phenanthrene and anthracene. Phenanthrene has two Clar structures, one with one aromatic sextet (the middle ring) and one with two (the first and third rings). The latter is the more characteristic electronic nature of the two, so the outer rings have more aromatic character and the central ring is more reactive.

Anthracene has a different story. The resonance structures have one sextet each, which can be at any of the three rings, so the aromaticity is spread out more evenly. This difference in number of sextets is reflected in the differing ultraviolet–visible spectra of these two isomers.


Chrysene has four rings and three Clar structures with two sextets each. Here’s the breakdown:

  • Sextets in the first and third rings
  • Sextets in the second and fourth rings
  • Sextets in the first and fourth rings

Superposition of these structures reveals that the aromaticity in the outer rings is greater than the inner rings.

Redox Potential of Polycyclic Aromatic Compounds

What is a Polycyclic Aromatic Compound?

Polycyclic Aromatic Compounds (PACs) are molecules made of multiple aromatic rings that are connected together. They’re found in nature and are also used in a variety of industrial applications.

What is Redox Potential?

Redox potential is a measure of how easily a substance can be oxidized or reduced. It’s used to measure the stability of a substance and can be used to predict how it will react with other substances.

How Does Redox Potential Relate to PACs?

When PACs are treated with alkali metals, they typically yield radical anions. Bigger PACs form dianions. The redox potential of PACs is related to their size, with bigger PACs having higher redox potentials. Here’s a look at the redox potentials of some common PACs:

  • Anthracene: -2.60 V (-3.18 Fc+/0)
  • Phenanthrene: -2.51 V (-3.1 Fc+/0)

Where Do Polycyclic Aromatic Hydrocarbons Come From?

Natural Sources

PAHs are all around us, and they come from a variety of sources! Here are some of the most common:

  • Bitumen: This sticky, black substance is a major source of PAHs.
  • Fossil fuels: When organic sediments are turned into oil and coal, PAHs are produced.
  • Wildfires: When organic matter is incompletely burned, PAHs are released into the air.
  • Interstellar medium: PAHs make up a big part of the mid-infrared wavelength range of galaxies.
  • Volcanic eruptions: PAHs are released into the atmosphere during eruptions.
  • Anaerobic sediments: Perylene can be generated in anaerobic sediments from existing organic material.

Human Activity

We humans are responsible for a lot of PAHs in the environment. Here’s how:

  • Wood-burning: This is the biggest source of PAHs, particularly in India and China.
  • Industrial processes: This accounts for more than a quarter of global PAH emissions.
  • Extraction and use of fossil fuels: This is the main source of PAHs in industrial countries.
  • Tobacco smoking: Low-temperature combustion like this produces low molecular weight PAHs.
  • High-temperature industrial processes: These typically generate PAHs with higher molecular weights.

What are PAHs and Where are They Found?

What are PAHs?

PAHs, or polycyclic aromatic hydrocarbons, are a group of chemicals found in things like coal, tar, and oil. They can also be found in smoke from burning wood, tobacco, and other materials.

Where are PAHs Found?

PAHs are mostly insoluble in water, so they don’t travel very far. They can stick to fine-grained organic-rich sediments, though. PAHs with two or three rings are more likely to dissolve in water, making them more available for biological uptake and degradation.

PAHs can also be found in the air, since two- to four-ringed PAHs can volatilize and become gaseous. Compounds with five or more rings, however, are usually in solid form and bound to particulate air pollution, soils, or sediments.

Human Exposure to PAHs

Human exposure to PAHs varies depending on a few factors, such as:

  • Smoking rates
  • Fuel types used in cooking
  • Pollution controls on power plants, industrial processes, and vehicles

In developed countries, people are exposed to lower levels of PAHs, while developing and undeveloped countries tend to have higher levels.

Wood-burning open-air cooking stoves are a large source of PAHs globally, since burning solid fuels like wood and coal can lead to high levels of exposure to indoor particulate air pollution containing PAHs.

People who smoke tobacco products, or who are exposed to second-hand smoke, are among the most highly exposed groups. For the general population in developed countries, the diet is the dominant source of PAH exposure, particularly from smoking or grilling meat or consuming PAHs deposited on plant foods.

Vehicles can also be a substantial outdoor source of PAHs in particulate air pollution. Major roadways are sources of PAHs, which may distribute in the atmosphere or deposit nearby.

People can also be occupationally exposed during work that involves fossil fuels or their derivatives, wood-burning, carbon electrodes, or exposure to diesel exhaust. Industrial activity that can produce and distribute PAHs includes aluminum, iron, and steel manufacturing; coal gasification, tar distillation, shale oil extraction; production of coke, creosote, carbon black, and calcium carbide; road paving and asphalt manufacturing; rubber tire production; manufacturing or use of metal working fluids; and activity of coal or natural gas power stations.

Oil spills, creosote, coal mining dust, and smog can also be sources of PAHs.

What are the Health Risks of PAHs?

What are PAHs?

PAHs are polycyclic aromatic hydrocarbons, which are a group of chemicals found in the environment. They’re found in things like coal, oil, gasoline, and tobacco smoke.

What are the Health Risks?

Nobody’s really sure what the health risks of indirect exposure to low levels of PAHs are. But, here’s what we do know:

  • If you breathe in a lot of naphthalene, it can make your eyes and airways feel irritated.
  • If you work with liquid naphthalene or breathe in its vapors, it could be bad news for your health. People have gotten sick from large amounts of exposure, with issues like blood and liver problems.
  • Some PAHs and mixtures of them have been linked to cancer. Yikes!

So, it’s probably best to avoid exposure to PAHs as much as possible.

PAH Metabolites: What You Need to Know

What Are PAH Metabolites?

PAH metabolites are chemicals that enter your body when you come into contact with certain substances. They can be found in the air, water, soil, and even in the food you eat.

How Are PAH Metabolites Measured?

PAH metabolites can be measured by testing the urine of people. Scientists from the CDC tested the urine of 2,504+ people aged 6 and up as part of the National Health and Nutrition Examination Survey (NHANES). This allowed them to estimate how much PAHs had been absorbed by each person.

What Do the Results Show?

The results showed that PAH metabolites were present in most of the participants, indicating that exposure to PAHs is widespread in the US. Smokers tend to have higher levels of PAH metabolites in their urine than non-smokers.

What Does This Mean For Me?

Just because PAH metabolites are present in your body doesn’t necessarily mean that they’re causing harm. But measuring these levels can give health officials a better understanding of what’s normal in a society. It can also help them plan and conduct research on exposure and health effects.

So, if you’re concerned about your exposure to PAHs, the best thing to do is to take steps to reduce your exposure. Here are a few tips:

  • Avoid smoking or being around secondhand smoke
  • Eat organic foods whenever possible
  • Filter your water and air
  • Limit your time outdoors in areas with high air pollution

What Are Polycyclic Aromatic Hydrocarbons?

What Are They Found In?

Ah, polycyclic aromatic hydrocarbons. Sounds like a mouthful, doesn’t it? Well, these little buggers are found in a lot of places. Here’s a quick rundown:

  • Tobacco smoke
  • Home heating (burning wood or oil)
  • Char-grilled food
  • Creosote treated wood products
  • Coal-tar production plants
  • Coking plants
  • Bitumen and asphalt production plants
  • Smoke houses
  • Aluminium production plants
  • Trash incinerators
  • Petroleum, petroleum products or coal
  • Wood or other plant materials
  • Soil where coal, wood, petrol or other products have been burned
  • Food produced from these soils

Where Can You Find Them?

Polycyclic aromatic hydrocarbons are everywhere! You can find them in your own home, at work, and even in the food you eat.

At home, they can be found in tobacco smoke, smoke from home heating, char-grilled food, and cresote treated wood products.

At work, they can be found in coal-tar production plants, coking plants, bitumen and asphalt production plants, smoke houses, aluminium production plants, and trash incinerators. Anywhere that manufactures or uses petroleum, petroleum products or coal, or where wood, or other plant materials are burned, you can find these pesky little particles.

And don’t forget about the soil! If coal, wood, petrol or other products have been burned, the soil can contain polycyclic aromatic hydrocarbons. And if food is produced from these soils, it can also contain these particles.

So, there you have it. Polycyclic aromatic hydrocarbons are everywhere, and you should watch out for them!

What Are the Workplace Exposure Standards for Naphthalene?

What is Naphthalene?

Naphthalene is a white, crystalline, aromatic hydrocarbon found in coal tar and petroleum products. It’s commonly used in the production of plastics, dyes, and pesticides.

What Are the Standards?

If you work with naphthalene, here’s what you need to know about the workplace exposure standards set by Safe Work Australia:

  • Maximum eight hour time weighted average (TWA): 10 parts per million (52 mg/m3)
  • Maximum short term exposure limit (STEL): 15 parts per million (79 mg/m3)

What Do These Standards Mean?

These standards are designed to keep workers safe when handling naphthalene. They’re not limited to any specific industry or operation, so it’s important to understand how to interpret them. That way, you can make sure you’re following the rules and keeping your workplace safe.


In conclusion, polycyclic aromatic hydrocarbons (PAHs) are a fascinating group of compounds that can be found in many everyday items. From the naphthalene in mothballs to the coronene in car exhaust, PAHs are everywhere! So, if you want to learn more about these fascinating compounds, don’t be afraid to take a deep dive into the world of PAHs.

Joost Nusselder, the founder of Lakeside Smokers is a content marketer, dad and loves trying out new food with BBQ Smoking (& Japanese food!) at the heart of his passion, and together with his team he's been creating in-depth blog articles since 2016 to help loyal readers with recipes and cooking tips.