Three representations ofhexabenzocoronene, 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. Middle:ball-and-stick model showing all carbon and hydrogen atoms. Bottom:atomic force microscopy image.
APolycyclic aromatic hydrocarbon (PAH) is any member of a class oforganic compounds that is composed of multiple fusedaromatic rings. Most are produced by the incomplete combustion oforganic matter— by engine exhaust fumes, tobacco, incinerators, in roasted meats and cereals,[1] or when biomass burns at lower temperatures as inforest fires.[2][3] The simplest representative isnaphthalene, having two aromatic rings, and the three-ring compoundsanthracene andphenanthrene. PAHs are uncharged, non-polar and planar. Many are colorless. Many of them are also found in fossil fuel deposits such ascoal and inpetroleum. Exposure to PAHs can lead to different types ofcancer, tofetal development complications, and tocardiovascular issues.[medical citation needed]
The termspolyaromatic hydrocarbon,[6] orpolynuclear aromatic hydrocarbon[7] (abbreviated as PNA) are also used for this concept.[8]
By definition, polycyclic aromatic hydrocarbons have multiple aromatic rings, precludingbenzene from being considered a PAH. Some sources, such as theUS EPA andCDC, considernaphthalene to be the simplest PAH.[9] Other authors consider PAHs to start with the tricyclic speciesphenanthrene andanthracene.[citation needed] Most authors exclude compounds that includeheteroatoms in the rings, or carrysubstituents.[10]
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 bealternant.[11]
The following are examples of PAHs that vary in the number and arrangement of their rings:
Most PAHs, like naphthalene, anthracene, and coronene, are planar. This geometry is a consequence of the fact that theσ-bonds that result from the merger of sp2hybrid orbitals of adjacent carbons lie on the same plane as the carbon atom. Those compounds areachiral, 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 thetopology of the molecule and the stiffness (in length and angle) of the carbon-carbon bonds. For example, unlikecoronene,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 11kcal/mol).[12]
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.[13]
Other PAHs that might seem to be planar, considering only the carbon skeleton, may be distorted by repulsion or steric hindrance between thehydrogen 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.[14] This effect also causes distortion of picene.[15]
Adding another benzene ring to form dibenzo[c,g]phenanthrene createssteric hindrance between the two extreme hydrogen atoms.[16] Adding two more rings on the same sense yieldsheptahelicene in which the two extreme rings overlap.[17] These non-planar forms are chiral, and theirenantiomers can be isolated.[18]
Thebenzenoid 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 liketriangulene orheptacene.[18]
As of 2012, over 300 benzenoid hydrocarbons had been isolated and characterized.[18]
Benzene-substructure resonance analysis for Clar's rule
Phenanthrene
Anthracene
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.[citation needed] In contrast, inanthracene 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.[citation needed] This difference in number of sextets is reflected in the differingultraviolet–visible spectra of these two isomers, as higher Clar pi-sextets are associated with largerHOMO-LUMO gaps;[21] the highest-wavelength absorbance of phenanthrene is at 293 nm, while anthracene is at 374 nm.[22] Three Clar structures with two sextets each are present in the four-ringchrysene 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.[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).
PAHs arenonpolar andlipophilic. Larger PAHs are generallyinsoluble in water, although some smaller PAHs are soluble.[23][24] The larger members are also poorly soluble inorganic solvents and inlipids. The larger members, e.g. perylene, are strongly colored.[18]
Polycyclic aromatic compounds characteristically yieldradicals andanions upon treatment with alkali metals. The large PAH form dianions as well.[25] Theredox potential correlates with the size of the PAH.
Half-cell potential of aromatic compounds against theSCE (Fc+/0)[26]
The dominant sources of PAHs in the environment are from human activity: wood-burning and combustion of otherbiofuels such as dung or crop residues contribute more than half of annual global PAH emissions, particularly due to biofuel use in India and China.[28][29] As of 2004, industrial processes and the extraction and use offossil fuels made up slightly more than one quarter of global PAH emissions, dominating outputs in industrial countries such as the United States.[28]
A year-long sampling campaign in Athens, Greece found a third (31%) of PAH urbanair 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.[30][31]
Lower-temperature combustion, such astobacco smoking orwood-burning, tends to generate low molecular weight PAHs, whereas high-temperature industrial processes typically generate PAHs with higher molecular weights.[32] Incense is also a source.[33]
PAHs are typically found as complex mixtures.[34][32]
PAHs may result from the incompletecombustion oforganic matter in naturalwildfires.[29][28] 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.[28][relevant?]
Polycyclic aromatic hydrocarbons are primarily found in natural sources such asbitumen.[35][36]
PAHs can also be produced geologically when organic sediments are chemically transformed intofossil fuels such as oil andcoal.[34] The rare mineralsidrialite,curtisite, andcarpathite consist almost entirely of PAHs that originated from such sediments, that were extracted, processed, separated, and deposited by very hot fluids.[37][15][38]High levels of such PAHs have been detected in theCretaceous-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.[39]
PAHs are prevalent in theinterstellar 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.[40] PAHs generally trace regions of cold molecular gas, which are optimum environments for the formation of stars.[40]
Certain PAHs such asperylene can also be generated inanaerobic sediments from existing organic material, although it remains undetermined whether abiotic or microbial processes drive their production.[43][44][45]
Most PAHs are insoluble in water, which limits their mobility in the environment, although PAHssorb to fine-grained organic-richsediments.[46][47][48][49] Aqueous solubility of PAHs decreases approximatelylogarithmically asmolecular mass increases.[50]
Two-ringed PAHs, and to a lesser extent three-ringed PAHs, dissolve in water, making them more available for biological uptake anddegradation.[49][50][51] Further, two- to four-ringed PAHsvolatilize sufficiently to appear in the atmosphere predominantly in gaseous form, although the physical state of four-ring PAHs can depend on temperature.[52][53] In contrast, compounds with five or more rings have low solubility in water and low volatility; they are therefore predominantly in solidstate, bound toparticulateair pollution,soils, orsediments.[49] In solid state, these compounds are less accessible for biological uptake or degradation, increasing their persistence in the environment.[50][54]
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.[34][28][55] 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.[34][28][55]Surgical smoke plumes have been proven to contain PAHs in several independent research studies.[56]
A wood-burning open-air cookingstove.Smoke from solid fuels likewood is a large source of PAHs globally.
Burning solid fuels such ascoal andbiofuels 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 toindoor particulate air pollution containing PAHs, particularly for women and children who spend more time in the home or cooking.[28][57]
Smog inCairo. 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.[34][28] Geographically, major roadways are thus sources of PAHs, which may distribute in the atmosphere or deposit nearby.[58]Catalytic converters are estimated to reduce PAH emissions from gasoline-fired vehicles by 25-fold.[34]
In industrial countries, people who smoke tobacco products, or who are exposed tosecond-hand smoke, are among the most highly exposed groups;tobacco smoke contributes to 90% of indoor PAH levels in the homes of smokers.[55] 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.[61] Exposure also occurs through drinking alcohol aged in charred barrels, flavored with peat smoke, or made with roasted grains.[62] PAHs are typically at low concentrations in drinking water.[55]
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 thewater column.[34] PAHs have a strong affinity fororganic carbon, and thus highly organic sediments inrivers,lakes, and theocean can be a substantial sink for PAHs.[58]
Algae and someinvertebrates such asprotozoans,mollusks, and manypolychaetes have limited ability tometabolize PAHs andbioaccumulate disproportionate concentrations of PAHs in their tissues; however, PAH metabolism can vary substantially across invertebrate species.[64][66] Mostvertebrates metabolize and excrete PAHs relatively rapidly.[64] Tissue concentrations of PAHs do not increase (biomagnify) from the lowest to highest levels of food chains.[64]
PAHs transform slowly to a wide range of degradation products. Biological degradation bymicrobes is a dominant form of PAH transformation in the environment.[54][67]Soil-consuming invertebrates such asearthworms speed PAH degradation, either through direct metabolism or by improving the conditions for microbial transformations.[67] 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.[64][68][69]
TheBritish Geological Survey reported the amount and distribution of PAH compounds including parent and alkylated forms in urban soils at 76 locations inGreater London.[70] 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 andfluoranthene andpyrene were generally the most abundant PAHs.[70]Benzo[a]pyrene (BaP), the most toxic of the parent PAHs, is widely considered a key marker PAH for environmental assessments;[71] the normal background concentration of BaP in the London urban sites was 6.9 mg/kg (dry soil weight).[70]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 microbialbiodegradation.
Managedburning ofmoorland vegetation in the UK has been shown to generate PAHs which become incorporated into thepeat surface.[72] Burning of moorland vegetation such asheather 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 lowermolecular weight PAHs are attenuated by biotic decay andphotodegradation.[72] 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).[72]
Concentrations of PAHs in river and estuarinesediments 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.[63][73][74] Consequently, the concentrations ofpollutants in estuaries tends to decrease at the river mouth.[75] Understanding of sediment hosted PAHs in estuaries is important for the protection of commercialfisheries (such asmussels) and general environmental habitat conservation because PAHs can impact the health of suspension and sediment feeding organism.[76] 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.[74] Typical PAH concentrations in UK estuaries range from about 19 to 16,163 µg/kg (dry sediment weight) in theRiver Clyde and 626 to 3,766 µg/kg in theRiver Mersey.[74][77] In general estuarine sediments with a higher naturaltotal organic carbon content (TOC) tend to accumulate PAHs due to highsorption capacity of organic matter.[77] A similar correspondence between PAHs and TOC has also been observed in the sediments of tropicalmangroves located on the coast of southern China.[78]
Cancer is a primary human health risk of exposure to PAHs.[79] Exposure to PAHs has also been linked with cardiovascular disease and poor fetal development.
PAHs have been linked toskin,lung,bladder,liver, andstomach cancers in well-established animal model studies.[79] Specific compounds classified by various agencies as possible or probable human carcinogens are identified in the section "Regulation and Oversight" below.
Historically, PAHs contributed substantially to our understanding of adverse health effects from exposures toenvironmental contaminants, including chemicalcarcinogenesis.[80] In 1775,Percivall Pott, a surgeon atSt. Bartholomew's Hospital in London, observed thatscrotal cancer was unusually common in chimney sweepers and proposed the cause as occupational exposure tosoot.[81] A century later,Richard von Volkmann reported increased skin cancers in workers of thecoal 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 andIchicawa were the first to experimentally produce cancers, specifically of the skin, by topically applying coal tar to rabbit ears.[81]
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 characteristicfluorescent pattern that was similar but not identical tobenz[a]anthracene, a PAH that was subsequently demonstrated to causetumors.[81] Cook, Hewett andHieger then linked the specific spectroscopic fluorescent profile ofbenzo[a]pyrene to that of the carcinogenic component of coal tar,[81] 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, includingRichard Doll and various others, reported greater rates of death fromlung cancer following occupational exposure to PAH-rich environments among workers incoke ovens andcoal carbonization andgasification processes.[82]
The structure of a PAH influences whether and how the individual compound is carcinogenic.[79][83] Some carcinogenic PAHs aregenotoxic and inducemutations that initiate cancer; others are not genotoxic and instead affect cancer promotion or progression.[83][84]
PAHs that affectcancer initiation are typically first chemically modified byenzymes into metabolites that react with DNA, leading to mutations. When the DNA sequence is altered in genes that regulatecell 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.[85] Mutagenic metabolites of PAHs includediol epoxides,quinones, andradical PAHcations.[85][86][87] These metabolites can bind to DNA at specific sites, forming bulky complexes calledDNA adducts that can be stable or unstable.[81][88] Stable adducts may lead toDNA replication errors, while unstable adducts react with the DNA strand, removing apurine base (eitheradenine orguanine).[88] Such mutations, if they are not repaired, can transform genes encoding for normalcell signaling proteins into cancer-causingoncogenes.[83] Quinones can also repeatedly generatereactive oxygen species that may independently damage DNA.[85]
Enzymes in thecytochrome family (CYP1A1,CYP1A2,CYP1B1) metabolize PAHs to diol epoxides.[89] PAH exposure can increase production of the cytochrome enzymes, allowing the enzymes to convert PAHs into mutagenic diol epoxides at greater rates.[89] In this pathway, PAH molecules bind to thearyl hydrocarbon receptor (AhR) and activate it as atranscription 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.[89]
Low molecular weight PAHs, with two to four aromatic hydrocarbon rings, are more potent asco-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.[90] Low-molecular-weight PAHs that have bay or bay-like regions can dysregulategap junction channels, interfering with intercellular communication, and also affectmitogen-activated protein kinases that activate transcription factors involved in cell proliferation.[90] 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.
In laboratory experiments, animals exposed to certain PAHs have shown increased development of plaques (atherogenesis) within arteries.[93] Potential mechanisms for thepathogenesis and development of atherosclerotic plaques may be similar to the mechanisms involved in the carcinogenic and mutagenic properties of PAHs.[93] A leading hypothesis is that PAHs may activate the cytochrome enzymeCYP1B1 invascular 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 inplaque formation.[92][93] These quinone metabolites also generatereactive oxygen species that may alter the activity of genes that affect plaque formation.[93]
Oxidative stress following PAH exposure could also result in cardiovascular disease by causinginflammation, which has been recognized as an important factor in the development of atherosclerosis and cardiovascular disease.[94][95]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.[96]
Multipleepidemiological studies of people living in Europe, the United States, and China have linkedin utero exposure to PAHs, through air pollution or parental occupational exposure, with poor fetal growth, reduced immune function, and poorerneurological development, including lowerIQ.[97][98][99][100]
PAHs possess very characteristicUV absorbance spectra. These often possess many absorbance bands and are unique for each ring structure. Thus, for a set ofisomers, each isomer has a different UV absorbance spectrum than the others. This is particularly useful in the identification of PAHs. Most PAHs are alsofluorescent, 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 exhibitingsemi-conducting and other behaviors.
PAHs may be abundant in the universe.[5][109][110][111] They seem to have been formed as early as a couple of billion years after theBig Bang, and are associated withnew stars andexoplanets.[4] More than 20% of thecarbon in the universe may be associated with PAHs.[4] PAHs are considered possiblestarting material for theearliest forms of life.[4][5]Light emitted by theRed Rectangle nebula possesses spectral signatures that suggest the presence ofanthracene andpyrene.[112][113] 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[112] that PAHs—which may have been vital in the formation ofearly life on Earth—can only originate in nebulae.[113]
Low-temperature chemical pathways from simpleorganic compounds to complex PAHs are of interest. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere ofSaturn's moonTitan, and may be significant pathways, in terms of thePAH world hypothesis, in producing precursors to biochemicals related to life as we know it.[117][118]
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