Instruct: Given a question, retrieve relevant passages that answer the question.
Query: What is the most common source of polycyclic aromatic hydrocarbons?
Polycyclic aromatic hydrocarbons are primarily found in natural sources such as creosote.[13][14] They can result from the incomplete combustion of organic matter. PAHs can also be produced geologically when organic sediments are chemically transformed into fossil fuels such as oil and coal.[15] PAHs are considered ubiquitous in the environment and can be formed from either natural or manmade combustion sources.[16] The dominant sources of PAHs in the environment are thus from human activity: wood-burning and combustion of other biofuels such as dung or crop residues contribute more than half of annual global PAH emissions, particularly due to biofuel use in India and China.[17] 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.[17] Wildfires are another notable source.[17] 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.[17]
Polycyclic aromatic hydrocarbons are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life.
Shale oil contains polycyclic aromatic hydrocarbons which are carcinogenic. It has been described that raw shale oil has a mild carcinogenic potential which is comparable to some intermediate refinery products, while upgraded shale oil has lower carcinogenic potential as most of the polycyclic aromatics are believed to broken down by hydrogenation. The World Health Organization classifies shale oil as Group 1 carcinogens to humans.
The most likely candidate molecules for producing DIBs are thought to be large carbon-bearing molecules, which are common in the interstellar medium. Polycyclic aromatic hydrocarbons, long carbon-chain molecules such as polyynes, and fullerenes are all potentially important.
Contorted aromatics or more precisely contorted polycyclic aromatic hydrocarbons represent a unique class of polycyclic aromatic hydrocarbons (PAHs) in which the fused aromatic molecules deviate from the usual planarity.
A benzopyrene is an organic compound with the formula CH. Structurally speaking, the colorless isomers of benzopyrene are pentacyclic hydrocarbons and are fusion products of pyrene and a phenylene group. Two isomeric species of benzopyrene are benzo["a"]pyrene and the less common benzo["e"]pyrene. They belong to the chemical class of polycyclic aromatic hydrocarbons.
Polycyclic aromatic hydrocarbons are molecules containing two or more simple aromatic rings fused together by sharing two neighboring carbon atoms (see also simple aromatic rings). Examples are naphthalene, anthracene, and phenanthrene. In fused aromatics, not all carbon–carbon bonds are necessarily equivalent, as the electrons are not delocalized over the entire molecule. The aromaticity of these molecules can be explained using their orbital picture. Like benzene and other monocyclic aromatic molecules, polycyclics have a cyclic conjugated pi system with p-orbital overlap above and below the plane of the ring.
The aromaticity varies for PAHs. According to Clar's rule,[7] the resonance structure of a PAH that has the largest number of disjoint aromatic pi sextets—i.e. benzene-like moieties—is the most important for the characterization of the properties of that PAH.[8]
U.S. Department of Health and Human Services NASA Spitzer Space Telescope Aromatic World An interview with Professor Pascale Ehrenfreund on PAH origin of life. Accessed June 2006 focused on new technologies and emerging health risks of Polycyclic Aromatic Hydrocarbons (PAHs) . U.S. Environmental Protection Agency, Office of Solid Waste, January 2008.
An aromatic hydrocarbon or arene (or sometimes aryl hydrocarbon) is a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming a circle. In contrast, aliphatic hydrocarbons lack this delocalization. The term "aromatic" was assigned before the physical mechanism determining aromaticity was discovered; the term was coined as such simply because many of the compounds have a sweet or pleasant odour. The configuration of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible such hydrocarbon, benzene. Aromatic hydrocarbons can be "monocyclic" (MAH) or "polycyclic" (PAH).
Polycyclic aromatic hydrocarbons (PAHs, also polyaromatic hydrocarbons or polynuclear aromatic hydrocarbons[1]) are hydrocarbons—organic compounds containing only carbon and hydrogen—that are composed of multiple aromatic rings (organic rings in which the electrons are delocalized). The simplest such chemicals are naphthalene, having two aromatic rings, and the three-ring compounds anthracene and phenanthrene.
Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant of the known polyatomic molecules in the observable universe, and are considered a likely constituent of the primordial sea. In 2010, PAHs, along with fullerenes (or "buckyballs"), have been detected in nebulae.
Arenibacter algicola is a Gram-negative, hydrocarbon-degrading and strictly aerobic bacterium from the genus of Arenibacter. "Arenibacter algicola" uses polycyclic aromatic hydrocarbons as only sources fore carbon and energy.
A spectral database exists[79] for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe.[80] Detection of PAHs in materials is often done using gas chromatography-mass spectrometry or liquid chromatography with ultraviolet-visible or fluorescence spectroscopic methods or by using rapid test PAH indicator strips.
Erich Clar (August 23, 1902 – March 27, 1987) was an organic chemist who studied polycyclic aromatic hydrocarbon chemistry. He is considered as the father of that field. He authored the two-volume "Polycyclic Hydrocarbons", which described the syntheses, properties, and UV-visible absorption spectra of hundreds of PAHs. He created the Sextet Theory, now eponymously called "Clar's rule", to describe the behavior of PAH isomers. This was described in his book "The Aromatic Sextet". He was awarded the August Kekulé Medal by the Chemical Society of the GDR in 1965, the highest award given by that society to foreign scientists, and the first Polycyclic Aromatic Hydrocarbon Research Award of the International Symposium on Polynuclear Aromatic Hydrocarbons in 1987.
Chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) are a group of compounds comprising polycyclic aromatic hydrocarbons with two or more aromatic rings and one or more chlorine atoms attached to the ring system. Cl-PAHs can be divided into two groups: chloro-substituted PAHs, which have one or more hydrogen atoms substituted by a chlorine atom, and chloro-added Cl-PAHs, which have two or more chlorine atoms added to the molecule. They are products of incomplete combustion of organic materials. They have many congeners, and the occurrences and toxicities of the congeners differ. Cl-PAHs are hydrophobic compounds and their persistence within ecosystems is due to their low water solubility. They are structurally similar to other halogenated hydrocarbons such as polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs). Cl-PAHs in the environment are strongly susceptible to the effects of gas/particle partitioning, seasonal sources, and climatic conditions.
Acenaphthylene is a polycyclic aromatic hydrocarbon. The molecule resembles naphthalene with positions 1 and 8 connected by a CH unit. It is a yellow solid. Unlike many polycyclic aromatic hydrocarbons, it has no fluorescence. 
AhR ligands have been generally classified into two categories, synthetic or naturally occurring. The first ligands to be discovered were synthetic and members of the halogenated aromatic hydrocarbons (polychlorinated dibenzodioxins, dibenzofurans and biphenyls) and polycyclic aromatic hydrocarbons (3-methylcholanthrene, benzo["a"]pyrene, benzanthracenes and benzoflavones).
Thermal rearrangements of aromatic hydrocarbons are considered to be unimolecular reactions that directly involve the atoms of an aromatic ring structure and require no other reagent than heat. These reactions can be categorized in two major types: one that involves a complete and permanent skeletal reorganization (isomerization), and one in which the atoms are scrambled but no net change in the aromatic ring occurs (automerization). The general reaction schemes of the two types are illustrated in Figure 1.
This class of reactions was uncovered through studies on the automerization of naphthalene as well as the isomerization of unsubstituted azulene, to naphthalene. Research on thermal rearrangements of aromatic hydrocarbons has since been expanded to isomerizations and automerizations of benzene and polycyclic aromatic hydrocarbons.
Oil sand tailings or oil sands process-affected water (OSPW), have a highly variable composition and a complex mixture of compounds. In his oft-cited 2008 journal article, E. W. Allen wrote that typically tailings ponds consist of c. 75% water, c. 25% sand, silt and clay, c.2% of residual bitumen, as well as dissolved salts, organics, and minerals. Although many of the components of TPW "occur naturally in adjacent landscapes, the mining process increases their concentrations", for example, sodium, chloride, sulphate, bicarbonate, and ammonia. Citing research from the 1978 onwards, Allen included naphthenic acids (NAs), bitumen, asphaltenes, creosols, phenols, humic and fulvic acids, benzene, phthalates, toluene, polycyclic aromatic hydrocarbons (PAHs), in the list of organic compounds in TPW. Allen names aromatic hydrocarbons [including polycyclic aromatic hydrocarbons (PAHs), benzene, phenols and toluene], naphthenic acids (NAs) and dissolved solids, as those that were most harmful to fish and birds. As well as toxic metals considered to be priority pollutants such as chromium, arsenic, nickel, cadmium, copper, lead, and zinc, OSPW also contains "common, low-toxicity metals" including titanium aluminum, molybdenum, iron, and vanadium.
PAHs with five or six-membered rings are most common. Those composed only of six-membered rings are called alternant PAHs, which include benzenoid PAHs.[4] The following are examples of PAHs that vary in the number and arrangement of their rings:
Polycyclic aromatic compounds characteristically reduce to the radical anions. The redox potential correlates with the size of the PAH.
9,10-Dihydroanthracene is an organic compound that is derived from the polycyclic aromatic hydrocarbon anthracene. Several isomers of dihydroanthracene are known, but the 9,10 derivative is most common. It is a colourless solid that is used as a carrier of H in various chemical reactions. 
Many of the polycyclic aromatic hydrocarbons known to be tumorigenic or mutagenic are found in atmospheric aerosols, which is connected to the thermal rearrangement of polycyclic aromatic hydrocarbons in fast soot formation during combustion.
In general, the term polycyclic includes polycyclic aromatic compounds, including polycyclic aromatic hydrocarbons, as well as heterocyclic aromatic compounds with multiple rings (where heteroaromatic compounds are aromatic compounds that contain sulfur, nitrogen, oxygen, or another non-carbon atoms in their rings in addition to carbon).