What are polycyclic aromatic hydrocarbons

what are polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbon

Polycyclic aromatic hydrocarbons (PAHs) are a group of over different chemicals that are formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances like tobacco or charbroiled meat. PAHs are usually found as a mixture containing two or more of these compounds, such as soot. Some PAHs are manufactured. Polycyclic aromatic hydrocarbons (PAHs) are a group of over different chemicals that are formed during the incomplete burning of coal, oil and gas, garbage, or other.

Polycyclic aromatic hydrocarbons PAHs are a class of chemicals that occur naturally in coal, crude oil, and gasoline. They also are produced when coal, oil, gas, wood, garbage, and tobacco are burned. PAHs generated from these sources can bind to or form small particles in the air.

High-temperature cooking will form PAHs in meat and in other foods. Naphthalene is a PAH that is produced commercially in the United States to make other chemicals and mothballs. Cigarette smoke contains many PAHs. People are usually exposed to mixtures of PAHs.

Breathing air contaminated with motor vehicle exhaust, cigarette smoke, wood smoke, or fumes from asphalt roads are common ways exposure occurs. People take in PAHs when they eat grilled or charred meats or foods or foods on which PAH particles have settled from the air. After PAHs are swallowed, breathed in, or in some cases, passed through the skin, the body converts PAHs into breakdown products called metabolites that pass out of the body in the urine and feces.

Human health effects from environmental exposure to low levels of PAHs are unknown. Large amounts of naphthalene in what are polycyclic aromatic hydrocarbons can irritate eyes and breathing passages. Workers who have been exposed to large amounts of naphthalene from skin contact with the liquid form and from breathing naphthalene vapor have the like what i say and what i mean blood and liver abnormalities.

The Fourth Report includes results from the earlier survey period of for several PAH metabolites. PAHs were measured in most participants, indicating widespread exposure in the U. Research has found that urinary PAH metabolites are higher in adults who smoke than in nonsmoking adults.

Finding a measurable amount of one or more PAH metabolites in the urine does not imply that the levels of one or more PAH metabolites or PAHs cause an adverse health effect. Biomonitoring studies on levels of PAH metabolites provide physicians and public health officials with reference values so that they can determine whether people have been exposed to higher levels of these chemicals than are found in the general population.

Biomonitoring data can also help scientists plan and conduct research on exposure and health effects. Skip directly to site content Skip directly to page options Skip directly to A-Z link. National Biomonitoring Program. Section Navigation. Facebook Twitter LinkedIn Syndicate. Minus Related Pages. Links with this icon indicate that you are leaving the CDC website. Linking to a non-federal website does not constitute an endorsement by CDC or any of its employees of the sponsors or the information and products presented on the website.

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Polycyclic aromatic hydrocarbons (PAHs) are a large group of diverse organic compounds that contain two or more fused aromatic rings ranging from the two-ring naphthalene and naphthalene derivates to complex ring structures containing up to 10 rings. PAHs with up to six fused aromatic rings are often known as small PAHs, whereas those containing more than six aromatic rings are . POLYCYCLIC AROMATIC HYDROCARBONS (PAHs) Division of Toxicology August attached to dust particles or as solids in soil or sediment. Although the health effects of individual PAHs are not exactly alike, the following 17 PAHs are considered as a group in this profile: acenaphthene acenaphthylene anthracene benz[a]anthracene benzo[a]pyrene. Polycyclic Aromatic Hydrocarbons (PAHs) What are PAHs? Short for polycyclic aromatic hydrocarbons, PAHs describe chemicals that are often found together in groups of two or more. PAHs are found naturally in the environment but they can also be man-made. In their purest form, PAHs are solid and range in appearance from colorless to white or pale yellow-File Size: KB.

A polycyclic aromatic hydrocarbon PAH is a hydrocarbon a chemical compound containing only carbon and hydrogenthat is composed of multiple aromatic rings.

The group is a major subset of the aromatic hydrocarbons. The simplest of such chemicals are naphthalene , having two aromatic rings, and the three-ring compounds anthracene and phenanthrene. The terms polyaromatic hydrocarbon [1] or polynuclear aromatic hydrocarbon [2] are also used for this concept. PAHs are uncharged, non-polar molecules, with distinctive properties due in part to the delocalized electrons in their aromatic rings. Many of them are found in coal and in oil deposits, and are also produced by the thermal decomposition of organic matter for example, in engines and incinerators or when biomass burns in forest fires.

Polycyclic aromatic hydrocarbons are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life. By definition, polycyclic aromatic hydrocarbons have multiple cycles, precluding benzene from being considered a PAH.

A polyaromatic hydrocarbon may have rings of various sizes, including some that are not aromatic. Those that have only six-membered rings are said to be alternant. In some PAHs, like naphthalene, anthracene, and coronene, all carbon and hydrogen atoms lie on the same plane. Those compounds are achiral , since the plane of the molecule is a symmetry plane. However, some other PAHs are not planar.

In some cases, the non-planarity may be forced by the topology of the molecule and the stiffness in length and angle of the carbon-carbon bonds. For example, unlike coronene , corannulene adopts a bowl shape in order to reduce the bond stress. In theory, there are 51 structural isomers of coronene that have six fused benzene rings in a cyclic sequence, with two edge carbons shared between successive rings.

Other PAHs that might seem to be planar, considering only the carbon skeleton, may be distorted by repulsion or steric hindrance between the hydrogen atoms in their periphery. Benzo[c]phenantrene, with four rings fused in a "C" shape, has a slight helical distortion due to repulsion between the closest pair of hydrogen atoms in the two extremal rings. Adding another benzene ring to form dibenzo[c,g]phenantrene creates steric hindrance between the two extreme hydrogen atoms.

The benzenoid hydrocarbons have been defined as condensed polycyclic unsaturated fully-conjugated hydrocarbons whose molecules are essentially planar with all rings six-membered. Full conjugation means that all carbon atoms and carbon-carbon bonds must have the sp 2 structure of benzene.

This class is largely a subset of the alternant PAHs, but is considered to include unstable or hypothetical compounds like triangulene or heptacene. As of , over benzenoid hydrocarbons had been isolated and characterized. The aromaticity varies for PAHs. According to Clar's rule, [17] the resonance structure of a PAH that has the largest number of disjoint aromatic pi sextets i.

For example, phenanthrene has two Clar structures: one with just one aromatic sextet the middle ring , and the other with two the first and third rings. The latter case is therefore the more characteristic electronic nature of the two. Therefore, in this molecule the outer rings have greater aromatic character whereas the central ring is less aromatic and therefore more reactive.

PAHs are nonpolar and lipophilic. The larger members, e. Polycyclic aromatic compounds characteristically yield radical anions upon treatment with alkali metals.

The large PAH form dianions as well. Polycyclic aromatic hydrocarbons are primarily found in natural sources such as bitumen. PAHs can also be produced geologically when organic sediments are chemically transformed into fossil fuels such as oil and coal. PAHs may result from the incomplete combustion of organic matter in natural wildfires.

High levels of such pyrogenetic PAHs have been detected in the Cretaceous-Tertiary K-T boundary , more than times the level in adjacent layers.

These images can trace the surface of star-forming clouds in our own galaxy or identify star forming galaxies in the distant universe.

Volcanic eruptions may emit PAHs. Certain PAHs such as perylene can also be generated in anaerobic sediments from existing organic material, although it remains undetermined whether abiotic or microbial processes drive their production. The dominant sources of PAHs in the environment are thus from human activity: wood-burning and combustion of other biofuels such as dung or crop residues contribute more than half of annual global PAH emissions, particularly due to biofuel use in India and China.

Lower-temperature combustion, such as tobacco smoking or wood-burning , tends to generate low molecular weight PAHs, whereas high-temperature industrial processes typically generate PAHs with higher molecular weights. PAHs are typically found as complex mixtures.

Most PAHs are insoluble in water, which limits their mobility in the environment, although PAHs sorb to fine-grained organic-rich sediments. Two-ringed PAHs, and to a lesser extent three-ringed PAHs, dissolve in water, making them more available for biological uptake and degradation.

Human exposure varies across the globe and depends on factors such as smoking rates, fuel types in cooking, and pollution controls on power plants, industrial processes, and vehicles. Burning solid fuels such as coal and biofuels in the home for cooking and heating is a dominant global source of PAH emissions that in developing countries leads to high levels of exposure to indoor particulate air pollution containing PAHs, particularly for women and children who spend more time in the home or cooking.

Emissions from vehicles such as cars and trucks can be a substantial outdoor source of PAHs in particulate air pollution. People can also be occupationally exposed during work that involves fossil fuels or their derivatives, wood-burning, carbon electrodes , or exposure to diesel exhaust. PAHs typically disperse from urban and suburban non-point sources through road run-off , sewage , and atmospheric circulation and subsequent deposition of particulate air pollution.

Two- and three-ringed PAHs can disperse widely while dissolved in water or as gases in the atmosphere, while PAHs with higher molecular weights can disperse locally or regionally adhered to particulate matter that is suspended in air or water until the particles land or settle out of the water column. Algae and some invertebrates such as protozoans , mollusks , and many polychaetes have limited ability to metabolize PAHs and bioaccumulate disproportionate concentrations of PAHs in their tissues; however, PAH metabolism can vary substantially across invertebrate species.

PAHs transform slowly to a wide range of degradation products. Biological degradation by microbes is a dominant form of PAH transformation in the environment. The British Geological Survey reported the amount and distribution of PAH compounds including parent and alkylated forms in urban soils at 76 locations in Greater London. However, the overall distribution also suggested that the PAHs in London soils had undergone weathering and been modified by a variety of pre-and post-depositional processes such as volatilization and microbial biodegradation.

Managed burning of moorland vegetation in the UK has been shown to generate PAHs which become incorporated into the peat surface. Concentrations of PAHs in river and estuarine sediments vary according to a variety of factors including proximity to municipal and industrial discharge points, wind direction and distance from major urban roadways, as well as tidal regime which controls the diluting effect of generally cleaner marine sediments relative to freshwater discharge.

Cancer is a primary human health risk of exposure to PAHs. PAHs have been linked to skin , lung , bladder , liver , and stomach cancers in well-established animal model studies.

Historically, PAHs contributed substantially to our understanding of adverse health effects from exposures to environmental contaminants , including chemical carcinogenesis. Bartholomew's Hospital in London, observed that scrotal cancer was unusually common in chimney sweepers and proposed the cause as occupational exposure to soot. In , Yamigawa and Ichicawa were the first to experimentally produce cancers, specifically of the skin, by topically applying coal tar to rabbit ears.

In , Ernest Kennaway determined that the carcinogenic component of coal tar mixtures was an organic compound consisting of only carbon and hydrogen. This component was later linked to a characteristic fluorescent pattern that was similar but not identical to benz[ a ]anthracene , a PAH that was subsequently demonstrated to cause tumors. In the s and later, epidemiologists from Japan, the UK, and the US, including Richard Doll and various others, reported greater rates of death from lung cancer following occupational exposure to PAH-rich environments among workers in coke ovens and coal carbonization and gasification processes.

The structure of a PAH influences whether and how the individual compound is carcinogenic. PAHs that affect cancer initiation are typically first chemically modified by enzymes into metabolites that react with DNA, leading to mutations. When the DNA sequence is altered in genes that regulate cell replication , cancer can result. Mutagenic PAHs, such as benzo[ a ]pyrene, usually have four or more aromatic rings as well as a "bay region", a structural pocket that increases reactivity of the molecule to the metabolizing enzymes.

The activity of these enzymes may at times conversely protect against PAH toxicity, which is not yet well understood. Low molecular weight PAHs, with two to four aromatic hydrocarbon rings, are more potent as co-carcinogens during the promotional stage of cancer. In this stage, an initiated cell a cell that has retained a carcinogenic mutation in a key gene related to cell replication is removed from growth-suppressing signals from its neighboring cells and begins to clonally replicate.

Excessive closure of these channels after exposure to PAHs results in removing a cell from the normal growth-regulating signals imposed by its local community of cells, thus allowing initiated cancerous cells to replicate.

These PAHs do not need to be enzymatically metabolized first. Low molecular weight PAHs are prevalent in the environment, thus posing a significant risk to human health at the promotional phases of cancer. Adult exposure to PAHs has been linked to cardiovascular disease.

In laboratory experiments, animals exposed to certain PAHs have shown increased development of plaques atherogenesis within arteries. This enzyme then metabolically processes the PAHs to quinone metabolites that bind to DNA in reactive adducts that remove purine bases.

The resulting mutations may contribute to unregulated growth of vascular smooth muscle cells or to their migration to the inside of the artery, which are steps in plaque formation. Oxidative stress following PAH exposure could also result in cardiovascular disease by causing inflammation , which has been recognized as an important factor in the development of atherosclerosis and cardiovascular disease.

Multiple epidemiological studies of people living in Europe, the United States, and China have linked in utero exposure to PAHs, through air pollution or parental occupational exposure, with poor fetal growth, reduced immune function, and poorer neurological development, including lower IQ. A spectral database exists [4] for tracking polycyclic aromatic hydrocarbons PAHs in the universe. Structures of PAHs have been analyzed using infrared spectroscopy. PAHs possess very characteristic UV absorbance spectra.

These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of isomers , each isomer has a different UV absorbance spectrum than the others. This is particularly useful in the identification of PAHs.

Most PAHs are also fluorescent , emitting characteristic wavelengths of light when they are excited when the molecules absorb light. The extended pi-electron electronic structures of PAHs lead to these spectra, as well as to certain large PAHs also exhibiting semi-conducting and other behaviors. PAHs may be abundant in the universe. As they cool, the atoms supposedly bond to each other in various ways and eventually form particles of a million or more atoms.

Adolf Witt and his team inferred [] that PAHswhich may have been vital in the formation of early life on Earth can only originate in nebulae. PAHs, subjected to interstellar medium ISM conditions, are transformed, through hydrogenation , oxygenation , and hydroxylation , to more complex organic compounds "a step along the path toward amino acids and nucleotides , the raw materials of proteins and DNA , respectively".

Low-temperature chemical pathways from simple organic compounds to complex PAHs are of interest. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Saturn 's moon Titan , and may be significant pathways, in terms of the PAH world hypothesis , in producing precursors to biochemicals related to life as we know it. From Wikipedia, the free encyclopedia. Hydrocarbon composed of multiple aromatic rings. Three representations of hexabenzocoronene , a polycyclic aromatic hydrocarbon.

Top: standard line-angle schematic, where carbon atoms are represented by the vertices of the hexagons and hydrogen atoms are inferred.


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