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Author
Aleksandra Buha

Lead Editor
Maria M. Williams

Note: we also offer a longer version of this article that includes more scientific details on the toxicology of this chemical group!

Overview


The term polycyclic aromatic hydrocarbons (PAHs) refers to a ubiquitous group of several hundred chemically-related and environmentally persistent organic compounds of various structures and varied toxicity. Most of them are formed by a process of thermal decomposition and then recombination of these organic molecules. PAHs enter the environment through various routes and can usually be found as a mixture containing two or more of these compounds, e.g. soot. However, some PAHs are manufactured and these pure PAHs usually exist as colorless, white, or pale yellow solids.

Polycyclic aromatic hydrocarbons affect organisms through various toxic actions. They have been shown to cause carcinogenic and mutagenic effects and also have been found to be potent immunosuppressants (decreasing immune function). The most extensively studied PAHs are 7,12-dimethylbenzo anthracene (DMBA) and benzo(a)pyrene (BaP).

Chemical Characteristics



Polycyclic aromatic hydrocarbons have two or more single or fused aromatic rings with a pair of carbon atoms shared between rings in their molecules. In particular, the term "PAH" refers to the compounds consisting of only carbon and hydrogen atoms. PAHs containing up to six fused aromatic rings are often known as "small" PAHs, and those containing more than six aromatic rings are called "large" PAHs. Due to the availability of samples of the various small PAHs, the majority of research on PAHs has been conducted on small PAHs.
The general characteristics of PAHs are high melting and boiling points (therefore they are solid), low vapor, pressure and very low aqueous solubility, which both tend to decrease with increasing molecular weight, whereas resistance to oxidation and reduction increases. PAHs are highly lipophilic and therefore very soluble in various organic solvents. Aqueous solubility decreases for each additional ring.

The simplest PAHs, as defined by the International Union of Pure and Applied Chemistry (IUPAC), are phenanthrene and anthracene, which both contain three fused aromatic rings. Smaller molecules, such as benzene, are not PAHs. Naphthalene, which consists of two coplanar six-membered rings sharing an edge, is another aromatic hydrocarbon. By formal convention, it is not a true PAH, though is referred to as a bicyclic aromatic hydrocarbon.

(Image of selected PAHs from ATSDR.)

Sources



Sources of PAHs can be both natural and anthropogenic.
Natural sources include:

  • forest and grass fires
  • oil seeps
  • volcanoes
  • chlorophyllous plants, fungi, and bacteria

Anthropogenic sources of PAHs include:

  • petroleum
  • electric power generation
  • refuse incineration
  • home heating
  • production of coke, carbon black, coal tar, and asphalt;
  • internal combustion engines

Uses


PAHs are not synthesized chemically for industrial purposes. Rather than industrial sources, the major source of PAHs is the incomplete combustion of organic material such as coal, oil, and wood. However, there are few commercial uses for many PAHs. They are mostly used as intermediaries in pharmaceuticals, agricultural products, photographic products, thermosetting plastics, lubricating materials, and other chemical industries. Other PAHs may be contained in asphalt used for the construction of roads, as well as roofing tar. Precise PAHs, specific refined products, are used also in the field of electronics, functional plastics and liquid crystals.

Routes of Exposure


The major route of exposure to PAHs in the general population is from breathing ambient and indoor air, eating food containing PAHs, smoking cigarettes, or breathing smoke from open fireplaces. Tobacco smoke contains a variety of PAHs, such as benzo(a)pyrene, and more than 40 known or suspected human carcinogens. For non-smokers the main route of exposure is through food. PAH concentrations in foodstuffs vary. Charring meat or barbecuing food over a charcoal, wood, or other type of fire greatly increases the concentration of PAHs. Some crops, such as wheat, rye, and lentils, may synthesize PAHs or absorb them via water, air, or soil.

Water can also contain substantional amounts of PAHs since these chemicals can leach from soil into water, or they can enter water from industrial effluents and accidental spills during oil shipment at sea. Soil also contains PAHs, primarily from airborne fallout. Therefore, PAH exposure occurs on a regular basis for most people. Occupational exposure may also occur, mainly in workers breathing in exhaust fumes, such as mechanics, street vendors, motor vehicle drivers, as well as in those involved in mining, metal working or oil refining. Routes of exposure include ingestion, inhalation, and dermal contact in both occupational and non-occupational settings.

The formation of PAHs in Meat Cooked at High Temperatures

Polycyclic aromatic hydrocarbons (PAHs) are chemicals formed when various meat, including beef, pork, fish, or poultry, is cooked using high-temperature methods, such as pan frying or grilling directly over an open flame. Since PAHs have been found to be mutagenic, eating of meat which is cooked at high temperatures can increase the risk of cancer.
PAHs can be formed when fat and juices from meat grilled directly over an open fire drip onto the fire, causing flames. These flames contain PAHs that then adhere to the surface of the meat. PAHs can also be formed during other food preparation processes, such as smoking of meats. The formation of PAHs varies by meat type and cooking method, for example cooking methods that expose meat to smoke or charring contribute to PAH formation. PAHs become capable of damaging DNA only after they are metabolized by specific enzymes in the body in the process called "bioactivation." Studies have shown that exposure to PAHs can cause cancer in animal models while population studies have not established a definitive link between PAH exposure from cooked meats and cancer in humans. One difficulty with conducting such studies is that it can be difficult to determine the exact level of PAH exposure a person gets from cooked meats. Nevertheless, researchers found that high consumption of well-done, fried, or barbecued meats was associated with increased risks of colorectal, pancreatic, and prostate cancer.
Currently, no Federal guidelines address the consumption of foods containing PAHs. Even though no specific guidelines for PAH consumption exist, concerned individuals can reduce their exposure by using several cooking methods:

  • Avoiding direct exposure of meat to an open flame or a hot metal surface and avoiding prolonged cooking times (especially at high temperatures) can help reduce HCA and PAH formation.
  • Using a microwave oven to cook meat prior to exposure to high temperatures can also substantially reduce HCA formation by reducing the time that meat must be in contact with high heat to finish cooking.
  • Continuously turning meat over on a high heat source can substantially reduce HCA formation compared with just leaving the meat on the heat source without flipping it often.
  • Removing charred portions of meat and refraining from using gravy made from meat drippings can also reduce HCA and PAH exposure.

Metabolism


Since exposure to PAHs is never exposure to single PAHs, only by understanding what differences may occur in mixtures of PAHs can we accurately assess the dangers of PAHs. Due to the high lipophilicity (affinity for lipids or fats) of this class of compounds, after ingestion and inhalation they can be easily absorbed and occur in almost all internal organs, particularly in those rich in adipose tissue (fat-rich tissues). These organs can even serve as storage depots from which the hydrocarbons can be gradually released.

Once they enter the organism, polycyclic aromatic hydrocarbons require activation by specific enzymes. The enzyme system primarily responsible for PAH metabolism is the system that performs oxidation. After oxidation these metabolites are conjugated and excreted in faeces and urine.

Human Health Effects


Acute or Short-term Health Effects

The effects on human health will depend mainly on the length and route of exposure, the amount or concentration of PAHs one is exposed to, and of course the innate toxicity of PAHs. A variety of other factors can also affect health impacts including subjective factors such as pre-existing health status and age. The ability of PAHs to induce short-term health effects in humans is not clear. Occupational exposures to high levels of pollutant mixtures containing PAHs has resulted in symptoms such as eye irritation, nausea, vomiting, diarrhea and confusion. However, it is not known which components of the mixture is responsible for these effects and other compounds commonly found with PAHs may be the cause of these symptoms.

Mixtures of PAHs are also known to cause skin irritation and inflammation. Anthracene, benzo(a)pyrene and naphthalene are direct skin irritants while anthracene and benzo(a)pyrene are reported to be skin sensitizers, i.e. cause an allergic skin response in animals and humans (IPCS, 1998).

Chronic or Long-term Health Effects

Health effects from chronic or long-term exposure to PAHs may include decreased immune function, cataracts, kidney and liver damage (e.g. jaundice), breathing problems, asthma-like symptoms, and lung function abnormalities, and repeated contact with skin may induce redness and skin inflammation. The harmful effects that may occur also largely depend on the way people are exposed.

Carcinogenicity

Although unmetabolized PAHs can have toxic effects, a major concern is the ability of their reactive metabolites to bind to cellular proteins and DNA. The resulting biochemical disruptions and cell damage lead to mutations, developmental malformations, tumors, and cancer. Evidence indicates that mixtures of PAHs are carcinogenic to humans. The evidence comes primarily from occupational studies of workers exposed to mixtures containing PAHs and these long-term studies have shown an increased risk of predominantly skin and lung, but as well as bladder and gastrointestinal cancers. However, it is not clear from these studies whether exposure to PAHs was the main cause as workers were simulatneoulsy exposed to other cancer-causing agents (e.g. aromatic amines).

Based on the available evidence, both the International Agency for Research on Cancer(IARC, 1987) and US EPA (1994) classified a number of PAH as carcinogenic to animals and some PAH rich mixtures as carcinogenic to humans. The EPA has classified seven PAH compounds as probable human carcinogens: benzo(a)anthracene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, chrysene, dibenz(ah)anthracene, and indeno(1,2,3-cd)pyrene.

Teratogenicity

Embryotoxic effects of PAHs have been described in experimental animals exposed to PAH such as benzaanthracene, benzoapyrene, and naphtalene. Laboratory studies conducted on mice have demonstrated that ingestion of high levels of benzoapyrene during pregnancy resulted in birth defects and decreased body weight in the offspring. It is not known whether these effects can occur in humans.

However, the Center for Children's Environmental Health reports studies that demonstrate that exposure to PAH pollution during pregnancy is related to adverse birth outcomes including low birth weight, premature delivery, and heart malformations. High prenatal exposure to PAH is also associated with lower IQ at age three, increased behavorial problems at ages six and eight and childhood asthma.

Genotoxicity

Genotoxic effects for some PAH have been demonstrated both in rodents and in vitro tests using mammalian (including human) cell lines. Most of the PAH are not genotoxic by themselves and they need to be metabolized to the diol epoxides which react with DNA, thus inducing genotoxic damage.

Immunotoxicity

PAHs have also been reported to suppress immune reaction in rodents. The precise mechanisms of PAH-induced immunotoxicity are still not clear; however, it appears that immunosuppression may be involved in the mechanisms by which PAH induce cancer.


(Photo by Zakysant from de.wikipedia.org)

Environmental Fate and Ecotoxic Effects


PAHs are usually released into the air, or they evaporate into the air when they are released to soil or water. PAHs often adsorb to dust particles in the atmosphere, where they undergo photo oxidation in the presence of sunlight, especially when they are adsorbed to particles. This oxidation process can break down the chemicals over a period of days to weeks. PAHs are generally insoluble in water and therefore are generally found adsorbed to particulates and precipitated in the bottom of lakes and rivers, or solubilized in any oily matter which may contaminate water, sediments and soil.

Mixed microbial populations in sediment/water systems may degrade some PAHs over a period of weeks to months. The toxicity of PAH to aquatic organisms is affected by metabolism and photo-oxidation, and they are generally more toxic in the presence of ultraviolet light. PAHs have moderate to high acute toxicity to aquatic life and birds. PAH in soil are unlikely to exert toxic effects on terrestrial invertebrates, except when the soil is highly contaminated. Mammals can absorb PAHs by various routes e.g. inhalation, dermal contact, and ingestion. Plants can absorb PAHs from soils through their roots and translocate them to other plant parts. PAHs are moderately persistent in the environment, and can bioaccumulate. The concentrations of PAHs found in fish and shellfish are expected to be much higher than in the environment from which they were taken.

Recommendations for the Protection of Human Health and the Environment

The International Programme on Chemical Safety offers these general guidelines for protecting human health:

  1. owning to their proven immunotoxic effects, coal-tar shampoos should be used for anti-dandruff therapy only if no other treatment is available.
  2. in view of the proven immunotoxic and carcinogenic effects of PAH in coke-oven workers, exposure to PAH in occupational settings should be eliminated or minimized by reducing emissions to the extent possible or, when they cannot be sufficiently reduced, by providing effective personal protection.
  3. public education about the sources and health effects of exposure to PAH should be improved:
  • use of unvented indoor fires, as in many developing countries, should be discouraged, and they should be replaced by more efficient, well-vented combustion devices
  • the risk of exposure to PAH from passive smoking should be stressed and measures taken to avoid it
  • urban air pollution should be monitored all year round and not only seasonally.

This programme also provides suggestions on ways to reduce PAH emissions:

  • filtration and scrubbing of industrial emissions,
  • treatment of effluents,
  • use of catalytic converters and particle traps on motor vehicles.

Relevant Topics on Toxipedia


References


1. Agency for Toxic Substances and Disease Registry(ATSDR)Toxicological Profile for Polycyclic Aromatic Hydrocarbons (PAHs) August 1995. Accessed 12.09.2010.
2. Agency for Toxic Substances and Disease Registry (ATSDR). Public Health Statement August 1995. Accessed 12.09.2010.
3. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans Volume 92 (2010) Some Non-heterocyclic Polycyclic Aromatic Hydrocarbons and Some Related Exposures Accessed 12.10.2010.
4. International Programme On Chemical Safety (INCHEM) Polycyclic aromatic hydrocarbons, selected non-heterocyclic (EHC 202, 1998) Accessed 12.11.2010
5. Occupational Safety & Health Administration (OSHA) Coal Tar Pitch Volatiles Accessed 14.11.2010.
6. Public Health SA Polycyclic Public Health Fact Sheet . Accessed 13.11.2010.
7. Norbert E. Kaminski, Barbara L. Faubert Kaplan and Michael P. Holsapple In Curtis D. Klaassen editor. Casarett and Doull's Toxicology, the basic science of poisons; seventh edition. Mc-Graw-Hill, Inc. 2008, 526.
8. Peter H. Albers Petroleum and Individual Polycyclic Aromatic Hydrocarbons In D. J. Hoffman, B. A. Rattner, G. A. Burton, J. Cairns editors. Hanbook of Ecotoxicology. Lewis Publishers; 2003.
9. Chemicals in Meat Cooked at High Temperatures and Cancer Risk.Fact Sheet.National Cancer Institute. Accessed 05.03.2011.

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