View source for Polycyclic aromatic hydrocarbon

{{Short description|Hydrocarbon composed of multiple aromatic rings}}
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 | caption3 = 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. Middle: [[ball-and-stick model]] showing all carbon and hydrogen atoms. Bottom: [[atomic force microscopy]] image.}}

A '''Polycyclic aromatic hydrocarbon''' ('''PAH''') is any member of a class of [[organic compound]]s that is composed of multiple fused [[aromatic ring]]s. 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.{{medical citation needed|date=April 2025}}

Polycyclic aromatic hydrocarbons are discussed as possible [[PAH world hypothesis|starting materials]] for [[abiotic]] syntheses of [[material]]s required by the [[Abiogenesis#PAH world hypothesis|earliest forms of life]].<ref name="NASA-20140221"/><ref name="NASA-20110413"/>

==Nomenclature and structure==
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 {{doi|10.1021/ac00253a023}}</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. {{doi|10.1021/es00178a012}}</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 [[United States Environmental Protection Agency|US EPA]] and [[Centers for Disease Control and Prevention|CDC]] consider [[naphthalene]] to be the simplest PAH.<ref>{{Cite web|url=https://www.epa.gov/sites/production/files/2014-03/documents/pahs_factsheet_cdc_2013.pdf|title=Polycyclic Aromatic Hydrocarbons (PAHs)|quote=Naphthalene is a PAH that is produced commercially in the US}}</ref> Most authors exclude compounds that include [[heteroatom]]s in the rings, or carry [[substituent]]s.<ref>{{cite journal |last1=Fetzer |first1=John C. |title=The chemistry and analysis of large PAHs |journal=Polycyclic Aromatic Compounds |date=16 April 2007 |volume=27 |issue=2 |pages=143–162 |doi=10.1080/10406630701268255 |s2cid=97930473 }}</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>{{Cite book| publisher = Springer| isbn = 978-3-540-49697-7| pages = 1–54| last = Harvey| first = R. G.| title = PAHs and Related Compounds: Chemistry| chapter = Environmental Chemistry of PAHs| series = The Handbook of Environmental Chemistry| date = 1998}}</ref>

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

<gallery caption="List of polycyclic aromatic hydrocarbons">
File:Naphthalene-2D-Skeletal.svg|[[Naphthalene]]
File:Diphenylene structure 2.svg|[[Biphenylene]]
File:Fluorene.svg|[[Fluorene]]
File:Acenaphthene.svg|[[Acenaphthene]]
File:Acenaphthylene.svg|[[Acenaphthylene]]
File:Anthracene.svg|[[Anthracene]]
File:Phenanthrene.svg|[[Phenanthrene]]
File:Phenalene.svg|[[Phenalene]]
File:Fluoranthene.svg|[[Fluoranthene]]
File:Tetracene.svg|[[Tetracene]]
File:Benzanthracene.png|[[Benz(a)anthracene|Benz[''a'']anthracene]]
File:Chrysene.svg|[[Chrysene]]
File:Triphenylene.svg|[[Triphenylene]]
File:Pyrene.svg|[[Pyrene]]
File:Pentacene.svg|[[Pentacene]]
File:Benzo-a-pyrene.svg|[[Benzo(a)pyrene|Benzo[''a'']pyrene]]
File:Perylene.svg|[[Perylene]]
File:Corannulene.svg|[[Corannulene]]
File:Benzo(ghi)perilene.png|[[Benzo(ghi)perylene|Benzo[''ghi'']perylene]]
File:Coronene.svg|[[Coronene]]
</gallery>

===Geometry===
Most PAHs, like naphthalene, anthracene, and coronene, are planar. This geometry is a consequence of the fact that the [[σ-bond]]s that result from the merger of sp<sup>2</sup> [[Orbital hybridisation|hybrid orbitals]] of adjacent carbons lie on the same plane as the carbon atom. Those compounds are [[chirality|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 [[kilocalorie|kcal]]/[[mole (chemistry)|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. {{doi|10.1016/j.molstruc.2004.02.027}}</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 {{doi|10.1021/ci0100853}}</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. {{doi|10.1039/JR9540003302}}</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 (C<sub>24</sub>H<sub>12</sub>) from the Picacho Peak Area, San Benito County, California: Evidences for the hydrothermal formation". ''American Mineralogist'', volume 92, issues 8-9, pages 1262–1269.
{{doi|10.2138/am.2007.2509}}</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. {{doi|10.1016/S0021-9673(00)82245-1}}</ref> Adding two more rings on the same sense yields [[helicene|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. {{doi|10.1039/C39760000099}}</ref> These non-planar forms are chiral, and their [[enantiomer]]s can be isolated.<ref name=gutm2012>Ivan Gutman and Sven J. Cyvin (2012): ''Introduction to the Theory of Benzenoid Hydrocarbons''. 152 pages. {{isbn|9783642871436}}</ref>

===Benzenoid hydrocarbons===

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<sup>2</sup> 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 aromaticity==
{{See also|Clar's rule}}
The [[aromaticity]] varies for PAHs. According to [[Clar's rule]],<ref>{{ cite book | last = Clar | first = E. | date = 1964 | title = Polycyclic Hydrocarbons | url = https://archive.org/details/polycyclichydroc0001clar | url-access = registration | location = New York, NY | publisher = [[Academic Press]] | lccn = 63012392 }}</ref> the [[resonance structure]] of a PAH that has the largest number of disjoint aromatic [[pi bond|pi sextets]]—i.e. [[benzene]]-like moieties—is the most important for the characterization of the properties of that PAH.<ref>{{ cite journal | last1 = Portella | first1 = G. | last2 = Poater | first2 = J. | last3 = Solà | first3 = M. | title = Assessment of Clar's aromatic π-sextet rule by means of PDI, NICS and HOMA indicators of local aromaticity | journal = Journal of Physical Organic Chemistry | date = 2005 | volume = 18 | issue = 8 | pages = 785–791 | doi = 10.1002/poc.938 }}</ref>

<gallery caption="Benzene-substructure resonance analysis for Clar's rule" class="center">
File:Phenanthrene Clar rule.svg|Phenanthrene
File:Anthracene Clar rule.svg|Anthracene
File:Chrysene Clar rule.svg|Chrysene
</gallery>

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|date=June 2014}} 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.{{citation needed|date=June 2014}} This difference in number of sextets is reflected in the differing [[Ultraviolet–visible spectroscopy|ultraviolet–visible spectra]] of these two isomers, as higher Clar pi-sextets are associated with larger [[HOMO-LUMO]] gaps;<ref>{{cite journal|last1=Chen |first1=T.-A. |last2=Liu |first2=R.-S.|title=Synthesis of Polyaromatic Hydrocarbons from Bis(biaryl)diynes: Large PAHs with Low Clar Sextets|journal=Chemistry: A European Journal|year=2011|volume=17|issue=21 |pages=8023–8027|doi=10.1002/chem.201101057|pmid=21656594 }}</ref> the highest-wavelength absorbance of phenanthrene is at 293&nbsp;nm, while anthracene is at 374&nbsp;nm.<ref>{{cite journal |title= The ultraviolet spectra of aromatic hydrocarbons: Predicting substitution and isomerism changes |first= Philip E. |last= Stevenson |journal= Journal of Chemical Education |year= 1964 |volume= 41 |issue= 5 |pages= 234–239 |doi= 10.1021/ed041p234 |bibcode= 1964JChEd..41..234S }}</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.{{citation needed|date=June 2014}} 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).

==Properties==

===Physicochemical===
PAHs are [[Chemical polarity#Nonpolar molecules|nonpolar]] and [[lipophilicity|lipophilic]]. Larger PAHs are generally [[solubility|insoluble]] in water, although some smaller PAHs are soluble.<ref>{{cite journal|first1=Xinliang |last1=Feng |first2=Wojciech |last2=Pisula |first3=Klaus |last3=Müllen |title=Large polycyclic aromatic hydrocarbons: Synthesis and discotic organization |journal=Pure and Applied Chemistry |date=2009 |volume=81 |issue=2 |pages=2203–2224 |doi=10.1351/PAC-CON-09-07-07|s2cid=98098882 |doi-access=free }}</ref><ref>{{citation|title=Guidelines for drinking-water quality |edition=2nd |chapter=Addendum to Vol. 2. Health criteria and other supporting information |publisher=World Health Organization |location=Geneva |date=1998}}</ref> The larger members are also poorly soluble in [[organic solvent]]s and in [[lipid]]s. The larger members, e.g. perylene, are strongly colored.<ref name=gutm2012/>

===Redox===
Polycyclic aromatic compounds characteristically yield [[free radical|radical]]s and [[anion]]s upon treatment with alkali metals. The large PAH form dianions as well.<ref>{{cite journal |doi=10.1039/C5NJ02841H|title=Isolation of gravimetrically quantifiable alkali metal arenides using 18-crown-6|year=2016|last1=Castillo|first1=Maximiliano|last2=Metta-Magaña|first2=Alejandro J.|last3=Fortier|first3=Skye|journal=New Journal of Chemistry|volume=40|issue=3|pages=1923–1926}}</ref> The [[redox potential]] correlates with the size of the PAH.
:{| class="wikitable"
|+[[Half-cell]] potential of aromatic compounds against the [[saturated calomel electrode|SCE]] (Fc<sup>+/0</sup>)<ref>{{cite journal|last1=Ruoff |first1=R. S. |last2=Kadish |first2=K. M. |last3=Boulas |first3=P. |last4=Chen |first4=E. C. M.|title=Relationship between the electron affinities and half-wave reduction potentials of fullerenes, aromatic hydrocarbons, and metal complexes|journal=The Journal of Physical Chemistry|year=1995|volume=99|issue=21 |pages=8843–8850|doi=10.1021/j100021a060}}</ref>
!Compound
!Potential (V)
|-
|[[benzene]]
| −3.42
|-
|[[biphenyl]]<ref>{{cite journal|title=Highly reactive calcium for the preparation of organocalcium reagents: 1-adamantyl calcium halides and their addition to ketones: 1-(1-adamantyl)cyclohexanol|first1=Reuben D. |last1=Rieke |first2=Tse-Chong |last2=Wu |first3=Loretta I. |last3=Rieke|journal=Organic Syntheses|year=1995|volume=72|page=147|doi=10.15227/orgsyn.072.0147}}</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
|}

==Sources==

===Artificial===
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>{{Cite book| publisher = CRC Press| isbn = 978-1-4398-3831-0| pages = 97–126| last1 = Ramesh| first1 = A.| last2 = Archibong| first2 = A.| last3 = Hood| first3 = D. B.| last4 = Guo| first4 = Z.| last5 = Loganathan| first5 = B. G.|display-authors=3| title = Global Contamination Trends of Persistent Organic Chemicals| chapter = Global environmental distribution and human health effects of polycyclic aromatic hydrocarbons| location = Boca Raton, FL| date = 2011}}</ref><ref name=abdel2016>{{cite journal |last1=Abdel-Shafy|first1=Hussein I.|title=A review on polycyclic aromatic hydrocarbons: Source, environmental impact, effect on human health and remediation|journal=Egyptian Journal of Petroleum |date=2016 |volume=25 |issue=1 |pages=107–123 |doi=10.1016/j.ejpe.2015.03.011|doi-access=free|bibcode=2016EgyJP..25..107A }}</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 ([[carcinogen]]ic 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>{{cite news |title=Wood burners cause nearly half of urban air pollution cancer risk – study |url=https://www.theguardian.com/environment/2021/dec/17/wood-burners-urban-air-pollution-cancer-risk-study |access-date=16 January 2022 |work=The Guardian |date=17 December 2021 |language=en}}</ref><ref>{{cite journal |last1=Tsiodra |first1=Irini |last2=Grivas |first2=Georgios |last3=Tavernaraki |first3=Kalliopi |last4=Bougiatioti |first4=Aikaterini |last5=Apostolaki |first5=Maria |last6=Paraskevopoulou |first6=Despina |last7=Gogou |first7=Alexandra |last8=Parinos |first8=Constantine |last9=Oikonomou |first9=Konstantina |last10=Tsagkaraki |first10=Maria |last11=Zarmpas |first11=Pavlos |last12=Nenes |first12=Athanasios |last13=Mihalopoulos |first13=Nikolaos |display-authors=3|title=Annual exposure to polycyclic aromatic hydrocarbons in urban environments linked to wintertime wood-burning episodes |journal=Atmospheric Chemistry and Physics |date=7 December 2021 |volume=21 |issue=23 |pages=17865–17883 |doi=10.5194/acp-21-17865-2021 |bibcode=2021ACP....2117865T |s2cid=245103794 |language=English |issn=1680-7316|doi-access=free }}</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>{{Cite journal |last1=Bootdee |first1=Susira |last2=Chantara |first2=Somporn |last3=Prapamontol |first3=Tippawan |date=2016-07-01 |title=Determination of PM2.5 and polycyclic aromatic hydrocarbons from incense burning emission at shrine for health risk assessment |journal=Atmospheric Pollution Research |volume=7 |issue=4 |pages=680–689 |doi=10.1016/j.apr.2016.03.002 |bibcode=2016AtmPR...7..680B |issn=1309-1042|doi-access=free }}</ref>

PAHs are typically found as complex mixtures.<ref name=ravi2008 /><ref name=Tobis>{{Cite journal |doi = 10.1016/j.envpol.2011.10.025 |pmid = 22243855 |issn = 0269-7491 |volume = 162 |pages = 110–119 |last1 = Tobiszewski |first1 = M. |last2 = Namieśnik |first2 = J. |title = PAH diagnostic ratios for the identification of pollution emission sources |journal = Environmental Pollution |date = 2012|bibcode = 2012EPoll.162..110T }}</ref>

===Natural===
====Natural fires====
PAHs may result from the incomplete [[combustion]] of [[organic matter]] in natural [[wildfire]]s.<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 />{{relevance|reason=What do these specific countries have to do with natural fires?  California burns more than the average volcano and huge parts of Africa and  Asia are desert and can't burn|date=April 2025}}

====Fossil carbon====
Polycyclic aromatic hydrocarbons are primarily found in natural sources such as [[bitumen]].<ref>{{Ullmann | first=Anja | last=Sörensen | first2=Bodo | last2=Wichert | title=Asphalt and Bitumen | DOI=10.1002/14356007.a03_169.pub2}}</ref><ref>{{Cite web|url=http://www.qrpoil.com/site/?bitumen|title=QRPOIL |website=www.qrpoil.com|access-date=2018-07-19|archive-url=https://web.archive.org/web/20160304042118/http://www.qrpoil.com/site/?bitumen|archive-date=2016-03-04}}</ref>

PAHs can also be produced geologically when organic sediments are chemically transformed into [[fossil fuel]]s such as oil and [[coal]].<ref name=ravi2008>{{Cite journal| doi = 10.1016/j.atmosenv.2007.12.010| issn = 1352-2310| volume = 42| issue = 13| pages = 2895–2921| last1 = Ravindra| first1 = K.| last2 = Sokhi| first2 = R.| last3 = Van Grieken| first3 = R.| title = Atmospheric polycyclic aromatic hydrocarbons: Source attribution, emission factors and regulation| journal = Atmospheric Environment| date = 2008| bibcode = 2008AtmEn..42.2895R| hdl = 2299/1986| s2cid = 2388197| hdl-access = free}}</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. {{doi|10.1016/0009-2541(86)90148-8}}</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. {{doi|10.1016/0009-2541(75)90064-9}}</ref>
High levels of such PAHs have been detected in the [[Cretaceous-Tertiary boundary|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 δ<sup>13</sup>C of a terrestrial biomarker at the Cretaceous-Tertiary boundary at Caravaca, Spain ". ''Geology'', volume 27, issue 8, pages 723–726 {{doi|10.1130/0091-7613(1999)027<0723:SOPPAH>2.3.CO;2}}</ref>

====Extraterrestrial====
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">{{Citation|title=Svea Hernandez: Shining Light on the CO-dark Molecular Gas in the Heart of M83| date=13 December 2021 |url=https://www.youtube.com/watch?v=uJ4wCL_hfiw|language=en|access-date=2022-01-09}}</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 [[molecular cloud|clouds]] in our own galaxy or identify star forming galaxies in the distant universe.<ref>{{cite web|url=http://legacy.spitzer.caltech.edu/features/articles/20050627.shtml|title=Understanding Polycyclic Aromatic Hydrocarbons|date= 2005-06-27|author=Robert Hurt|website=Spitzer Space Telescope|access-date=2018-04-21}}</ref> In June 2013, PAHs were detected in the [[upper atmosphere]] of [[Titan (moon)|Titan]], the largest [[moon]] of the [[planet]] [[Saturn]].<ref>{{cite news |last=López Puertas |first=Manuel |url=http://www.iaa.es/content/pahs-titans-upper-atmosphere |title=PAHs in Titan's Upper Atmosphere |date=2013-06-06 |work=[[Spanish National Research Council|CSIC]] |access-date=2013-06-06 |archive-date=2013-12-03 |archive-url=https://web.archive.org/web/20131203030521/http://www.iaa.es/content/pahs-titans-upper-atmosphere  }}</ref>

====Minor sources====
[[Volcanic eruption]]s may emit PAHs.<ref name=ravi2008 />

Certain PAHs such as [[perylene]] can also be generated in [[Hypoxia (environmental)|anaerobic]] sediments from existing organic material, although it remains undetermined whether abiotic or microbial processes drive their production.<ref>{{cite journal |last1=Meyers |first1=Philip A. |last2=Ishiwatari |first2=Ryoshi |title=Lacustrine organic geochemistry—an overview of indicators of organic matter sources and diagenesis in lake sediments |journal=Organic Geochemistry |date=September 1993 |volume=20 |issue=7 |pages=867–900 |doi=10.1016/0146-6380(93)90100-P |bibcode=1993OrGeo..20..867M |hdl=2027.42/30617 |s2cid=36874753 |url=https://deepblue.lib.umich.edu/bitstream/2027.42/30617/1/0000257.pdf |hdl-access=free }}</ref><ref>{{Cite journal| doi = 10.1016/S0009-2541(00)00415-0| issn = 0009-2541| volume = 177| issue = 3–4| pages = 309–322| last1 = Silliman| first1 = J. E.| last2 = Meyers| first2 = P. A.| last3 = Eadie| first3 = B. J.| last4 = Val Klump| first4 = J.| title = A hypothesis for the origin of perylene based on its low abundance in sediments of Green Bay, Wisconsin| journal = Chemical Geology| date = 2001| bibcode = 2001ChGeo.177..309S}}</ref><ref>{{cite journal |last1=Wakeham |first1=Stuart G. |last2=Schaffner |first2=Christian |last3=Giger |first3=Walter |title=Poly cyclic aromatic hydrocarbons in Recent lake sediments—II. Compounds derived from biogenic precursors during early diagenesis |journal=Geochimica et Cosmochimica Acta |date=March 1980 |volume=44 |issue=3 |pages=415–429 |doi=10.1016/0016-7037(80)90041-1 |bibcode=1980GeCoA..44..415W }}</ref>

==Distribution in the environment==

===Aquatic environments===
Most PAHs are insoluble in water, which limits their mobility in the environment, although PAHs [[sorption|sorb]] to fine-grained organic-rich [[sediment]]s.<ref>{{cite journal|last1=Walker |first1=T. R. |last2=MacAskill |first2=D. |last3=Rushton |first3=T. |last4=Thalheimer |first4=A. |last5=Weaver |first5=P. |display-authors=3|date=2013 |title=Monitoring effects of remediation on natural sediment recovery in Sydney Harbour, Nova Scotia |journal=Environmental Monitoring and Assessment |volume=185 |issue=10 |pages=8089–107 |doi=10.1007/s10661-013-3157-8 |pmid=23512488 |bibcode=2013EMnAs.185.8089W |s2cid=25505589 }}</ref><ref>{{cite journal|last1=Walker |first1=T. R. |last2=MacAskill |first2=D. |last3=Weaver |first3=P. |date=2013 |title=Environmental recovery in Sydney Harbour, Nova Scotia: Evidence of natural and anthropogenic sediment capping |journal=Marine Pollution Bulletin |volume=74 |issue=1 |pages=446–52 |doi=10.1016/j.marpolbul.2013.06.013 |pmid=23820194 |bibcode=2013MarPB..74..446W }}</ref><ref>{{cite journal|last1=Walker |first1=T. R. |last2=MacAskill |first2=N. D. |last3=Thalheimer |first3=A. H. |last4=Zhao |first4=L. |date=2017 |title=Contaminant mass flux and forensic assessment of polycyclic aromatic hydrocarbons: Tools to inform remediation decision making at a contaminated site in Canada |journal=Remediation Journal |volume=27 |issue=4 |pages=9–17|doi=10.1002/rem.21525 |bibcode=2017RemJ...27d...9W }}</ref><ref name=Choi-2007>{{Cite book| publisher = World Health Organization| last1 = Choi| first1 = H.| last2 = Harrison| first2 = R.| last3 = Komulainen| first3 = H.| last4 = Delgado Saborit| first4 = J.| title = WHO Guidelines for Indoor Air Quality: Selected Pollutants| chapter = Polycyclic aromatic hydrocarbons| location = Geneva| date = 2010| chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK138709/}}</ref> Aqueous solubility of PAHs decreases approximately [[Logarithmic growth|logarithmically]] as [[molecular mass]] increases.<ref name=Johnsen-2005>{{Cite journal| doi = 10.1016/j.envpol.2004.04.015| pmid = 15327858| issn = 0269-7491| volume = 133| issue = 1| pages = 71–84| last1 = Johnsen| first1 = Anders R.| last2 = Wick| first2 = Lukas Y.| last3 = Harms| first3 = Hauke| title = Principles of microbial PAH degradation in soil| journal = Environmental Pollution| date = 2005| bibcode = 2005EPoll.133...71J}}</ref>

Two-ringed PAHs, and to a lesser extent three-ringed PAHs, dissolve in water, making them more available for biological uptake and [[biodegradation|degradation]].<ref name=Choi-2007 /><ref name=Johnsen-2005 /><ref>{{Cite book| publisher = Springer Berlin Heidelberg| isbn = 978-3-642-08286-3| pages = 325–345| editor-first = A. |editor-last= Neilson| last1 = Mackay| first1 = D.| last2 = Callcott| first2 = D.| chapter = Partitioning and Physical Chemical Properties of PAHs| title = PAHs and Related Compounds| series = The Handbook of Environmental Chemistry| date = 1998| volume = 3 / 3I| doi = 10.1007/978-3-540-49697-7_8}}</ref> Further, two- to four-ringed PAHs [[volatility (chemistry)|volatilize]] sufficiently to appear in the atmosphere predominantly in gaseous form, although the physical state of four-ring PAHs can depend on temperature.<ref>{{Cite journal| doi = 10.2307/3431940| pmid = 7821285| pmc = 1566940| issn = 0091-6765| volume = 102| pages = 117–126| last1 = Atkinson| first1 = R.| last2 = Arey| first2 = J.| title = Atmospheric chemistry of gas-phase polycyclic aromatic hydrocarbons: formation of atmospheric mutagens| journal = Environmental Health Perspectives| date = 1994-10-01| issue = Suppl 4| jstor = 3431940}}</ref><ref>{{Cite journal| doi = 10.1007/s10311-007-0095-0| pmid = 29033701| pmc = 5614912| issn = 1610-3661| volume = 5| issue = 4| pages = 169–195| last = Srogi| first = K.| title = Monitoring of environmental exposure to polycyclic aromatic hydrocarbons: a review| journal = Environmental Chemistry Letters| date = 2007-11-01| bibcode = 2007EnvCL...5..169S}}</ref> In contrast, compounds with five or more rings have low solubility in water and low volatility; they are therefore predominantly in solid [[state of matter|state]], bound to [[particulates|particulate]] [[air pollution]], [[soils]], or [[sediment]]s.<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">{{Cite journal| doi = 10.1016/j.jhazmat.2009.03.137| pmid = 19442441| issn = 0304-3894| volume = 169| issue = 1–3| pages = 1–15| last1 = Haritash| first1 = A. K.| last2 = Kaushik| first2 = C. P.| title = Biodegradation aspects of polycyclic aromatic hydrocarbons (PAHs): A review| journal = Journal of Hazardous Materials| date = 2009| bibcode = 2009JHzM..169....1H}}</ref>

===Human exposure===
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>{{Cite book| publisher = World Health Organization| last1 = Choi| first1 = H.| last2 = Harrison| first2 = R.| last3 = Komulainen| first3 = H.| last4 = Delgado Saborit| first4 = J.| title = WHO Guidelines for Indoor Air Quality: Selected Pollutants| chapter = Polycyclic aromatic hydrocarbons| location = Geneva| date = 2010| chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK138709/?report=classic}}</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>{{Cite journal|last1=Dobrogowski|first1=Miłosz|last2=Wesołowski|first2=Wiktor|last3=Kucharska|first3=Małgorzata|last4=Sapota|first4=Andrzej|last5=Pomorski|first5=Lech|display-authors=3|date=2014-01-01|title=Chemical composition of surgical smoke formed in the abdominal cavity during laparoscopic cholecystectomy – Assessment of the risk to the patient|journal=International Journal of Occupational Medicine and Environmental Health|volume=27|issue=2|pages=314–25|doi=10.2478/s13382-014-0250-3|pmid=24715421|issn=1896-494X|doi-access=free}}</ref>

[[File:Kochen_%C3%BCber_offenem_Feuer.JPG|thumb|left|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 [[biofuel]]s 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 air pollution in developing nations|indoor particulate air pollution]] containing PAHs, particularly for women and children who spend more time in the home or cooking.<ref name=Ramesh2011 /><ref>{{Cite journal| doi = 10.1016/j.jhazmat.2011.05.087| pmid = 21705140| issn = 0304-3894| volume = 192| issue = 2| pages = 425–431| last1 = Kim| first1 = K.-H.| last2 = Jahan| first2 = S. A.| last3 = Kabir| first3 = E.| title = A review of diseases associated with household air pollution due to the use of biomass fuels| journal = Journal of Hazardous Materials| date = 2011| bibcode = 2011JHzM..192..425K}}</ref>

[[File:Cairo in smog.jpg|thumb|[[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>{{Cite journal| doi = 10.1007/s10311-007-0095-0| pmid = 29033701| pmc = 5614912| issn = 1610-3661| volume = 5| issue = 4| pages = 169–195| last = Srogi| first = K.| title = Monitoring of environmental exposure to polycyclic aromatic hydrocarbons: a review| journal = Environmental Chemistry Letters| date = 2007| bibcode = 2007EnvCL...5..169S}}</ref> [[Catalytic converter]]s 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 electrode]]s, or exposure to [[diesel exhaust]].<ref name=Boffetta1997>{{Cite journal| doi = 10.1023/A:1018465507029| issn = 1573-7225| volume = 8| issue = 3| pages = 444–472| last1 = Boffetta| first1 = P.| last2 = Jourenkova| first2 = N.| last3 = Gustavsson| first3 = P.| title = Cancer risk from occupational and environmental exposure to polycyclic aromatic hydrocarbons| journal = Cancer Causes & Control| date = 1997| pmid = 9498904| s2cid = 35174373}}</ref><ref name=Wagner2015>{{Cite journal| doi = 10.1136/oemed-2014-102317| issn =1470-7926| volume = 72| issue = 3| pages = 226–233| last1 = Wagner| first1 = M.| last2 = Bolm-Audorff| first2 = U.| last3 = Hegewald| first3 = J.| last4 = Fishta| first4 = A.| last5 = Schlattmann| first5 = P.| last6 = Schmitt| first6 = J.| last7 = Seidler| first7 = A.|display-authors=3| title = Occupational polycyclic aromatic hydrocarbon exposure and risk of larynx cancer: a systematic review and meta-analysis| journal = Occupational and Environmental Medicine| access-date = 2015-04-13| date = 2015| url = http://oem.bmj.com/content/72/3/226| pmid = 25398415| s2cid =25991349}}</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 (fuel)|coke]], [[creosote]], [[carbon black]], and [[calcium carbide]]; road paving and [[Asphalt concrete|asphalt]] manufacturing; [[rubber]] [[tire]] production; manufacturing or use of [[metalworking|metal working]] fluids; and activity of coal or [[natural gas]] [[power station]]s.<ref name=ravi2008 /><ref name=Boffetta1997 /><ref name=Wagner2015 />

In industrial countries, people who smoke tobacco products, or who are exposed to [[passive smoking|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>{{Cite journal| doi = 10.1016/S1383-5742(99)00016-2| pmid = 10415437| issn = 1383-5718| volume = 443| issue = 1–2| pages = 139–147| last = Phillips| first = D. H.| title = Polycyclic aromatic hydrocarbons in the diet| journal = Mutation Research/Genetic Toxicology and Environmental Mutagenesis| date = 1999| bibcode = 1999MRGTE.443..139P}}</ref> Exposure also occurs through drinking alcohol aged in charred barrels, flavored with peat smoke, or made with roasted grains.<ref>{{Cite journal |last1=Berrigan |first1=David |last2=Freedman |first2=Neal D. |date=January 2024 |title=Invited Perspective: Polyaromatic Hydrocarbons in Alcohol—An Unappreciated Carcinogenic Mechanism? |journal=Environmental Health Perspectives |language=en |volume=132 |issue=1 |page=11302 |doi=10.1289/EHP14255 |issn=0091-6765 |pmc=10798426 |pmid=38241190|bibcode=2024EnvHP.132a1302B }}</ref> PAHs are typically at low concentrations in drinking water.<ref name=Choi2010 />

===Environmental pollution and degradation===
[[File:Manripo071210 3.jpg|thumb|left|alt=A worker's glove touches a dense patch of black oil on a sandy beach.|[[Crude oil]] on a beach after a 2007 [[oil spill]] in Korea.]]
PAHs typically disperse from [[urban area|urban]] and [[suburban]] [[nonpoint source pollution|non-point sources]] through road [[surface runoff|runoff]], [[sewage]], and [[atmospheric circulation]] and subsequent deposition of particulate air pollution.<ref name="Davis">{{cite journal |last1=Davis |first1=Emily |last2=Walker |first2=Tony R. |last3=Adams |first3=Michelle |last4=Willis |first4=Rob |last5=Norris |first5=Gary A. |last6=Henry |first6=Ronald C. |display-authors=3 |title=Source apportionment of polycyclic aromatic hydrocarbons (PAHs) in small craft harbor (SCH) surficial sediments in Nova Scotia, Canada |journal=Science of the Total Environment |volume=691 |pages=528–537 |date=July 2019 |doi=10.1016/j.scitotenv.2019.07.114 |pmid=31325853 |bibcode=2019ScTEn.691..528D |pmc=8190821 }}</ref><ref name=Hylland2006>{{Cite journal| doi = 10.1080/15287390500259327| issn = 1528-7394| volume = 69| issue = 1–2| pages = 109–123| last = Hylland| first = K.| title = Polycyclic aromatic hydrocarbon (PAH) ecotoxicology in marine ecosystems| journal = Journal of Toxicology and Environmental Health, Part A| date = 2006| pmid = 16291565| bibcode = 2006JTEHA..69..109H| s2cid = 23704718}}</ref> [[Soil]] and river [[sediment]] near industrial sites such as creosote manufacturing facilities can be highly contaminated with PAHs.<ref name=ravi2008 /> [[Oil spill]]s, creosote, [[coal mining]] dust, and other fossil fuel sources can also distribute PAHs in the environment.<ref name=ravi2008 /><ref>{{Cite journal| doi = 10.1016/j.scitotenv.2008.12.008| pmid = 19195680| issn = 0048-9697| volume = 407| issue = 8| pages = 2461–2473| last1 = Achten| first1 = C.| last2 = Hofmann| first2 = T.| title = Native polycyclic aromatic hydrocarbons (PAH) in coals – A hardly recognized source of environmental contamination| journal = Science of the Total Environment| date = 2009| bibcode = 2009ScTEn.407.2461A}}</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 [[river]]s, [[lake]]s, and the [[ocean]] can be a substantial sink for PAHs.<ref name=Srogi2007 />

[[Algae]] and some [[invertebrate]]s such as [[protozoans]], [[mollusks]], and many [[polychaete]]s have limited ability to [[metabolism|metabolize]] PAHs and [[bioaccumulate]] disproportionate concentrations of PAHs in their tissues; however, PAH metabolism can vary substantially across invertebrate species.<ref name=Hylland2006 /><ref>{{Cite journal| doi = 10.1016/j.marenvres.2007.10.001| pmid = 18023473| issn = 0141-1136| volume = 65| issue = 2| pages = 171–186| last1 = Jørgensen| first1 = A.| last2 = Giessing| first2 = A. M. B.| last3 = Rasmussen| first3 = L. J.| last4 = Andersen| first4 = O.| title = Biotransformation of polycyclic aromatic hydrocarbons in marine polychaetes| journal = Marine Environmental Research| date = 2008| bibcode = 2008MarER..65..171J| s2cid = 6404851| url = https://hal.archives-ouvertes.fr/hal-00501926/file/PEER_stage2_10.1016%252Fj.marenvres.2007.10.001.pdf}}</ref> Most [[vertebrate]]s 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 [[Microorganism|microbe]]s is a dominant form of PAH transformation in the environment.<ref name="Haritash-2009"/><ref name=Johnsen2005>{{Cite journal| doi = 10.1016/j.envpol.2004.04.015| pmid = 15327858| issn = 0269-7491| volume = 133| issue = 1| pages = 71–84| last1 = Johnsen| first1 = A. R.| last2 = Wick| first2 = L. Y.| last3 = Harms| first3 = H.| title = Principles of microbial PAH-degradation in soil| journal = Environmental Pollution| date = 2005| bibcode = 2005EPoll.133...71J}}</ref> [[Detritivore|Soil-consuming invertebrates]] such as [[earthworm]]s 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>{{Cite journal| doi = 10.1579/0044-7447(2007)36[475:SFATHO]2.0.CO;2| issn = 0044-7447| volume = 36| issue = 6| pages = 475–485| last1 = Lundstedt| first1 = S.| last2 = White| first2 = P. A.| last3 = Lemieux| first3 = C. L.| last4 = Lynes| first4 = K. D.| last5 = Lambert| first5 = I. B.| last6 = Öberg| first6 = L.| last7 = Haglund| first7 = P.| last8 = Tysklind| first8 = M. |display-authors=3| title = Sources, fate, and toxic hazards of oxygenated polycyclic aromatic hydrocarbons (PAHs) at PAH- contaminated sites| journal = Ambio: A Journal of the Human Environment| year = 2007| pmid = 17985702| s2cid = 36295655}}</ref><ref>{{Cite journal| doi = 10.1080/10590501.2012.653887| issn = 1059-0501| volume = 30| issue = 1| pages = 1–41| last1 = Fu| first1 = P. P.| last2 = Xia| first2 = Q.| last3 = Sun| first3 = X.| last4 = Yu| first4 = H.| title = Phototoxicity and Environmental Transformation of Polycyclic Aromatic Hydrocarbons (PAHs)—Light-Induced Reactive Oxygen Species, Lipid Peroxidation, and DNA Damage| journal = Journal of Environmental Science and Health, Part C| date = 2012| pmid = 22458855| bibcode = 2012JESHC..30....1F| s2cid = 205722865}}</ref>

===Urban soils===
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>{{cite journal|last1=Vane|first1=Christopher H.|last2=Kim|first2=Alexander W.|last3=Beriro|first3=Darren J.|last4=Cave|first4=Mark R.|last5=Knights|first5=Katherine|last6=Moss-Hayes|first6=Vicky|last7=Nathanail|first7=Paul C.|display-authors=3| title=Polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) in urban soils of Greater London, UK| journal=Applied Geochemistry| volume=51|year=2014|pages=303–314|issn=0883-2927|doi=10.1016/j.apgeochem.2014.09.013|bibcode=2014ApGC...51..303V|doi-access=free}}</ref> The study showed that parent (16 PAH) content ranged from 4 to 67&nbsp;mg/kg (dry soil weight) and an average PAH concentration of 18&nbsp;mg/kg (dry soil weight) whereas the total PAH content (33 PAH) ranged from 6 to 88&nbsp;mg/kg and [[fluoranthene]] and [[pyrene]] were generally the most abundant PAHs.<ref name=VaneKim2014/> [[Benzo(a)pyrene|Benzo[''a'']pyrene]] (B''a''P), the most toxic of the parent PAHs, is widely considered a key marker PAH for environmental assessments;<ref>{{cite journal|last1=Cave|first1=Mark R.|last2=Wragg|first2=Joanna|last3=Harrison|first3=Ian|last4=Vane|first4=Christopher H.|last5=Van de Wiele|first5=Tom|last6=De Groeve|first6=Eva|last7=Nathanail|first7=C. Paul|last8=Ashmore|first8=Matthew|last9=Thomas|first9=Russell|last10=Robinson|first10=Jamie|last11=Daly|first11=Paddy|display-authors=3|title=Comparison of Batch Mode and Dynamic Physiologically Based Bioaccessibility Tests for PAHs in Soil Samples|journal=Environmental Science & Technology|volume=44|issue=7|year=2010|pages=2654–2660|issn=0013-936X|doi=10.1021/es903258v|pmid=20201516|bibcode=2010EnST...44.2654C|url=http://nora.nerc.ac.uk/id/eprint/9833/1/EST_V9.pdf}}</ref> the normal background concentration of B''a''P in the London urban sites was 6.9&nbsp;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]].

===Peatlands===
Managed [[burning]] of [[moorland]] vegetation in the UK has been shown to generate PAHs which become incorporated into the [[peat]] surface.<ref name=VaneRawlins2013>{{cite journal|last1=Vane|first1=Christopher H.|last2=Rawlins|first2=Barry G.|last3=Kim|first3=Alexander W.|last4=Moss-Hayes|first4=Vicky|last5=Kendrick|first5=Christopher P.|last6=Leng|first6=Melanie J.|display-authors=3|title=Sedimentary transport and fate of polycyclic aromatic hydrocarbons (PAH) from managed burning of moorland vegetation on a blanket peat, South Yorkshire, UK|journal=Science of the Total Environment|volume=449|year=2013|pages=81–94|issn=0048-9697|doi=10.1016/j.scitotenv.2013.01.043|pmid=23416203|bibcode=2013ScTEn.449...81V}}</ref> Burning of moorland vegetation such as [[Calluna|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 sediments===
Concentrations of PAHs in river and estuarine [[sediment]]s 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>{{cite journal|last1=Vane|first1=C. H.|last2=Harrison|first2=I.|last3=Kim|first3=A. W.|last4=Moss-Hayes|first4=V.|last5=Vickers|first5=B.P.|last6=Horton|first6=B. P.|display-authors=3|title=Status of organic pollutants in surface sediments of Barnegat Bay-Little Egg Harbor Estuary, New Jersey, USA|journal=Marine Pollution Bulletin | volume=56|issue=10|year=2008|pages=1802–1808|issn=0025-326X|doi=10.1016/j.marpolbul.2008.07.004|pmid=18715597|bibcode=2008MarPB..56.1802V |url=http://nora.nerc.ac.uk/id/eprint/4452/1/Vane_MPB_2008_for_NORA__2_.pdf}}</ref><ref name=VaneChenery2011>{{cite journal|last1=Vane|first1=C. H.|last2=Chenery|first2=S. R.|last3=Harrison|first3=I.|last4=Kim|first4=A. W.|last5=Moss-Hayes|first5=V.|last6=Jones|first6=D. G.|display-authors=3|title=Chemical signatures of the Anthropocene in the Clyde estuary, UK: sediment-hosted Pb, <sup>207/206</sup>Pb, total petroleum hydrocarbon, polyaromatic hydrocarbon and polychlorinated biphenyl pollution records|journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=369|issue=1938|year=2011|pages=1085–1111|issn=1364-503X|doi=10.1098/rsta.2010.0298|pmid=21282161|bibcode=2011RSPTA.369.1085V|s2cid=1480181|url=http://nora.nerc.ac.uk/id/eprint/13301/1/Vane_et_al__Clyde_Anthropocene_2011.pdf}}</ref> Consequently, the concentrations of [[pollutants]] in estuaries tends to decrease at the river mouth.<ref>{{cite journal|last1=Vane|first1=Christopher H.|last2=Beriro|first2=Darren J.|last3=Turner|first3=Grenville H.|title=Rise and fall of mercury (Hg) pollution in sediment cores of the Thames Estuary, London, UK|journal=Earth and Environmental Science Transactions of the Royal Society of Edinburgh| volume=105|issue=4|year=2015|pages=285–296|issn=1755-6910|doi=10.1017/S1755691015000158|url=http://nora.nerc.ac.uk/id/eprint/511959/1/Vane%20etal.%202015_RSE_Thames%20Hg.pdf|doi-access=free}}</ref> Understanding of sediment hosted PAHs in estuaries is important for the protection of commercial [[fisheries]] (such as [[mussel]]s) and general environmental habitat conservation because PAHs can impact the health of suspension and sediment feeding organism.<ref>{{cite journal|last1=Langston|first1=W. J.|last2=O'Hara|first2=S.|last3=Pope|first3=N. D.|last4=Davey|first4=M.|last5=Shortridge|first5=E.|last6=Imamura|first6=M.|last7=Harino|first7=H.|last8=Kim|first8=A.|last9=Vane|first9=C. H.|display-authors=3|title=Bioaccumulation surveillance in Milford Haven Waterway|journal=Environmental Monitoring and Assessment|volume=184|issue=1|year=2011|pages=289–311|issn=0167-6369|doi=10.1007/s10661-011-1968-z|pmid=21432028|s2cid=19881327|url=http://nora.nerc.ac.uk/id/eprint/17113/1/Milford_Haven_Pollution_Draft_Manuscript.pdf}}</ref> River-estuary surface sediments in the UK tend to have a lower PAH content than sediments buried 10–60&nbsp;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&nbsp;µg/kg (dry sediment weight) in the [[River Clyde]] and 626 to 3,766&nbsp;µg/kg in the [[River Mersey]].<ref name=VaneChenery2011/><ref name=VaneHarrison2007>{{cite journal|last1=Vane|first1=C.|last2=Harrison|first2=I.|last3=Kim|first3=A.|title=Polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) in sediments from the Mersey Estuary, U.K|journal=Science of the Total Environment| volume=374|issue=1|year=2007|pages=112–126|issn=0048-9697|doi=10.1016/j.scitotenv.2006.12.036|pmid=17258286|bibcode=2007ScTEn.374..112V|url=http://nora.nerc.ac.uk/id/eprint/992/1/Mersey_Contamination-abstract_%28NORA%29.pdf}}</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 [[mangrove]]s located on the coast of southern China.<ref>{{cite journal|last1=Vane|first1=C. H.|last2=Harrison|first2=I.|last3=Kim|first3=A. W.|last4=Moss-Hayes|first4=V.|last5=Vickers|first5=B. P.|last6=Hong|first6=K.|display-authors=3|title=Organic and metal contamination in surface mangrove sediments of South China|journal=Marine Pollution Bulletin|volume=58|issue=1|year=2009|pages=134–144|issn=0025-326X|doi=10.1016/j.marpolbul.2008.09.024|pmid=18990413|bibcode=2009MarPB..58..134V |url=http://nora.nerc.ac.uk/id/eprint/5547/1/Vane_et_al.%2C_2009.pdf}}</ref>

==Human health==
[[Cancer]] is a primary human health risk of exposure to PAHs.<ref name=Bostrom2002>{{Cite journal| issn = 0091-6765| volume = 110| issue = Suppl. 3| pages = 451–488| last1 = Bostrom| first1 = C.-E.| last2 = Gerde| first2 = P.| last3 = Hanberg| first3 = A.| last4 = Jernstrom| first4 = B.| last5 = Johansson| first5 = C.| last6 = Kyrklund| first6 = T.| last7 = Rannug| first7 = A.| last8 = Tornqvist| first8 = M.| last9 = Victorin| first9 = K.| last10 = Westerholm| first10 = R.|display-authors=3| title = Cancer risk assessment, indicators, and guidelines for polycyclic aromatic hydrocarbons in the ambient air| journal = Environmental Health Perspectives| date = 2002| pmid = 12060843| pmc = 1241197| doi=10.1289/ehp.02110s3451| bibcode = 2002EnvHP.110S.451B}}</ref> Exposure to PAHs has also been linked with cardiovascular disease and poor fetal development.

===Cancer===
PAHs have been linked to [[skin cancer|skin]], [[lung cancer|lung]], [[bladder cancer|bladder]], [[liver cancer|liver]], and [[stomach cancer|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 "[[Polycyclic aromatic hydrocarbon#Regulation and oversight|Regulation and Oversight]]" below.

====History====
[[File:Chimney sweeps.jpg|thumbnail|upright=.7|alt=A line drawing of an 18th-century man and boy, the man carrying long tools such as a broom|An 18th-century drawing of [[chimney sweep]]s.]]
Historically, PAHs contributed substantially to our understanding of adverse health effects from exposures to [[pollution|environmental contaminants]], including chemical [[carcinogenesis]].<ref>{{Cite journal| doi = 10.1158/0008-5472.CAN-08-2852| issn = 0008-5472| volume = 68| issue = 17| pages = 6863–6872| last1 = Loeb| first1 = L. A.| last2 = Harris| first2 = C. C.| title = Advances in Chemical Carcinogenesis: A Historical Review and Prospective| journal = Cancer Research| date = 2008| pmid = 18757397| pmc = 2583449}}</ref> In 1775, [[Percivall Pott]], a surgeon at [[St Bartholomew's Hospital|St. Bartholomew's Hospital]] in London, observed that [[chimney sweeps' carcinoma|scrotal cancer]] was unusually common in chimney sweepers and proposed the cause as occupational exposure to [[soot]].<ref name=Dipple1985>{{Cite book| publisher = American Chemical Society| isbn = 978-0-8412-0924-4| volume = 283| pages = 1–17| last = Dipple| first = A.| title = Polycyclic Hydrocarbons and Carcinogenesis| chapter = Polycyclic Aromatic Hydrocarbon Carcinogenesis| series = ACS Symposium Series| date = 1985| doi = 10.1021/bk-1985-0283.ch001}}</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, [[Yamagiwa Katsusaburō|Yamigawa]] and [[Kōichi Ichikawa|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 [[fluorescence|fluorescent]] pattern that was similar but not identical to [[benz(a)anthracene|benz[''a'']anthracene]], a PAH that was subsequently demonstrated to cause [[tumor]]s.<ref name=Dipple1985 /> Cook, Hewett and [[Izrael Hieger|Hieger]] then linked the specific spectroscopic fluorescent profile of [[benzo(a)pyrene|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 oven]]s and [[coal carbonization]] and [[gasification]] processes.<ref>{{Cite report| publisher = World Health Organization| last = International Agency for Research on Cancer| title = Polynuclear Aromatic Compounds, Part 3, Industrial Exposures in Aluminium Production, Coal Gasification, Coke Production, and Iron and Steel Founding| location = Lyon, France| series = IARC Monographs on the Evaluation of Carcinogenic Risks to Humans| access-date = 2016-02-13| date = 1984|pages=89–92, 118–124| url = http://apps.who.int/bookorders/anglais/detart1.jsp?sesslan=1&codlan=1&codcol=72&codcch=34| archive-url = https://web.archive.org/web/20100407053627/http://apps.who.int/bookorders/anglais/detart1.jsp?sesslan=1&codlan=1&codcol=72&codcch=34| archive-date = April 7, 2010}}</ref>

====Mechanisms of carcinogenesis====
[[File:Benzopyrene DNA adduct 1JDG.png| thumb|upright=0.7|An [[adduct]] formed between a [[DNA]] strand and an [[(+)-Benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide|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>{{Cite journal| doi = 10.1002/em.20095| pmid = 15688365| issn = 1098-2280| volume = 45| issue = 2–3| pages = 106–114| last1 = Baird| first1 = W. M.| last2 = Hooven| first2 = L. A.| last3 = Mahadevan| first3 = B.| title = Carcinogenic polycyclic aromatic hydrocarbon-DNA adducts and mechanism of action| journal = Environmental and Molecular Mutagenesis| date = 2015-02-01| s2cid = 4847912}}</ref> Some carcinogenic PAHs are [[genotoxicity|genotoxic]] and induce [[mutation]]s that initiate cancer; others are not genotoxic and instead affect cancer promotion or progression.<ref name=Baird2005 /><ref>{{Cite journal| doi = 10.1111/j.1600-0773.1984.tb02485.x| pmid = 6385617| issn = 1600-0773| volume = 55| issue = S2| pages = 107–124| last = Slaga| first = T. J.| title = Chapter 7: Multistage skin carcinogenesis: A useful model for the study of the chemoprevention of cancer| journal = Acta Pharmacologica et Toxicologica| date = 1984}}</ref>

PAHs that affect [[Cancer#Pathophysiology|cancer initiation]] are typically first chemically modified by [[enzyme]]s into metabolites that react with DNA, leading to mutations. When the DNA sequence is altered in genes that regulate [[mitosis|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>{{Cite journal| doi = 10.1016/j.taap.2004.11.006| pmid = 15963346| issn = 0041-008X| volume = 206| issue = 1| pages = 73–93| last1 = Xue| first1 = W.| last2 = Warshawsky| first2 = D.| title = Metabolic activation of polycyclic and heterocyclic aromatic hydrocarbons and DNA damage: A review| journal = Toxicology and Applied Pharmacology| date = 2005| bibcode = 2005ToxAP.206...73X}}</ref> Mutagenic metabolites of PAHs include [[diol]] epoxides, [[quinones]], and [[radical (chemistry)|radical]] PAH [[cation]]s.<ref name=Xue2005 /><ref>{{Cite journal| doi = 10.1111/j.1349-7006.2004.tb03162.x| pmid = 14720319| issn = 1349-7006| volume = 95| issue = 1| pages = 1–6| last1 = Shimada| first1 = T.| last2 = Fujii-Kuriyama| first2 = Y.| title = Metabolic activation of polycyclic aromatic hydrocarbons to carcinogens by cytochromes P450 1A1 and 1B1| journal = Cancer Science| date = 2004-01-01| s2cid = 26021902| doi-access = free| pmc = 11158916}}</ref><ref>{{Cite journal| doi = 10.1186/1471-2407-9-187| issn = 1471-2407| volume = 9| issue = 1| page = 187| last1 = Androutsopoulos| first1 = V. P.| last2 = Tsatsakis| first2 = A. M.| last3 = Spandidos| first3 = D. A.| title = Cytochrome P450 CYP1A1: wider roles in cancer progression and prevention| journal = BMC Cancer| date = 2009| pmid = 19531241| pmc=2703651| doi-access = free}}</ref> These metabolites can bind to DNA at specific sites, forming bulky complexes called [[DNA adduct]]s that can be stable or unstable.<ref name=Dipple1985 /><ref name=Henkler2012>{{Cite book| publisher = Springer Basel| isbn = 978-3-7643-8340-4| pages = 107–131| editor-first= A. |editor-last=Luch| last1 = Henkler| first1 = F.| last2 = Stolpmann| first2 = K.| last3 = Luch| first3 = Andreas| title = Molecular, Clinical and Environmental Toxicology|year=2012| volume = 101| chapter = Exposure to Polycyclic Aromatic Hydrocarbons: Bulky DNA Adducts and Cellular Responses| series = Experientia Supplementum| doi = 10.1007/978-3-7643-8340-4_5| pmid = 22945568}}</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 [[oncogene]]s.<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>{{Cite journal| doi = 10.1074/jbc.R400004200| issn = 1083-351X| volume = 279| issue = 23| pages = 23847–23850| last1 = Nebert| first1 = D. W.| last2 = Dalton| first2 = T. P.| last3 = Okey| first3 = A. B.| last4 = Gonzalez| first4 = F. J.| title = Role of Aryl Hydrocarbon Receptor-mediated Induction of the CYP1 Enzymes in Environmental Toxicity and Cancer| journal = Journal of Biological Chemistry| date = 2004| pmid = 15028720| doi-access = free}}</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-carcinogen]]s 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 >{{Cite journal| doi = 10.1080/10915810490517063| issn = 1092-874X| volume = 23| issue = 5| pages = 301–333| last1 = Ramesh| first1 = A.| last2 = Walker| first2 = S. A.| last3 = Hood| first3 = D. B.| last4 = Guillén| first4 = M. D.| last5 = Schneider| first5 = K.| last6 = Weyand| first6 = E. H.|display-authors=3| title = Bioavailability and risk assessment of orally ingested polycyclic aromatic hydrocarbons| journal = International Journal of Toxicology| date = 2004| pmid = 15513831| s2cid = 41215420| doi-access = free}}</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 kinase]]s 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 disease===
Adult exposure to PAHs has been linked to [[cardiovascular disease]].<ref>{{Cite journal| doi = 10.1080/03602530600632063| pmid = 16877260| issn = 0360-2532| volume = 38| issue = 3| pages = 411–450| last1 = Korashy| first1 = H. M.| last2 = El-Kadi| first2 = A. O. S.| title = The Role of Aryl Hydrocarbon Receptor in the Pathogenesis of Cardiovascular Diseases| journal = Drug Metabolism Reviews| date = 2006| s2cid = 30406435}}
</ref> PAHs are among the complex suite of contaminants in [[tobacco smoke]] and [[Particulates|particulate air pollution]] and may contribute to cardiovascular disease resulting from such exposures.<ref name=Lewtas2007>{{Cite journal| doi = 10.1016/j.mrrev.2007.08.003| pmid = 17951105| issn = 1383-5742| volume = 636| issue = 1–3| pages = 95–133| last = Lewtas| first = J.| title = Air pollution combustion emissions: Characterization of causative agents and mechanisms associated with cancer, reproductive, and cardiovascular effects| journal = Mutation Research/Reviews in Mutation Research| series = The Sources and Potential Hazards of Mutagens in Complex Environmental Matrices – Part II| date = 2007| bibcode = 2007MRRMR.636...95L}}</ref>

In laboratory experiments, animals exposed to certain PAHs have shown increased development of plaques ([[atherogenesis]]) within arteries.<ref name=Ramos2005>{{Cite journal| doi = 10.1080/03602530500251253| pmid = 16393887| issn = 0360-2532| volume = 37| issue = 4| pages = 595–610| last1 = Ramos| first1 = Kenneth S.| last2 = Moorthy| first2 = Bhagavatula| title = Bioactivation of Polycyclic Aromatic Hydrocarbon Carcinogens within the vascular Wall: Implications for Human Atherogenesis| journal = Drug Metabolism Reviews| date = 2005| s2cid = 25713047}}</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 [[atheroma|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>{{Cite journal| volume = 135| issue = 47–48| pages = 697–702| last1 = Kunzli| first1 = N.| last2 = Tager| first2 = I.| title = Air pollution: from lung to heart| journal = [[Swiss Medical Weekly]]| access-date = 2015-12-16| date = 2005| doi = 10.4414/smw.2005.11025| url = http://www.smw.ch/docs/pdf200x/2005/47/smw-11025.pdf| pmid = 16511705| s2cid = 28408634| archive-date = 2016-10-09| archive-url = https://web.archive.org/web/20161009084342/http://www.smw.ch/docs/pdf200x/2005/47/smw-11025.pdf}}</ref><ref>{{Cite journal| doi = 10.1373/clinchem.2008.119214| issn = 1530-8561| volume = 55| issue = 2| pages = 209–215| last = Ridker| first = P. M.| title = C-Reactive Protein: Eighty Years from Discovery to Emergence as a Major Risk Marker for Cardiovascular Disease| journal = Clinical Chemistry| date = 2009| pmid = 19095723| doi-access = }}</ref> [[Biomarker]]s 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>{{Cite journal| doi = 10.3109/10715762.2011.632415| pmid = 22034834| issn = 1071-5762| volume = 46| issue = 4| pages = 492–522| last1 = Rossner| first1 = P. Jr. | last2 = Sram| first2 = R. J.| title = Immunochemical detection of oxidatively damaged DNA| journal = Free Radical Research| date = 2012| s2cid = 44543315}}</ref>

===Fetal development impacts===
Multiple [[epidemiology|epidemiological]] studies of people living in Europe, the United States, and China have linked [[Uterus|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 [[intelligence quotient|IQ]].<ref>{{Cite journal| doi = 10.1289/ehp.6362| issn = 0091-6765| volume = 113| issue = 4| pages = 375–382| last1 = Sram| first1 = R. J.| last2 = Binkova| first2 = B.| last3 = Dejmek| first3 = J.| last4 = Bobak| first4 = M.| title = Ambient Air Pollution and Pregnancy Outcomes: A Review of the Literature| journal = Environmental Health Perspectives| date = 2005| pmid = 15811825| pmc = 1278474| bibcode = 2005EnvHP.113..375S}}</ref><ref>{{Cite journal| doi = 10.1016/j.reprotox.2010.09.004| pmid = 20851760| volume = 31| issue = 3| pages = 327–336| last1 = Winans| first1 = B.| last2 = Humble| first2 = M.| last3 = Lawrence| first3 = B. P.| title = Environmental toxicants and the developing immune system: A missing link in the global battle against infectious disease?| journal = Reproductive Toxicology| date = 2011| pmc=3033466| bibcode = 2011RepTx..31..327W}}</ref><ref>{{Cite journal| doi = 10.1016/j.taap.2004.01.016| pmid = 15126074| issn = 0041-008X| volume = 197| issue = 1| pages = 49–65| last1 = Wormley| first1 = D. D.| last2 = Ramesh| first2 = A.| last3 = Hood| first3 = D. B.| title = Environmental contaminant–mixture effects on CNS development, plasticity, and behavior| journal = Toxicology and Applied Pharmacology| date = 2004| bibcode = 2004ToxAP.197...49W}}</ref><ref>{{Cite journal| doi = 10.1210/en.2015-1403| pmid = 26241071| issn = 0013-7227| volume = 156| issue = 10| pages = 3473–3482| last1 = Suades-González| first1 = E.| last2 = Gascon| first2 = M.| last3 = Guxens| first3 = M.| last4 = Sunyer| first4 = J.| title = Air Pollution and Neuropsychological Development: A Review of the Latest Evidence| journal = Endocrinology| date = 2015| pmc=4588818}}</ref>

==Regulation and oversight==
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>{{Citation| last = European Union| title = Commission Regulation (EU) 1272/2013| access-date = 2016-02-01| date = 2013-12-06| url = http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32013R1272}}</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>{{Cite journal| doi = 10.1080/10406638.2014.892886| issn = 1040-6638| issue = 2–4| pages = 147–160| last = Keith| first = Lawrence H.| title = The Source of U.S. EPA's Sixteen PAH Priority Pollutants| journal = Polycyclic Aromatic Compounds| date = 2014-12-08| volume = 35| s2cid = 98493548}}</ref><ref>{{Cite report | publisher = U.S. Department of Health and Human Services, Public Health Service| last = Agency for Toxic Substances and Disease Registry (ATSDR)| title = Toxicological profile for Polycyclic Aromatic Hydrocarbons (PAHs)| location = Atlanta, GA| access-date = 2015-05-06| date = 1995| url = https://www.atsdr.cdc.gov/toxprofiledocs/index.html?id=122&tid=25}}</ref><ref name="EFSA">{{Cite report | publisher = European Food Safety Authority (EFSA)| pages = 1–4| last = EFSA Panel on Contaminants in the Food Chain (CONTAM)| title = Polycyclic Aromatic Hydrocarbons in Food: Scientific Opinion of the Panel on Contaminants in the Food Chain| location = Parma, Italy| date = 2008}}</ref>

{| align="center"
|- valign="top"
|
{| class="wikitable"
|-
! Compound !! Agency
![[United States Environmental Protection Agency|EPA]] [[Maximum Contaminant Level|MCL]] in water [<nowiki/>[[Milligram|mg]] [[Litre|L]]<sup>−1</sup>]<ref name=PAHReview>{{Cite journal|last1=Kim|first1=Ki-Hyun|last2=Jahan|first2=Shamin Ara|last3=Kabir|first3=Ehsanul|last4=Brown|first4=Richard J. C.|date=2013-10-01|title=A review of airborne polycyclic aromatic hydrocarbons (PAHs) and their human health effects|journal=Environment International|volume=60|pages=71–80|doi=10.1016/j.envint.2013.07.019|pmid=24013021|bibcode=2013EnInt..60...71K |issn=0160-4120}}</ref>
|-
| [[acenaphthene]] || EPA, ATSDR
|
|-
| [[acenaphthylene]] || EPA, ATSDR
|
|-
| [[anthracene]] || EPA, ATSDR
|
|-
| [[benz(a)anthracene|benz[''a'']anthracene]]{{ref label|cancer|A|A}} || EPA, ATSDR, EFSA
|0.0001
|-
| [[benzo(b)fluoranthene|benzo[''b'']fluoranthene]]{{ref label|cancer|A|A}} || EPA, ATSDR, EFSA
|0.0002
|-
| [[benzo(j)fluoranthene|benzo[''j'']fluoranthene]] || ATSDR, EFSA
|
|-
| [[benzo(k)fluoranthene|benzo[''k'']fluoranthene]]{{ref label|cancer|A|A}} || EPA, ATSDR, EFSA
|0.0002
|-
| [[benzo(c)fluorene|benzo[''c'']fluorene]] || EFSA
|
|-
| [[benzo(ghi)perylene|benzo[''ghi'']perylene]]{{ref label|cancer|A|A}} || EPA, ATSDR, EFSA
|
|-
| [[benzo(a)pyrene|benzo[''a'']pyrene]]{{ref label|cancer|A|A}} || EPA, ATSDR, EFSA
|0.0002
|-
| [[benzo(e)pyrene|benzo[''e'']pyrene]] || ATSDR
|
|-
| [[chrysene]]{{ref label|cancer|A|A}} || EPA, ATSDR, EFSA
|0.0002
|-
| [[coronene]] || ATSDR
|
|}

| ||
|
{| class="wikitable"
|-
! Compound !! Agency
![[United States Environmental Protection Agency|EPA]] [[Maximum Contaminant Level|MCL]] in water [<nowiki/>[[Milligram|mg]] [[Litre|L]]<sup>−1</sup>]<ref name=PAHReview/>
|-
| [[Cyclopenta(cd)pyrene|cyclopenta[''cd'']pyrene]]|| EFSA
|
|-
| [[dibenz(a,h)anthracene|dibenz[''a,h'']anthracene]]{{ref label|cancer|A|A}} || EPA, ATSDR, EFSA
|0.0003
|-
| [[dibenzopyrenes|dibenzo[''a,e'']pyrene]] || EFSA
|
|-
| [[dibenzopyrenes|dibenzo[''a,h'']pyrene]] || EFSA
|
|-
| [[dibenzopyrenes|dibenzo[''a,i'']pyrene]] || EFSA
|
|-
| [[dibenzopyrenes|dibenzo[''a,l'']pyrene]] || EFSA
|
|-
| [[fluoranthene]] || EPA, ATSDR
|
|-
| [[fluorene]] || EPA, ATSDR
|
|-
| [[Indeno(1,2,3-cd)pyrene|indeno[1,2,3-''cd'']pyrene]]{{ref label|cancer|A|A}} || EPA, ATSDR, EFSA
|0.0004
|-
| 5-methylchrysene || EFSA
|
|-
| [[naphthalene]] || EPA
|
|-
| [[phenanthrene]] || EPA, ATSDR
|
|-
| [[pyrene]] || EPA, ATSDR
|
|}

|}
::{{note label|cancer|A|A}}Considered 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>{{Cite web|first= Kim |last=Gehle |agency=Agency for Toxic Substances and Disease Registry |publisher=[[Centers for Disease Control and Prevention|CDC]] | title = Toxicity of Polycyclic Aromatic Hydrocarbons (PAHs): Health Effects Associated With PAH Exposure| access-date = 2016-02-01| url = https://www.atsdr.cdc.gov/csem/csem.html |url-status= live |archive-url=https://web.archive.org/web/20190806211601/http://www.atsdr.cdc.gov/csem/csem.asp?csem=13&po=11 |archive-date= 6 August 2019}}</ref>

==Detection and optical properties==
A spectral database exists<ref name="NASA-20140221"/> for tracking polycyclic aromatic hydrocarbons (PAHs) in the [[universe]].<ref>{{cite web|url=http://www.astrochem.org/pahdb/|title=NASA Ames PAH IR Spectroscopic Database|website=www.astrochem.org}}</ref> Detection of PAHs in materials is often done using [[gas chromatography-mass spectrometry]] or [[liquid chromatography]] with [[ultraviolet-visible spectroscopy|ultraviolet-visible]] or [[fluorescence spectroscopy|fluorescence]] spectroscopic methods or by using rapid test PAH indicator strips.<ref>{{cite journal |last1=Kim |first1=Na Yeong |last2=Loganathan |first2=Bommanna G. |last3=Kim |first3=Gi Beum |title=Assessment of toxicity potential of freely dissolved PAHs using passive sampler in Kentucky Lake and Ohio River |journal=Marine Pollution Bulletin |date=October 2024 |volume=207 |page=116833 |doi=10.1016/j.marpolbul.2024.116833|pmid=39159572 |bibcode=2024MarPB.20716833K }}</ref> Structures of PAHs have been analyzed using infrared spectroscopy.<ref>{{Cite journal|last1=Sasaki|first1=Tatsuya|last2=Yamada|first2=Yasuhiro|last3=Sato|first3=Satoshi|date=2018-09-18|title=Quantitative Analysis of Zigzag and Armchair Edges on Carbon Materials with and without Pentagons Using Infrared Spectroscopy|journal=Analytical Chemistry|volume=90|issue=18|pages=10724–10731|doi=10.1021/acs.analchem.8b00949|pmid=30079720|s2cid=51920955|issn=0003-2700}}</ref>

PAHs possess very characteristic [[ultraviolet-visible spectroscopy|UV absorbance spectra]]. These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of [[isomer]]s, 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 [[semiconductor|semi-conducting]] and other behaviors.

===Origins of life===
{{Main|PAH world hypothesis}}
[[File:PIA22568-CatsPawNebula-Spitzer-20181023.jpg|thumb|right|300px|{{center|The [[Cat's Paw Nebula]] lies inside the [[Milky Way Galaxy]] and is located in the [[constellation]] [[Scorpius]].<br />Green areas show regions where radiation from hot stars collided with large molecules and small dust grains called "polycyclic aromatic hydrocarbons" (PAHs), causing them to [[fluoresce]].<br />([[Spitzer Space Telescope]], 2018)}}]]
<!---[[File:W40_star-forming_region.jpg|thumb|200px|right|The [[Spitzer Space Telescope]] detects light emitted by PAHs at 3.28, 6.2, 7.7 and 8.6&nbsp;μm, producing this image of nebulosity in a [[Westerhout 40|stellar nursery]].<ref>{{cite journal| last=Kuhn | first=M. A. | last2=Povich | first2=M. S. | last3=Luhman | first3= K. L. | last4=Getman | first4=K. V. | last5= Busk | first5=H. A.| last6=Feigelson | first6=E. D. | display-authors=etal | year=2013 | title=The Massive Young Star-Forming Complex Study in Infrared and X-Ray: Mid-infrared Observations and Catalogs |
journal=Astrophysical Journal Supplement| volume=209 | issue=2 | page=29 | doi=10.1088/0067-0049/209/2/29 | bibcode=2013ApJS..209...29K| arxiv=1309.4490 }}</ref>]]--->
PAHs may be abundant in the universe.<ref name="NASA-20110413">{{cite web|author=Allamandola, Louis|display-authors=etal|title=Cosmic Distribution of Chemical Complexity|url=http://amesteam.arc.nasa.gov/Research/cosmic.html|date=2011-04-13|work=[[NASA]]|access-date=2014-03-03|archive-url=https://web.archive.org/web/20140227184503/http://amesteam.arc.nasa.gov/Research/cosmic.html|archive-date=2014-02-27}}</ref><ref>{{cite web |last=Carey|first=Bjorn |title=Life's Building Blocks 'Abundant in Space' |url=http://www.space.com/1686-life-building-blocks-abundant-space.html |date=2005-10-18 |work=[[Space.com]] |access-date=2014-03-03 }}</ref><ref>{{ cite journal | last1=Hudgins | first1=D. M. | last2=Bauschlicher | first2=C. W. Jr | last3=Allamandola | first3=L. J. | title=Variations in the Peak Position of the 6.2 μm Interstellar Emission Feature: A Tracer of N in the Interstellar Polycyclic Aromatic Hydrocarbon Population | date=2005 | journal= [[Astrophysical Journal]] | volume=632 | issue=1 | pages=316–332 | doi=10.1086/432495 | bibcode=2005ApJ...632..316H| citeseerx=10.1.1.218.8786 | s2cid=7808613 }}</ref><ref>{{cite web |last=Clavin |first=Whitney |title=Why Comets Are Like Deep Fried Ice Cream |url=http://www.jpl.nasa.gov/news/news.php?feature=4480 |date=2015-02-10 |work=[[NASA]] |access-date=2015-02-10 }}</ref> They seem to have been formed as early as a couple of billion years after the [[Big Bang]], and are associated with [[Star formation|new stars]] and [[exoplanet]]s.<ref name="NASA-20140221">{{cite web | last=Hoover | first=R. | title=Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That | url=http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ | date=2014-02-21 | publisher=[[NASA]] | access-date=2014-02-22 | archive-date=2020-05-10 | archive-url=https://web.archive.org/web/20200510124801/https://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ | url-status=dead }}</ref> More than 20% of the [[carbon]] in the universe may be associated with PAHs.<ref name="NASA-20140221"/> PAHs are considered possible [[PAH world hypothesis|starting material]] for the [[Abiogenesis#PAH world hypothesis|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>{{cite news| last=Battersby| first=S.| url=https://www.newscientist.com/article/dn4552-space-molecules-point-to-organic-origins.html|title=Space molecules point to organic origins|publisher=[[New Scientist]]|date=2004|access-date=2009-12-11}}</ref><ref name=ReferenceA>{{ cite journal | last1=Mulas | first1=G. | last2=Malloci | first2=G. | last3=Joblin | first3=C.|author3-link=Christine Joblin | last4=Toublanc | first4=D. | title=Estimated IR and phosphorescence emission fluxes for specific polycyclic aromatic hydrocarbons in the Red Rectangle | journal=Astronomy and Astrophysics | date=2006 | volume=446 | issue=2 | pages=537–549 | doi=10.1051/0004-6361:20053738 | bibcode=2006A&A...446..537M | arxiv=astro-ph/0509586 | s2cid=14545794 }}</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 [[Earliest known life forms|early life on Earth]]—can only originate in nebulae.<ref name=ReferenceA/>
[[File:Polycyclic Aromatic Hydrocarbons In Space.jpg|thumb|150px|left|Two extremely bright stars illuminate a mist of PAHs in this [[Spitzer Space Telescope]] image.<ref>{{cite web |author=Staff |title=Bright Lights, Green City |url=http://www.spitzer.caltech.edu/images/3230-sig10-013-Bright-Lights-Green-City |date=2010-07-28 |work=[[NASA]] |access-date=2014-06-13 }}</ref>]]

PAHs, subjected to [[interstellar medium|interstellar medium (ISM)]] conditions, are transformed, through [[hydrogenation]], [[oxygenate|oxygenation]], and [[hydroxylation]], to more complex [[organic compound]]s—"a step along the path toward [[amino acid]]s and [[nucleotide]]s, the raw materials of [[protein]]s and [[DNA]], respectively".<ref name="Space-20120920">{{cite web| author=Staff |title=NASA Cooks Up Icy Organics to Mimic Life's Origins| url=http://www.space.com/17681-life-building-blocks-nasa-organic-molecules.html |date=2012-09-20| publisher=[[Space.com]]|access-date=2012-09-22}}</ref><ref name="AJL-20120901">{{ cite journal | last1=Gudipati | first1=M. S. | last2=Yang | first2=R. | title=In-Situ Probing Of Radiation-Induced Processing Of Organics In Astrophysical Ice Analogs—Novel Laser Desorption Laser Ionization Time-Of-Flight Mass Spectroscopic Studies | journal=[[The Astrophysical Journal Letters]] | date=2012 | volume=756 | issue=1 | pages=L24 | doi=10.1088/2041-8205/756/1/L24 | bibcode=2012ApJ...756L..24G | s2cid=5541727 }}</ref> Further, as a result of these transformations, the PAHs lose their [[spectroscopy|spectroscopic signature]] which could be one of the reasons "for the lack of PAH detection in [[interstellar ice]] [[cosmic dust#Dust grain formation|grains]], particularly the outer regions of cold, dense clouds or the upper molecular layers of [[protoplanetary disk]]s."<ref name="Space-20120920"/><ref name="AJL-20120901"/>

Low-temperature chemical pathways from simple [[organic compound]]s 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 (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>{{cite web |title="A Prebiotic Earth" – Missing Link Found on Saturn's Moon Titan |url=https://dailygalaxy.com/2018/10/a-prebiotic-earth-missing-link-found-on-saturns-moon-titan/ |date=11 October 2018 |work=DailyGalaxy.com |access-date=11 October 2018 |archive-url=https://web.archive.org/web/20210814144308/https://dailygalaxy.com/2018/10/a-prebiotic-earth-missing-link-found-on-saturns-moon-titan/ |archive-date=14 Aug 2021}}</ref><ref>{{cite journal |author=Zhao, Long|display-authors=etal|title=Low-temperature formation of polycyclic aromatic hydrocarbons in Titan's atmosphere |date=8 October 2018 |journal=[[Nature Astronomy]] |volume=2 |issue=12 |pages=973–979 |doi=10.1038/s41550-018-0585-y |bibcode=2018NatAs...2..973Z|s2cid=105480354|url=https://escholarship.org/uc/item/1pj0q014}}</ref>
{{clear}}

==See also==
* [[Chicken wire (chemistry)]]
* [[F number (chemistry)|F number]]
* [[Graphene]]
* [[Total petroleum hydrocarbon]]

==References==
{{reflist}}

==External links==
{{Commons category|Polycyclic aromatic hydrocarbons}}
* [http://www.atsdr.cdc.gov/csem/pah/index.html ATSDR - Toxicity of Polycyclic Aromatic Hydrocarbons (PAHs)] {{Webarchive|url=https://web.archive.org/web/20200530145140/https://www.atsdr.cdc.gov/csem/pah/ |date=2020-05-30 }} U.S. Department of Health and Human Services
* [https://web.archive.org/web/20161012003445/http://www.chem.qmul.ac.uk/iupac/fusedring/ Fused Ring and Bridged Fused Ring Nomenclature]
* [https://web.archive.org/web/20050117084624/http://ois.nist.gov/pah/ Database of PAH structures]
* [https://web.archive.org/web/20130510152831/http://astrochemistry.ca.astro.it/database/ Cagliari PAH Theoretical Database]
* [http://www.astrochem.org/pahdb/ NASA Ames PAH IR Spectroscopic Database]
* [https://web.archive.org/web/20060518002254/http://www.npi.gov.au/database/substance-info/profiles/74.html National Pollutant Inventory: Polycyclic Aromatic Hydrocarbon Fact Sheet]
* [https://web.archive.org/web/20060623051810/http://www.spitzer.caltech.edu/features/articles/20050627.shtml Understanding Polycyclic Aromatic Hydrocarbons] NASA Spitzer Space Telescope
* [https://web.archive.org/web/20110628183505/http://www.astrobio.net/interview/1992/the-aromatic-world "The Aromatic World: An Interview with Professor Pascale Ehrenfreund"] from ''[[Astrobiology Magazine]]''
* [https://web.archive.org/web/20101206055302/http://oregonstate.edu/superfund/ Oregon State University Superfund Research Center] focused on new technologies and emerging health risks of Polycyclic Aromatic Hydrocarbons (PAHs)
* [http://www.epa.gov/osw/hazard/wastemin/minimize/factshts/pahs.pdf Polycyclic Aromatic Hydrocarbons (PAHs)--EPA Fact Sheet]. U.S. Environmental Protection Agency, Office of Solid Waste, January 2008.

{{PAHs}}
{{Hydrocarbons}}
{{Consumer Food Safety}}
{{Molecules detected in outer space}}
{{Aryl hydrocarbon receptor modulators}}
{{Authority control}}

{{DEFAULTSORT:Polycyclic Aromatic Hydrocarbon}}
[[Category:Polycyclic aromatic hydrocarbons| ]]
[[Category:Carcinogens]]
[[Category:Origin of life]]
[[Category:Astrochemistry]]
[[Category:Persistent organic pollutants under the Convention on Long-Range Transboundary Air Pollution]]