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Polycyclic aromatic hydrocarbon

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A Polycyclic aromatic hydrocarbon (PAH) is any member of a class of organic compounds that is composed of multiple fused aromatic rings. Most are produced by the incomplete combustion of organic matter— by engine exhaust fumes, tobacco, incinerators, in roasted meats and cereals,<ref>D H Phillips, "Polycyclic aromatic hydrocarbons in the diet", Mutat Res. 1999, https://pubmed.ncbi.nlm.nih.gov/10415437/</ref> or when biomass burns at lower temperatures as in forest fires.<ref>Oregon State University, Superfund Research Center, "All About PAHs", https://superfund.oregonstate.edu/resources/all-about-pahs</ref><ref>Wisconsin Department of Health Services, "Polycyclic Aromatic Hydrocarbons (PAHs)", https://www.dhs.wisconsin.gov/chemical/pah.htm</ref> The simplest representative is naphthalene, having two aromatic rings, and the three-ring compounds anthracene and phenanthrene. PAHs are uncharged, non-polar and planar. Many are colorless. Many of them are also found in fossil fuel deposits such as coal and in petroleum. Exposure to PAHs can lead to different types of cancer, to fetal development complications, and to cardiovascular issues.Template:Medical citation needed

Polycyclic aromatic hydrocarbons are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life.<ref name="NASA-20140221"/><ref name="NASA-20110413"/>

Nomenclature and structureEdit

The terms polyaromatic hydrocarbon,<ref>Gerald Rhodes, Richard B. Opsal, Jon T. Meek, and James P. Reilly (1983): "Analysis of polyaromatic hydrocarbon mixtures with laser ionization gas chromatography/mass spectrometry". Analytic Chemistry, volume 55, issue 2, pages 280–286 {{#invoke:doi|main}}</ref> or polynuclear aromatic hydrocarbon<ref>Kevin C. Jones, Jennifer A. Stratford, Keith S. Waterhouse, et al. (1989): "Increases in the polynuclear aromatic hydrocarbon content of an agricultural soil over the last century". Environmental Science and Technology, volume 23, issue 1, pages 95–101. {{#invoke:doi|main}}</ref> (abbreviated as PNA) are also used for this concept.<ref name=ATSDR2011a/>

By definition, polycyclic aromatic hydrocarbons have multiple aromatic rings, precluding benzene from being considered a PAH. Sources such as the US EPA and CDC consider naphthalene to be the simplest PAH.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Most authors exclude compounds that include heteroatoms in the rings, or carry substituents.<ref>Template:Cite journal</ref>

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.<ref>Template:Cite book</ref>

The following are examples of PAHs that vary in the number and arrangement of their rings:

GeometryEdit

Most PAHs, like naphthalene, anthracene, and coronene, are planar. This geometry is a consequence of the fact that the σ-bonds that result from the merger of sp2 hybrid orbitals of adjacent carbons lie on the same plane as the carbon atom. Those compounds are achiral, since the plane of the molecule is a symmetry plane.

In rare cases, 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. The two possible configurations, concave and convex, are separated by a relatively low energy barrier (about 11 kcal/mol).<ref>Marina V. Zhigalko, Oleg V. Shishkin, Leonid Gorb, and Jerzy Leszczynski (2004): "Out-of-plane deformability of aromatic systems in naphthalene, anthracene and phenanthrene". Journal of Molecular Structure, volume 693, issues 1–3, pages 153-159. {{#invoke:doi|main}}</ref>

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. All of them must be non-planar and have considerable higher bonding energy (computed to be at least 130 kcal/mol) than coronene; as of 2002, none of them had been synthesized.<ref>Jan Cz. Dobrowolski (2002): "On the belt and Moebius isomers of the coronene molecule". Journal of Chemical Information and Computer Science, volume 42, issue 3, pages 490–499 {{#invoke:doi|main}}</ref>

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]phenanthrene, 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.<ref>F. H. Herbstein and G. M. J. Schmidt (1954): "The structure of overcrowded aromatic compounds. Part III. The crystal structure of 3:4-benzophenanthrene". Journal of the Chemical Society (Resumed), volume 1954, issue 0, pages 3302-3313. {{#invoke:doi|main}}</ref> This effect also causes distortion of picene.<ref name=echi2007>Takuya Echigo, Mitsuyoshi Kimata, and Teruyuki Maruoka (2007): "Crystal-chemical and carbon-isotopic characteristics of karpatite (C24H12) from the Picacho Peak Area, San Benito County, California: Evidences for the hydrothermal formation". American Mineralogist, volume 92, issues 8-9, pages 1262–1269. {{#invoke:doi|main}}</ref>

Adding another benzene ring to form dibenzo[c,g]phenanthrene creates steric hindrance between the two extreme hydrogen atoms.<ref>František Mikeš, Geraldine Boshart, and Emanuel Gil-Av (1976): "Resolution of optical isomers by high-performance liquid chromatography, using coated and bonded chiral charge-transfer complexing agents as stationary phases". Journal of Chromatography A, volume 122, pages 205-221. {{#invoke:doi|main}}</ref> Adding two more rings on the same sense yields heptahelicene in which the two extreme rings overlap.<ref>František Mikeš, Geraldine Boshart, and Emanuel Gil-Av (1976): "Helicenes. Resolution on chiral charge-transfer complexing agents using high performance liquid chromatography". Journal of the Chemical Society, Chemical Communications, volume 1976, issue 3, pages 99-100. {{#invoke:doi|main}}</ref> These non-planar forms are chiral, and their enantiomers can be isolated.<ref name=gutm2012>Ivan Gutman and Sven J. Cyvin (2012): Introduction to the Theory of Benzenoid Hydrocarbons. 152 pages. Template:Isbn</ref>

Benzenoid hydrocarbonsEdit

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 sp2 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.<ref name=gutm2012/>

As of 2012, over 300 benzenoid hydrocarbons had been isolated and characterized.<ref name=gutm2012/>

Bonding and aromaticityEdit

Template:See also The aromaticity varies for PAHs. According to Clar's rule,<ref>Template:Cite book</ref> the resonance structure of a PAH that has the largest number of disjoint aromatic pi sextets—i.e. benzene-like moieties—is the most important for the characterization of the properties of that PAH.<ref>Template:Cite journal</ref>

  • Benzene-substructure resonance analysis for Clar's rule
  • Phenanthrene Clar rule.svg

    Phenanthrene

  • Anthracene Clar rule.svg

    Anthracene

  • Chrysene Clar rule.svg

    Chrysene

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.Template:Citation needed In contrast, in anthracene the resonance structures have one sextet each, which can be at any of the three rings, and the aromaticity spreads out more evenly across the whole molecule.Template:Citation needed This difference in number of sextets is reflected in the differing ultraviolet–visible spectra of these two isomers, as higher Clar pi-sextets are associated with larger HOMO-LUMO gaps;<ref>Template:Cite journal</ref> the highest-wavelength absorbance of phenanthrene is at 293 nm, while anthracene is at 374 nm.<ref>Template:Cite journal</ref> Three Clar structures with two sextets each are present in the four-ring chrysene structure: one having sextets in the first and third rings, one in the second and fourth rings, and one in the first and fourth rings.Template:Citation needed Superposition of these structures reveals that the aromaticity in the outer rings is greater (each has a sextet in two of the three Clar structures) compared to the inner rings (each has a sextet in only one of the three).

PropertiesEdit

PhysicochemicalEdit

PAHs are nonpolar and lipophilic. Larger PAHs are generally insoluble in water, although some smaller PAHs are soluble.<ref>Template:Cite journal</ref><ref>Template:Citation</ref> The larger members are also poorly soluble in organic solvents and in lipids. The larger members, e.g. perylene, are strongly colored.<ref name=gutm2012/>

RedoxEdit

Polycyclic aromatic compounds characteristically yield radicals and anions upon treatment with alkali metals. The large PAH form dianions as well.<ref>Template:Cite journal</ref> The redox potential correlates with the size of the PAH.

Half-cell potential of aromatic compounds against the SCE (Fc+/0)<ref>Template:Cite journal</ref>
Compound Potential (V)
benzene −3.42
biphenyl<ref>Template:Cite journal</ref> −2.60 (-3.18)
naphthalene −2.51 (-3.1)
anthracene −1.96 (-2.5)
phenanthrene −2.46
perylene −1.67 (-2.2)
pentacene −1.35

SourcesEdit

ArtificialEdit

The dominant sources of PAHs in the environment are 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.<ref name=Ramesh2011>Template:Cite book</ref><ref name=abdel2016>Template:Cite journal</ref> As of 2004, industrial processes and the extraction and use of fossil fuels made up slightly more than one quarter of global PAH emissions, dominating outputs in industrial countries such as the United States.<ref name=Ramesh2011 />

A year-long sampling campaign in Athens, Greece found a third (31%) of PAH urban air pollution to be caused by wood-burning, like diesel and oil (33%) and gasoline (29%). It also found that wood-burning is responsible for nearly half (43%) of annual PAH cancer-risk (carcinogenic potential) compared to the other sources and that wintertime PAH levels were 7 times higher than in other seasons, especially if atmospheric dispersion is low.<ref>Template:Cite news</ref><ref>Template:Cite journal</ref>

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.<ref name=Tobis /> Incense is also a source.<ref>Template:Cite journal</ref>

PAHs are typically found as complex mixtures.<ref name=ravi2008 /><ref name=Tobis>Template:Cite journal</ref>

NaturalEdit

Natural firesEdit

PAHs may result from the incomplete combustion of organic matter in natural wildfires.<ref name=abdel2016/><ref name=Ramesh2011 /> Substantially higher outdoor air, soil, and water concentrations of PAHs have been measured in Asia, Africa, and Latin America than in Europe, Australia, the U.S., and Canada.<ref name=Ramesh2011 />Template:Relevance

Fossil carbonEdit

Polycyclic aromatic hydrocarbons are primarily found in natural sources such as bitumen.<ref>Template:Ullmann</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

PAHs can also be produced geologically when organic sediments are chemically transformed into fossil fuels such as oil and coal.<ref name=ravi2008>Template:Cite journal</ref> The rare minerals idrialite, curtisite, and carpathite consist almost entirely of PAHs that originated from such sediments, that were extracted, processed, separated, and deposited by very hot fluids.<ref>Stephen A. Wise, Robert M. Campbell, W. Raymond West, et al. (1986): "Characterization of polycyclic aromatic hydrocarbon minerals curtisite, idrialite and pendletonite using high-performance liquid chromatography, gas chromatography, mass spectrometry and nuclear magnetic resonance spectroscopy". Chemical Geology, volume 54, issues 3–4, pages 339-357. {{#invoke:doi|main}}</ref><ref name=echi2007/><ref>Max Blumer (1975): "Curtisite, idrialite and pendletonite, polycyclic aromatic hydrocarbon minerals: Their composition and origin" Chemical Geology, volume 16, issue 4, pages 245-256. {{#invoke:doi|main}}</ref> High levels of such PAHs have been detected in the Cretaceous-Tertiary (K-T) boundary, more than 100 times the level in adjacent layers. The spike was attributed to massive fires that consumed about 20% of the terrestrial above-ground biomass in a very short time.<ref>Tetsuya Arinobu, Ryoshi Ishiwatari, Kunio Kaiho, and Marcos A. Lamolda (1999): "Spike of pyrosynthetic polycyclic aromatic hydrocarbons associated with an abrupt decrease in δ13C of a terrestrial biomarker at the Cretaceous-Tertiary boundary at Caravaca, Spain ". Geology, volume 27, issue 8, pages 723–726 {{#invoke:doi|main}}</ref>

ExtraterrestrialEdit

PAHs are prevalent in the interstellar medium (ISM) of galaxies in both the nearby and distant Universe and make up a dominant emission mechanism in the mid-infrared wavelength range, containing as much as 10% of the total integrated infrared luminosity of galaxies.<ref name="youtube.com">Template:Citation</ref> PAHs generally trace regions of cold molecular gas, which are optimum environments for the formation of stars.<ref name="youtube.com"/>

NASA's Spitzer Space Telescope and James Webb Space Telescope include instruments for obtaining both images and spectra of light emitted by PAHs associated with star formation. These images can trace the surface of star-forming clouds in our own galaxy or identify star forming galaxies in the distant universe.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In June 2013, PAHs were detected in the upper atmosphere of Titan, the largest moon of the planet Saturn.<ref>Template:Cite news</ref>

Minor sourcesEdit

Volcanic eruptions may emit PAHs.<ref name=ravi2008 />

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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Distribution in the environmentEdit

Aquatic environmentsEdit

Most PAHs are insoluble in water, which limits their mobility in the environment, although PAHs sorb to fine-grained organic-rich sediments.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name=Choi-2007>Template:Cite book</ref> Aqueous solubility of PAHs decreases approximately logarithmically as molecular mass increases.<ref name=Johnsen-2005>Template:Cite journal</ref>

Two-ringed PAHs, and to a lesser extent three-ringed PAHs, dissolve in water, making them more available for biological uptake and degradation.<ref name=Choi-2007 /><ref name=Johnsen-2005 /><ref>Template:Cite book</ref> Further, two- to four-ringed PAHs volatilize sufficiently to appear in the atmosphere predominantly in gaseous form, although the physical state of four-ring PAHs can depend on temperature.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In contrast, compounds with five or more rings have low solubility in water and low volatility; they are therefore predominantly in solid state, bound to particulate air pollution, soils, or sediments.<ref name=Choi-2007 /> In solid state, these compounds are less accessible for biological uptake or degradation, increasing their persistence in the environment.<ref name=Johnsen-2005 /><ref name="Haritash-2009">Template:Cite journal</ref>

Human exposureEdit

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.<ref name=ravi2008 /><ref name=Ramesh2011 /><ref name=Choi2010>Template:Cite book</ref> Developed countries with stricter air and water pollution controls, cleaner sources of cooking (i.e., gas and electricity vs. coal or biofuels), and prohibitions of public smoking tend to have lower levels of PAH exposure, while developing and undeveloped countries tend to have higher levels.<ref name=ravi2008 /><ref name=Ramesh2011 /><ref name=Choi2010 /> Surgical smoke plumes have been proven to contain PAHs in several independent research studies.<ref>Template:Cite journal</ref>

File:Kochen über offenem Feuer.JPG
A wood-burning open-air cooking stove. Smoke from solid fuels like wood is a large source of PAHs globally.

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.<ref name=Ramesh2011 /><ref>Template:Cite journal</ref>

File:Cairo in smog.jpg
Smog in Cairo. Particulate air pollution, including smog, is a substantial cause of human exposure to PAHs.

Emissions from vehicles such as cars and trucks can be a substantial outdoor source of PAHs in particulate air pollution.<ref name=ravi2008 /><ref name=Ramesh2011 /> Geographically, major roadways are thus sources of PAHs, which may distribute in the atmosphere or deposit nearby.<ref name=Srogi2007>Template:Cite journal</ref> Catalytic converters are estimated to reduce PAH emissions from gasoline-fired vehicles by 25-fold.<ref name=ravi2008 />

People can also be occupationally exposed during work that involves fossil fuels or their derivatives, wood-burning, carbon electrodes, or exposure to diesel exhaust.<ref name=Boffetta1997>Template:Cite journal</ref><ref name=Wagner2015>Template:Cite journal</ref> 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.<ref name=ravi2008 /><ref name=Boffetta1997 /><ref name=Wagner2015 />

In industrial countries, people who smoke tobacco products, or who are exposed to second-hand smoke, are among the most highly exposed groups; tobacco smoke contributes to 90% of indoor PAH levels in the homes of smokers.<ref name=Choi2010 /> For the general population in developed countries, the diet is otherwise the dominant source of PAH exposure, particularly from smoking or grilling meat or consuming PAHs deposited on plant foods, especially broad-leafed vegetables, during growth.<ref>Template:Cite journal</ref> Exposure also occurs through drinking alcohol aged in charred barrels, flavored with peat smoke, or made with roasted grains.<ref>Template:Cite journal</ref> PAHs are typically at low concentrations in drinking water.<ref name=Choi2010 />

Environmental pollution and degradationEdit

File:Manripo071210 3.jpg
Crude oil on a beach after a 2007 oil spill in Korea.

PAHs typically disperse from urban and suburban non-point sources through road runoff, sewage, and atmospheric circulation and subsequent deposition of particulate air pollution.<ref name="Davis">Template:Cite journal</ref><ref name=Hylland2006>Template:Cite journal</ref> Soil and river sediment near industrial sites such as creosote manufacturing facilities can be highly contaminated with PAHs.<ref name=ravi2008 /> Oil spills, creosote, coal mining dust, and other fossil fuel sources can also distribute PAHs in the environment.<ref name=ravi2008 /><ref>Template:Cite journal</ref>

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.<ref name=ravi2008 /> PAHs have a strong affinity for organic carbon, and thus highly organic sediments in rivers, lakes, and the ocean can be a substantial sink for PAHs.<ref name=Srogi2007 />

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.<ref name=Hylland2006 /><ref>Template:Cite journal</ref> Most vertebrates metabolize and excrete PAHs relatively rapidly.<ref name=Hylland2006 /> Tissue concentrations of PAHs do not increase (biomagnify) from the lowest to highest levels of food chains.<ref name=Hylland2006 />

PAHs transform slowly to a wide range of degradation products. Biological degradation by microbes is a dominant form of PAH transformation in the environment.<ref name="Haritash-2009"/><ref name=Johnsen2005>Template:Cite journal</ref> Soil-consuming invertebrates such as earthworms speed PAH degradation, either through direct metabolism or by improving the conditions for microbial transformations.<ref name=Johnsen2005 /> Abiotic degradation in the atmosphere and the top layers of surface waters can produce nitrogenated, halogenated, hydroxylated, and oxygenated PAHs; some of these compounds can be more toxic, water-soluble, and mobile than their parent PAHs.<ref name=Hylland2006 /><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Urban soilsEdit

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.<ref name=VaneKim2014>Template:Cite journal</ref> The study showed that parent (16 PAH) content ranged from 4 to 67 mg/kg (dry soil weight) and an average PAH concentration of 18 mg/kg (dry soil weight) whereas the total PAH content (33 PAH) ranged from 6 to 88 mg/kg and fluoranthene and pyrene were generally the most abundant PAHs.<ref name=VaneKim2014/> Benzo[a]pyrene (BaP), the most toxic of the parent PAHs, is widely considered a key marker PAH for environmental assessments;<ref>Template:Cite journal</ref> the normal background concentration of BaP in the London urban sites was 6.9 mg/kg (dry soil weight).<ref name=VaneKim2014/> London soils contained more stable four- to six-ringed PAHs which were indicative of combustion and pyrolytic sources, such as coal and oil burning and traffic-sourced particulates. 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.

PeatlandsEdit

Managed burning of moorland vegetation in the UK has been shown to generate PAHs which become incorporated into the peat surface.<ref name=VaneRawlins2013>Template:Cite journal</ref> Burning of moorland vegetation such as heather initially generates high amounts of two- and three-ringed PAHs relative to four- to six-ringed PAHs in surface sediments, however, this pattern is reversed as the lower molecular weight PAHs are attenuated by biotic decay and photodegradation.<ref name=VaneRawlins2013/> Evaluation of the PAH distributions using statistical methods such as principal component analyses (PCA) enabled the study to link the source (burnt moorland) to pathway (suspended stream sediment) to the depositional sink (reservoir bed).<ref name=VaneRawlins2013/>

Rivers, estuarine and coastal sedimentsEdit

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.<ref name="Davis"/><ref name=VaneHarrison2008>Template:Cite journal</ref><ref name=VaneChenery2011>Template:Cite journal</ref> Consequently, the concentrations of pollutants in estuaries tends to decrease at the river mouth.<ref>Template:Cite journal</ref> Understanding of sediment hosted PAHs in estuaries is important for the protection of commercial fisheries (such as mussels) and general environmental habitat conservation because PAHs can impact the health of suspension and sediment feeding organism.<ref>Template:Cite journal</ref> River-estuary surface sediments in the UK tend to have a lower PAH content than sediments buried 10–60 cm from the surface reflecting lower present day industrial activity combined with improvement in environmental legislation of PAH.<ref name=VaneChenery2011/> Typical PAH concentrations 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.<ref name=VaneChenery2011/><ref name=VaneHarrison2007>Template:Cite journal</ref> In general estuarine sediments with a higher natural total organic carbon content (TOC) tend to accumulate PAHs due to high sorption capacity of organic matter.<ref name=VaneHarrison2007/> A similar correspondence between PAHs and TOC has also been observed in the sediments of tropical mangroves located on the coast of southern China.<ref>Template:Cite journal</ref>

Human healthEdit

Cancer is a primary human health risk of exposure to PAHs.<ref name=Bostrom2002>Template:Cite journal</ref> Exposure to PAHs has also been linked with cardiovascular disease and poor fetal development.

CancerEdit

PAHs have been linked to skin, lung, bladder, liver, and stomach cancers in well-established animal model studies.<ref name=Bostrom2002 /> Specific compounds classified by various agencies as possible or probable human carcinogens are identified in the section "Regulation and Oversight" below.

HistoryEdit

File:Chimney sweeps.jpg
An 18th-century drawing of chimney sweeps.

Historically, PAHs contributed substantially to our understanding of adverse health effects from exposures to environmental contaminants, including chemical carcinogenesis.<ref>Template:Cite journal</ref> In 1775, Percivall Pott, a surgeon at St. Bartholomew's Hospital in London, observed that scrotal cancer was unusually common in chimney sweepers and proposed the cause as occupational exposure to soot.<ref name=Dipple1985>Template:Cite book</ref> A century later, Richard von Volkmann reported increased skin cancers in workers of the coal tar industry of Germany, and by the early 1900s increased rates of cancer from exposure to soot and coal tar was widely accepted. In 1915, Yamigawa and Ichicawa were the first to experimentally produce cancers, specifically of the skin, by topically applying coal tar to rabbit ears.<ref name=Dipple1985 />

In 1922, 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.<ref name=Dipple1985 /> Cook, Hewett and Hieger then linked the specific spectroscopic fluorescent profile of benzo[a]pyrene to that of the carcinogenic component of coal tar,<ref name=Dipple1985 /> the first time that a specific compound from an environmental mixture (coal tar) was demonstrated to be carcinogenic.

In the 1930s 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.<ref>Template:Cite report</ref>

Mechanisms of carcinogenesisEdit

File:Benzopyrene DNA adduct 1JDG.png
An adduct formed between a DNA strand and an epoxide derived from a benzo[a]pyrene molecule (center); such adducts may interfere with normal DNA replication.

The structure of a PAH influences whether and how the individual compound is carcinogenic.<ref name=Bostrom2002 /><ref name=Baird2005>Template:Cite journal</ref> Some carcinogenic PAHs are genotoxic and induce mutations that initiate cancer; others are not genotoxic and instead affect cancer promotion or progression.<ref name=Baird2005 /><ref>Template:Cite journal</ref>

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.<ref name=Xue2005>Template:Cite journal</ref> Mutagenic metabolites of PAHs include diol epoxides, quinones, and radical PAH cations.<ref name=Xue2005 /><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> These metabolites can bind to DNA at specific sites, forming bulky complexes called DNA adducts that can be stable or unstable.<ref name=Dipple1985 /><ref name=Henkler2012>Template:Cite book</ref> Stable adducts may lead to DNA replication errors, while unstable adducts react with the DNA strand, removing a purine base (either adenine or guanine).<ref name=Henkler2012 /> Such mutations, if they are not repaired, can transform genes encoding for normal cell signaling proteins into cancer-causing oncogenes.<ref name=Baird2005 /> Quinones can also repeatedly generate reactive oxygen species that may independently damage DNA.<ref name=Xue2005 />

Enzymes in the cytochrome family (CYP1A1, CYP1A2, CYP1B1) metabolize PAHs to diol epoxides.<ref name=Nebert2004>Template:Cite journal</ref> PAH exposure can increase production of the cytochrome enzymes, allowing the enzymes to convert PAHs into mutagenic diol epoxides at greater rates.<ref name=Nebert2004 /> In this pathway, PAH molecules bind to the aryl hydrocarbon receptor (AhR) and activate it as a transcription factor that increases production of the cytochrome enzymes. The activity of these enzymes may at times conversely protect against PAH toxicity, which is not yet well understood.<ref name=Nebert2004 />

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.<ref name=Ramesh2004 >Template:Cite journal</ref> Low-molecular-weight PAHs that have bay or bay-like regions can dysregulate gap junction channels, interfering with intercellular communication, and also affect mitogen-activated protein kinases that activate transcription factors involved in cell proliferation.<ref name=Ramesh2004 /> Closure of gap junction protein channels is a normal precursor to cell division. 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.

Cardiovascular diseaseEdit

Adult exposure to PAHs has been linked to cardiovascular disease.<ref>Template:Cite journal </ref> PAHs are among the complex suite of contaminants in tobacco smoke and particulate air pollution and may contribute to cardiovascular disease resulting from such exposures.<ref name=Lewtas2007>Template:Cite journal</ref>

In laboratory experiments, animals exposed to certain PAHs have shown increased development of plaques (atherogenesis) within arteries.<ref name=Ramos2005>Template:Cite journal</ref> Potential mechanisms for the pathogenesis and development of atherosclerotic plaques may be similar to the mechanisms involved in the carcinogenic and mutagenic properties of PAHs.<ref name=Ramos2005 /> A leading hypothesis is that PAHs may activate the cytochrome enzyme CYP1B1 in vascular smooth muscle cells. 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.<ref name=Lewtas2007 /><ref name=Ramos2005 /> These quinone metabolites also generate reactive oxygen species that may alter the activity of genes that affect plaque formation.<ref name=Ramos2005 />

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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Biomarkers of exposure to PAHs in humans have been associated with inflammatory biomarkers that are recognized as important predictors of cardiovascular disease, suggesting that oxidative stress resulting from exposure to PAHs may be a mechanism of cardiovascular disease in humans.<ref>Template:Cite journal</ref>

Fetal development impactsEdit

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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Regulation and oversightEdit

Some governmental bodies, including the European Union as well as NIOSH and the United States Environmental Protection Agency (EPA), regulate concentrations of PAHs in air, water, and soil.<ref name=PAHReview/> The European Commission has restricted concentrations of 8 carcinogenic PAHs in consumer products that contact the skin or mouth.<ref>Template:Citation</ref>

Priority polycyclic aromatic hydrocarbons identified by the US EPA, the US Agency for Toxic Substances and Disease Registry (ATSDR), and the European Food Safety Authority (EFSA) due to their carcinogenicity or genotoxicity and/or ability to be monitored are the following:<ref>Template:Cite journal</ref><ref>Template:Cite report</ref><ref name="EFSA">Template:Cite report</ref>

Compound Agency EPA MCL in water [mg L−1]<ref name=PAHReview>Template:Cite journal</ref>
acenaphthene EPA, ATSDR
acenaphthylene EPA, ATSDR
anthracene EPA, ATSDR
benz[a]anthraceneTemplate:Ref label EPA, ATSDR, EFSA 0.0001
benzo[b]fluorantheneTemplate:Ref label EPA, ATSDR, EFSA 0.0002
benzo[j]fluoranthene ATSDR, EFSA
benzo[k]fluorantheneTemplate:Ref label EPA, ATSDR, EFSA 0.0002
benzo[c]fluorene EFSA
benzo[ghi]peryleneTemplate:Ref label EPA, ATSDR, EFSA
benzo[a]pyreneTemplate:Ref label EPA, ATSDR, EFSA 0.0002
benzo[e]pyrene ATSDR
chryseneTemplate:Ref label EPA, ATSDR, EFSA 0.0002
coronene ATSDR
Compound Agency EPA MCL in water [mg L−1]<ref name=PAHReview/>
cyclopenta[cd]pyrene EFSA
dibenz[a,h]anthraceneTemplate:Ref label EPA, ATSDR, EFSA 0.0003
dibenzo[a,e]pyrene EFSA
dibenzo[a,h]pyrene EFSA
dibenzo[a,i]pyrene EFSA
dibenzo[a,l]pyrene EFSA
fluoranthene EPA, ATSDR
fluorene EPA, ATSDR
indeno[1,2,3-cd]pyreneTemplate:Ref label EPA, ATSDR, EFSA 0.0004
5-methylchrysene EFSA
naphthalene EPA
phenanthrene EPA, ATSDR
pyrene EPA, ATSDR
Template:Note labelConsidered probable or possible human carcinogens by the US EPA, the European Union, and/or the International Agency for Research on Cancer (IARC).<ref name="EFSA"/><ref name=ATSDR2011a>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

Detection and optical propertiesEdit

A spectral database exists<ref name="NASA-20140221"/> for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Detection of PAHs in materials is often done using gas chromatography-mass spectrometry or liquid chromatography with ultraviolet-visible or fluorescence spectroscopic methods or by using rapid test PAH indicator strips.<ref>Template:Cite journal</ref> Structures of PAHs have been analyzed using infrared spectroscopy.<ref>Template:Cite journal</ref>

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.

Origins of lifeEdit

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File:PIA22568-CatsPawNebula-Spitzer-20181023.jpg
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PAHs may be abundant in the universe.<ref name="NASA-20110413">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> They seem to have been formed as early as a couple of billion years after the Big Bang, and are associated with new stars and exoplanets.<ref name="NASA-20140221">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> More than 20% of the carbon in the universe may be associated with PAHs.<ref name="NASA-20140221"/> PAHs are considered possible starting material for the earliest forms of life.<ref name="NASA-20140221"/><ref name="NASA-20110413"/> Light emitted by the Red Rectangle nebula possesses spectral signatures that suggest the presence of anthracene and pyrene.<ref name=Battersby>Template:Cite news</ref><ref name=ReferenceA>Template:Cite journal</ref> This report was considered a controversial hypothesis that as nebulae of the same type as the Red Rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's cores to get caught in stellar winds, and radiate outward. 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<ref name=Battersby/> that PAHs—which may have been vital in the formation of early life on Earth—can only originate in nebulae.<ref name=ReferenceA/>

File:Polycyclic Aromatic Hydrocarbons In Space.jpg
CitationClass=web }}</ref>

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".<ref name="Space-20120920">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="AJL-20120901">Template:Cite journal</ref> Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks."<ref name="Space-20120920"/><ref name="AJL-20120901"/>

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref>

See alsoEdit

ReferencesEdit

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External linksEdit

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