What Is a Polynuclear Aromatic Hydrocarbon?

A Hydrocarbon Made Up of Fused Aromatic Ring Molecules

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PAHs can be found in fossil fuels.

DuKai photographer / Getty Images

A polynuclear aromatic hydrocarbon is a hydrocarbon made up of fused aromatic ring molecules. These rings share one or more sides and contain delocalized electrons. Another way to consider PAHs is molecules made by fusing two or more benzene rings.

Polynuclear aromatic hydrocarbon molecules contain only carbon and hydrogen atoms.

Also Known As: PAH, polycyclic aromatic hydrocarbon, polyaromatic hydrocarbon

Examples

There are numerous examples of polynuclear aromatic hydrocarbons. Typically, several different PAHs are found together. Examples of these molecules include:

  • anthracene
  • phenanthrene
  • tetracene
  • chrysene
  • pyrene (note benzo[a]pyrene was the first carcinogen to be discovered)
  • pentacene
  • corannulene
  • coronene
  • ovalene

Properties

Polynuclear aromatic hydrocarbons are lipophilic, nonpolar molecules. They tend to persist in the environment because PAHs are not very soluble in water. While 2- and 3-ring PAHs are somewhat soluble in aqueous solution, the solubility decreases nearly logarithmically as molecular mass increases. 2-, 3-, and 4-ring PAHs are sufficiently volatile to exist in the gas phases, while larger molecules exist as solids. Pure solid PAHs may be colorless, white, pale yellow, or pale green.

Sources

PAHs are organic molecules that form from a variety of natural and anthropogenic reactions. Natural PAHs form from forest fires and volcanic eruptions. The compounds are numerous in fossil fuels, such as coal and petroleum.

Man contributes PAHs by burning wood and by the incomplete combustion of fossil fuels. The compounds occur as a natural consequence of cooking food, particularly when food is cooked at a high temperature, grilled, or smoked. The chemicals are released in cigarette smoke and from burning waste.

Health Effects

Polynuclear aromatic hydrocarbons are extremely important because they are associated with genetic damage and diseases. Also, the compounds persist in the environment, leading to increased problems over time. PAHs are toxic to aquatic life. In addition to toxicity, these compounds are often mutagenic, carcinogenic, and teratogenic. Prenatal exposure to these chemicals is associated with lowered IQ and childhood asthma.

People get exposed to PAHs from breathing contaminated air, eating food that contains the compounds, and from skin contact. Unless a person works in an industrial setting with these chemicals, exposure tends to be long-term and low-level, so there aren't medical treatments to address the effects. The best defense against health effects from PAH exposure is to become aware of situations that elevate risk: breathing smoke, eating charred meat, and touching petroleum products.

PAHs Classified as Carcinogens

The Environmental Protection Agency has identified seven polynuclear aromatic hydrocarbons as likely human carcinogens, or cancer-causing agents:

  • benzo[a]anthracene
  • benzo[a]pyrene
  • benzo[b]fluoranthene
  • benzo[k]fluoranthene
  • chrysene
  • dibenzo(a,h)anthracene
  • indeno(1,2,3-cd)pyrene

Although the emphasis is on avoiding exposure to PAHs, these molecules are useful for making medicines, plastics, dyes, and pesticides.