Polycyclic aromatic hydrocarbon

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Hexabenzocoronene.svg
Hexabenzocoronene-3D-balls.png
Hexabenzocoronene AFM.jpg
Three representations of hexabenzocoronene, a polycylic 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.

Polycyclic aromatic hydrocarbons (PAHs, also polyaromatic hydrocarbons or polynuclear aromatic hydrocarbons [1] ) are hydrocarbonsorganic 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.

Hydrocarbon organic compound consisting entirely of hydrogen and carbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons from which one hydrogen atom has been removed are functional groups called hydrocarbyls. Because carbon has 4 electrons in its outermost shell carbon has exactly four bonds to make, and is only stable if all 4 of these bonds are used.

Organic compound Chemical compound that contains carbon (except for several compounds traditionally classified as inorganic compounds)

In chemistry, organic compounds are generally any chemical compounds that contain carbon. Due to carbon's ability to catenate, millions of organic compounds are known. The study of the properties, reactions, and syntheses of organic compounds comprises the discipline known as organic chemistry. For historical reasons, a few classes of carbon-containing compounds, along with a handful of other exceptions, are not classified as organic compounds and are considered inorganic. Other than those just named, little consensus exists among chemists on precisely which carbon-containing compounds are excluded, making any rigorous definition of an organic compound elusive.

Electron subatomic particle with negative electric charge

The electron is a subatomic particle, symbol
e
or
β
, whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substructure. The electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the electron include an intrinsic angular momentum (spin) of a half-integer value, expressed in units of the reduced Planck constant, ħ. Being fermions, no two electrons can occupy the same quantum state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of both particles and waves: they can collide with other particles and can be diffracted like light. The wave properties of electrons are easier to observe with experiments than those of other particles like neutrons and protons because electrons have a lower mass and hence a longer de Broglie wavelength for a given energy.

Contents

PAHs are uncharged, non-polar molecules found in coal and in tar deposits. They are also produced by the thermal decomposition of organic matter (for example, in engines and incinerators or when biomass burns in forest fires).

Coal A combustible sedimentary rock composed primarily of carbon

Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements; chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed if dead plant matter decays into peat and over millions of years the heat and pressure of deep burial converts the peat into coal. Vast deposits of coal originates in former wetlands—called coal forests—that covered much of the Earth's tropical land areas during the late Carboniferous (Pennsylvanian) and Permian times.

Tar substance

Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. Production and trade in pine-derived tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving wooden sailing vessels against rot. The largest user was the Royal Navy of the United Kingdom. Demand for tar declined with the advent of iron and steel ships. It is hot to the touch, and can cause burns if one comes in contact with the substance.

Organic matter, organic material, or natural organic matter (NOM) refers to the large source of carbon-based compounds found within natural and engineered, terrestrial and aquatic environments. It is matter composed of organic compounds that have come from the remains of organisms such as plants and animals and their waste products in the environment. Organic molecules can also be made by chemical reactions that don't involve life. Basic structures are created from cellulose, tannin, cutin, and lignin, along with other various proteins, lipids, and carbohydrates. Organic matter is very important in the movement of nutrients in the environment and plays a role in water retention on the surface of the planet.

PAHs are abundant in the universe, and have recently been found to have formed possibly as early as the first couple of billion years after the Big Bang, in association with formation of new stars and exoplanets. Some studies suggest that PAHs account for a significant percentage of all carbon in the universe.

Universe Universe events since the Big Bang 13.8 billion years ago

The Universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire Universe is unknown, it is possible to measure the size of the observable universe, which is currently estimated to be 93 billion light-years in diameter. In various multiverse hypotheses, a universe is one of many causally disconnected constituent parts of a larger multiverse, which itself comprises all of space and time and its contents.

Big Bang The prevailing cosmological model for the observable universe

The Big Bang theory is the prevailing cosmological model for the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from a very high-density and high-temperature state, and offers a comprehensive explanation for a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB), large-scale structure and Hubble's law. If the observed conditions are extrapolated backwards in time using the known laws of physics, the prediction is that just before a period of very high density there was a singularity which is typically associated with the Big Bang. Current knowledge is insufficient to determine if the singularity was primordial.

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.

Polycyclic aromatic hydrocarbons are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life. [2] [3]

PAH world hypothesis

The PAH world hypothesis is a speculative hypothesis that proposes that polycyclic aromatic hydrocarbons (PAHs), known to be abundant in the universe, including in comets, and assumed to be abundant in the primordial soup of the early Earth, played a major role in the origin of life by mediating the synthesis of RNA molecules, leading into the RNA world. However, as yet, the hypothesis is untested.

A material is a chemical substance or mixture of substances that constitutes an object. Materials can be pure or impure, living or non-living matter. Materials can be classified based on their physical and chemical properties, or on their geological origin or biological function. Materials science is the study of materials and their applications.

Nomenclature, structure, properties

Nomenclature and structure

By definition, polycyclic aromatic hydrocarbons have multiple cycles, precluding benzene from being considered a PAH. Naphthalene is considered the simplest polycyclic aromatic hydrocarbon by the US EPA and US CDC for policy contexts. [4] Other authors consider PAHs to start with the tricyclic species phenanthrene and anthracene. [5]

Benzene Organic chemical compound

Benzene is an organic chemical compound with the chemical formula C6H6. The benzene molecule is composed of six carbon atoms joined in a ring with one hydrogen atom attached to each. As it contains only carbon and hydrogen atoms, benzene is classed as a hydrocarbon.

Naphthalene chemical compound

Naphthalene is an organic compound with formula C
10
H
8
. It is the simplest polycyclic aromatic hydrocarbon, and is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings. It is best known as the main ingredient of traditional mothballs.

Phenanthrene Polycyclic aromatic hydrocarbon composed of three fused benzene rings

Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings. The name 'phenanthrene' is a composite of phenyl and anthracene. In its pure form, it is found in cigarette smoke and is a known irritant, photosensitizing skin to light. It appears as a colorless, crystal-like solid but can also look yellow.

PAHs are not generally considered to contain heteroatoms or carry substituents. [6]

Heteroatom kind of atom

In chemistry, a heteroatom is, strictly, any atom that is not carbon or hydrogen.

In organic chemistry and biochemistry, a substituent is an atom or group of atoms which replaces one or more hydrogen atoms on the parent chain of a hydrocarbon, becoming a moiety of the resultant new molecule. The terms substituent and functional group, as well as other ones are used almost interchangeably to describe branches from a parent structure, though certain distinctions are made in the context of polymer chemistry. In polymers, side chains extend from a backbone structure. In proteins, side chains are attached to the alpha carbon atoms of the amino acid backbone.

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

Physicochemical properties and bonding

PAHs are nonpolar and lipophilic. Larger PAHs are generally insoluble in water, although some smaller PAHs are soluble and known contaminants in drinking water. [8] [9] The larger members are also poorly soluble in organic solvents and in lipids. They are usually colorless.

The aromaticity varies for PAHs. According to Clar's rule, [10] 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. [11]

For example, in phenanthrene one Clar structure has two sextets—the first and third rings—while the other resonance structure has just one central sextet; therefore in this molecule the outer rings have greater aromatic character whereas the central ring is less aromatic and therefore more reactive.[ citation needed ] In contrast, 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 ] This difference in number of sextets is reflected in the differing ultraviolet–visible spectra of these two isomers, as higher Clar pi-sextets are associated with larger HOMO-LUMO gaps; [12] the highest-wavelength absorbance of phenanthrene is at 293 nm, while anthracene is at 374 nm. [13] 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 ] 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).

Polycyclic aromatic compounds characteristically reduce to the radical anions. The redox potential correlates with the size of the PAH.

Half-cell potential of aromatic compounds against the SCE [14]
CompoundPotential (V)
benzene −3.42
biphenyl [15] −2.60
naphthalene −2.51
anthracene −1.96
phenanthrene −1.96
perylene −1.67
pentacene −1.35

Sources and distribution

Polycyclic aromatic hydrocarbons are primarily found in natural sources such as creosote. [16] [17] 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. [18] PAHs are considered ubiquitous in the environment and can be formed from either natural or manmade combustion sources. [19] 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. [20] 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. [20] Wildfires are another notable source. [20] 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. [20]

PAHs are typically found as complex mixtures. [18] [21] 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. [21]

In the aqueous environment

Most PAHs are insoluble in water, which limits their mobility in the environment, although PAHs sorb to fine-grained organic-rich sediments. [22] [23] [24] [25] Aqueous solubility of PAHs decreases approximately logarithmically as molecular mass increases. [26] Two-ringed PAHs, and to a lesser extent three-ringed PAHs, dissolve in water, making them more available for biological uptake and degradation. [25] [26] [27] Further, two- to four-ringed PAHs volatilize sufficiently to appear in the atmosphere predominantly in gaseous form, although the physical state of four-ring PAHs can depend on temperature. [28] [29] In contrast, compounds with five or more rings have low solubility in water and low volatility; they are therefore predominantly in solid state, bound to particulate air pollution, soils, or sediments. [25] In solid state, these compounds are less accessible for biological uptake or degradation, increasing their persistence in the environment. [26] [30]

In galaxies

Spiral galaxy NGC 5529 has been found to have amounts of PAHs.

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. [18] [20] [31] 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. [18] [20] [31] Surgical smoke plume have been proven to contain PAHs in several independent research studies. [32] For indoor contaminants, surgical plume needs to be noticed as a serious potential health risk for the 59 million health care workers around the world.

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

Burning solid fuels such as coal and biofuels in the home for cooking and heating is a dominant global source of PAH emissions that in developing countries leads to high levels of exposure to indoor particulate air pollution containing PAHs, particularly for women and children who spend more time in the home or cooking. [20] [33]

In industrial countries, people who smoke tobacco products, or who are exposed to second-hand smoke, are among the most highly exposed groups; tobacco smoke contributes to 90% of indoor PAH levels in the homes of smokers. [31] 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. [34] PAHs are typically at low concentrations in drinking water. [31]

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

Emissions from vehicles such as cars and trucks can be a substantial outdoor source of PAHs in particulate air pollution. [18] [20] Geographically, major roadways are thus sources of PAHs, which may distribute in the atmosphere or deposit nearby. [35] Catalytic converters are estimated to reduce PAH emissions from gasoline-fired vehicles by 25-fold. [18]

People can also be occupationally exposed during work that involves fossil fuels or their derivatives, wood-burning, carbon electrodes, or exposure to diesel exhaust. [36] [37] Industrial activity that can produce and distribute PAHs includes aluminum, iron, and steel manufacturing; coal gasification, tar distillation, shale oil extraction; production of coke, creosote, carbon black, and calcium carbide; road paving and asphalt manufacturing; rubber tire production; manufacturing or use of metal working fluids; and activity of coal or natural gas power stations. [18] [36] [37]

Environmental distribution and degradation

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

PAHs typically disperse from urban and suburban non-point sources through road run-off, sewage, and atmospheric circulation and subsequent deposition of particulate air pollution. [38] [39] Soil and river sediment near industrial sites such as creosote manufacturing facilities can be highly contaminated with PAHs. [18] Oil spills, creosote, coal mining dust, and other fossil fuel sources can also distribute PAHs in the environment. [18] [40]

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. [18] PAHs have a strong affinity for organic carbon, and thus highly organic sediments in rivers, lakes, and the ocean can be a substantial sink for PAHs. [35]

Algae and some invertebrates such as protozoans, mollusks, and many polychaetes have limited ability to metabolize PAHs and bioaccumulate disproportionate concentrations of PAHs in their tissues; however, PAH metabolism can vary substantially across invertebrate species. [39] [41] Most vertebrates metabolize and excrete PAHs relatively rapidly. [39] Tissue concentrations of PAHs do not increase (biomagnify) from the lowest to highest levels of food chains. [39]

PAHs transform slowly to a wide range of degradation products. Biological degradation by microbes is a dominant form of PAH transformation in the environment. [30] [42] Soil-consuming invertebrates such as earthworms speed PAH degradation, either through direct metabolism or by improving the conditions for microbial transformations. [42] 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. [39] [43] [44]

PAHs in 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. [45] The study showed that parent (16 PAH) content ranged from 4 to 67 mg/kg (dry soil weight) and an average PAH concentration of 18 mg/kg (dry soil weight) whereas the total PAH content (33 PAH) ranged from 6 to 88 mg/kg and fluoranthene and pyrene were generally the most abundant PAHs. [45] Benzo[a]pyrene (BaP), the most toxic of the parent PAHs, is widely considered a key marker PAH for environmental assessments; [46] the normal background concentration of BaP in the London urban sites was 6.9 mg/kg (dry soil weight). [45] 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.

PAHs in peatlands

Managed burning of moorland vegetation in the UK has been shown to generate PAHs which become incorporated into the peat surface. [47] Burning of moorland vegetation such as heather initially generates high amounts of two- and three-ringed PAHs relative to four- to six-ringed PAHs in surface sediments, however, this pattern is reversed as the lower molecular weight PAHs are attenuated by biotic decay and photodegradation. [47] 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). [47]

PAHs in rivers, estuarine and coastal sediments

Concentrations of PAHs in river and estuarine sediments vary according to a variety of factors including proximity to municipal and industrial discharge points, wind direction and distance from major urban roadways, as well as tidal regime which controls the diluting effect of generally cleaner marine sediments relative to freshwater discharge. [38] [48] [49] Consequently, the concentrations of pollutants in estuaries tends to decrease at the river mouth. [50] Understanding of sediment hosted PAHs in estuaries is important for the protection of commercial fisheries (such as mussels) and general environmental habitat conservation because PAHs can impact the health of suspension and sediment feeding organism. [51] River-estuary surface sediments in the UK tend to have a lower PAH content than sediments buried 10–60 cm from the surface reflecting lower present day industrial activity combined with improvement in environmental legislation of PAH. [49] Typical PAH concentrations in UK estuaries range from about 19 to 16,163 µg/kg (dry sediment weight) in the River Clyde and 626 to 3,766 µg/kg in the River Mersey. [49] [52] 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. [52] A similar correspondence between PAHs and TOC has also been observed in the sediments of tropical mangroves located on the coast of southern China. [53]

Minor sources

Volcanic eruptions may emit PAHs. [18] Certain PAHs such as perylene can also be generated in anaerobic sediments from existing organic material, although it remains undetermined whether abiotic or microbial processes drive their production. [54] [55] [56]

Human health

Cancer is a primary human health risk of exposure to PAHs. [57] Exposure to PAHs has also been linked with cardiovascular disease and poor fetal development.

Cancer

PAHs have been linked to skin, lung, bladder, liver, and stomach cancers in well-established animal model studies. [57] Specific compounds classified by various agencies as possible or probable human carcinogens are identified in the section "Regulation and Oversight" below.

Historical significance

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

Historically, PAHs contributed substantially to our understanding of adverse health effects from exposures to environmental contaminants, including chemical carcinogenesis. [58] In 1775, Percivall Pott, a surgeon at St. Bartholomew's Hospital in London, observed that scrotal cancer was unusually common in chimney sweepers and proposed the cause as occupational exposure to soot. [59] A century later, Richard von Volkmann reported increased skin cancers in workers of the coal tar industry of Germany, and by the early 1900s increased rates of cancer from exposure to soot and coal tar was widely accepted. In 1915, Yamigawa and Ichicawa were the first to experimentally produce cancers, specifically of the skin, by topically applying coal tar to rabbit ears. [59]

In 1922, Ernest Kennaway determined that the carcinogenic component of coal tar mixtures was an organic compound consisting of only carbon and hydrogen. This component was later linked to a characteristic fluorescent pattern that was similar but not identical to benz[a]anthracene, a PAH that was subsequently demonstrated to cause tumors. [59] Cook, Hewett and Hieger then linked the specific spectroscopic fluorescent profile of benzo[a]pyrene to that of the carcinogenic component of coal tar, [59] the first time that a specific compound from an environmental mixture (coal tar) was demonstrated to be carcinogenic.

In the 1930s and later, epidemiologists from Japan, the UK, and the US, including Richard Doll and various others, reported greater rates of death from lung cancer following occupational exposure to PAH-rich environments among workers in coke ovens and coal carbonization and gasification processes. [60]

Mechanisms of carcinogenesis

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

The structure of a PAH influences whether and how the individual compound is carcinogenic. [57] [61] Some carcinogenic PAHs are genotoxic and induce mutations that initiate cancer; others are not genotoxic and instead affect cancer promotion or progression. [61] [62]

PAHs that affect cancer initiation are typically first chemically modified by enzymes into metabolites that react with DNA, leading to mutations. When the DNA sequence is altered in genes that regulate cell replication, cancer can result. Mutagenic PAHs, such as benzo[a]pyrene, usually have four or more aromatic rings as well as a "bay region", a structural pocket that increases reactivity of the molecule to the metabolizing enzymes. [63] Mutagenic metabolites of PAHs include diol epoxides, quinones, and radical PAH cations. [63] [64] [65] These metabolites can bind to DNA at specific sites, forming bulky complexes called DNA adducts that can be stable or unstable. [59] [66] Stable adducts may lead to DNA replication errors, while unstable adducts react with the DNA strand, removing a purine base (either adenine or guanine). [66] Such mutations, if they are not repaired, can transform genes encoding for normal cell signaling proteins into cancer-causing oncogenes. [61] Quinones can also repeatedly generate reactive oxygen species that may independently damage DNA. [63]

Enzymes in the cytochrome family (CYP1A1, CYP1A2, CYP1B1) metabolize PAHs to diol epoxides. [67] PAH exposure can increase production of the cytochrome enzymes, allowing the enzymes to convert PAHs into mutagenic diol epoxides at greater rates. [67] 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. [67]

Low molecular weight PAHs, with two to four aromatic hydrocarbon rings, are more potent as co-carcinogens during the promotional stage of cancer. In this stage, an initiated cell (a cell that has retained a carcinogenic mutation in a key gene related to cell replication) is removed from growth-suppressing signals from its neighboring cells and begins to clonally replicate. [68] Low-molecular-weight PAHs that have bay or bay-like regions can dysregulate gap junction channels, interfering with intercellular communication, and also affect mitogen-activated protein kinases that activate transcription factors involved in cell proliferation. [68] 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. [69] PAHs are among the complex suite of contaminants in tobacco smoke and particulate air pollution and may contribute to cardiovascular disease resulting from such exposures. [70]

In laboratory experiments, animals exposed to certain PAHs have shown increased development of plaques (atherogenesis) within arteries. [71] 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. [71] A leading hypothesis is that PAHs may activate the cytochrome enzyme CYP1B1 in vascular smooth muscle cells. This enzyme then metabolically processes the PAHs to quinone metabolites that bind to DNA in reactive adducts that remove purine bases. The resulting mutations may contribute to unregulated growth of vascular smooth muscle cells or to their migration to the inside of the artery, which are steps in plaque formation. [70] [71] These quinone metabolites also generate reactive oxygen species that may alter the activity of genes that affect plaque formation. [71]

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. [72] [73] Biomarkers of exposure to PAHs in humans have been associated with inflammatory biomarkers that are recognized as important predictors of cardiovascular disease, suggesting that oxidative stress resulting from exposure to PAHs may be a mechanism of cardiovascular disease in humans. [74]

Developmental impacts

Multiple epidemiological studies of people living in Europe, the United States, and China have linked in utero exposure to PAHs, through air pollution or parental occupational exposure, with poor fetal growth, reduced immune function, and poorer neurological development, including lower IQ. [75] [76] [77] [78]

Regulation and oversight

Some governmental bodies, including the European Union as well as NIOSH and the Environmental Protection Agency (EPA) in the US, regulate concentrations of PAHs in air, water, and soil. [79] The European Commission has restricted concentrations of 8 carcinogenic PAHs in consumer products that contact the skin or mouth. [80]

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: [81] [82] [83]

CompoundAgency EPA MCL in water [ mg L -3] [84]
acenaphthene EPA, ATSDR
acenaphthylene EPA, ATSDR
anthracene EPA, ATSDR
benz[a]anthracene [A] EPA, ATSDR, EFSA0.0001
benzo[b]fluoranthene [A] EPA, ATSDR, EFSA0.0002
benzo[j]fluoranthene ATSDR, EFSA
benzo[k]fluoranthene [A] EPA, ATSDR, EFSA0.0002
benzo[c]fluorene EFSA
benzo[g,h,i]perylene [A] EPA, ATSDR, EFSA
benzo[a]pyrene [A] EPA, ATSDR, EFSA0.0002
benzo[e]pyrene ATSDR
chrysene [A] EPA, ATSDR, EFSA0.0002
coronene ATSDR
CompoundAgency EPA MCL in water [ mg L -3] [85]
cyclopenta[c,d]pyreneEFSA
dibenz[a,h]anthracene [A] EPA, ATSDR, EFSA0.0003
dibenzo[a,e]pyrene EFSA
dibenzo[a,h]pyrene EFSA
dibenzo[a,i]pyrene EFSA
dibenzo[a,l]pyrene EFSA
fluoranthene EPA, ATSDR
fluorene EPA, ATSDR
indeno[1,2,3-c,d]pyrene [A] EPA, ATSDR, EFSA0.0004
5-methylchryseneEFSA
naphthalene EPA
phenanthrene EPA, ATSDR
pyrene EPA, ATSDR
A Considered probable or possible human carcinogens by the US EPA, the European Union, and/or the International Agency for Research on Cancer (IARC). [83] [1]

Detection and optical properties

A spectral database exists [2] for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. [86] 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.

PAHs possess very characteristic UV absorbance spectra. These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of isomers, each isomer has a different UV absorbance spectrum than the others. This is particularly useful in the identification of PAHs. Most PAHs are also fluorescent, emitting characteristic wavelengths of light when they are excited (when the molecules absorb light). The extended pi-electron electronic structures of PAHs lead to these spectra, as well as to certain large PAHs also exhibiting semi-conducting and other behaviors.

Origins of life

The Cat's Paw Nebula lies inside the Milky Way Galaxy and is located in the constellation Scorpius.
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.
(Spitzer space telescope, 2018) PIA22568-CatsPawNebula-Spitzer-20181023.jpg
The Cat's Paw Nebula lies inside the Milky Way Galaxy and is located in the constellation Scorpius.
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.
(Spitzer space telescope, 2018)

PAHs may be abundant in the universe. [87] [88] [3] [89] They seem to have been formed as early as a couple of billion years after the Big Bang, and are associated with new stars and exoplanets. [2] More than 20% of the carbon in the universe may be associated with PAHs. [2] PAHs are considered possible starting material for the earliest forms of life. [2] [3] Light emitted by the Red Rectangle nebula and found spectral signatures that suggest the presence of anthracene and pyrene. [90] [91] 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 [90] that PAHs—which may have been vital in the formation of early life on Earth—can only originate in nebulae. [91]

Two extremely bright stars illuminate a mist of PAHs in this Spitzer image. Polycyclic Aromatic Hydrocarbons In Space.jpg
Two extremely bright stars illuminate a mist of PAHs in this Spitzer image.

More recently, fullerenes (or "buckyballs"), have been detected in other nebulae. [93] Fullerenes are also implicated in the origin of life; according to astronomer Letizia Stanghellini, "It's possible that buckyballs from outer space provided seeds for life on Earth." [94] In September 2012, NASA scientists reported results of analog studies in vitro that PAHs, subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation, and hydroxylation, to more complex organics—"a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". [95] [96] Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks." [95] [96]

NASA's Spitzer Space Telescope includes instruments for obtaining both images and spectra of light emitted by PAHs associated with star formation. These images can trace the surface of star-forming clouds in our own galaxy or identify star forming galaxies in the distant universe. [97]

In June 2013, PAHs were detected in the upper atmosphere of Titan, the largest moon of the planet Saturn. [98]

In October 2018, researchers reported low-temperature chemical pathways from simple organic compounds to complex PAHs. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Saturn 's moon Titan, and may be significant pathways, in terms of the PAH world hypothesis, in producing precursors to biochemicals related to life as we know it. [99] [100]

See also

Related Research Articles

An aromatic hydrocarbon or arene 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).

Coal tar is a thick dark liquid which is a by-product of the production of coke and coal gas from coal. It has both medical and industrial uses. It may be applied to the affected area to treat psoriasis and seborrheic dermatitis (dandruff). It may be used in combination with ultraviolet light therapy. Industrially it is a railway tie preservative and used in the surfacing of roads.

Mutagen Chemical agents that increase the rate of genetic mutation by interfering with the function of nucleic acids. A clastogen is a specific mutagen that causes breaks in chromosomes

In genetics, a mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level. As many mutations can cause cancer, mutagens are therefore also likely to be carcinogens, although not always necessarily so. All mutagens have characteristic mutational signatures with some chemicals becoming mutagenic through cellular processes. Not all mutations are caused by mutagens: so-called "spontaneous mutations" occur due to spontaneous hydrolysis, errors in DNA replication, repair and recombination.

Coronene polycyclic aromatic hydrocarbon (PAH) comprising six peri-fused benzene rings

Coronene is a polycyclic aromatic hydrocarbon (PAH) comprising six peri-fused benzene rings. Its chemical formula is C
24
H
12
. It is a yellow material that dissolves in common solvents including benzene, toluene, and dichloromethane. Its solutions emit blue light fluorescence under UV light. It has been used as a solvent probe, similar to pyrene.

Benzo(<i>a</i>)pyrene chemical compound

Benzo[a]pyrene is a polycyclic aromatic hydrocarbon and the result of incomplete combustion of organic matter at temperatures between 300 °C (572 °F) and 600 °C (1,112 °F). The ubiquitous compound can be found in coal tar, tobacco smoke and many foods, especially grilled meats. The substance with the formula C20H12 is one of the benzopyrenes, formed by a benzene ring fused to pyrene. Its diol epoxide metabolites (more commonly known as BPDE) react and bind to DNA, resulting in mutations and eventually cancer. It is listed as a Group 1 carcinogen by the IARC. In the 18th century a scrotal cancer of chimney sweepers, the chimney sweeps' carcinoma, was already known to be connected to soot.

Mycoremediation is a form of bioremediation in which fungi-based technology is used to decontaminate the environment. Fungi have been proven to be a very cheap, effective and environmentally sound way for helping to remove a wide array of toxins from damaged environments or wastewater. The toxins include heavy metals, persistent organic pollutants, textile dyes, leather tanning industry chemicals and wastewater, petroleum fuels, polycyclic aromatic hydrocarbon, pharmaceuticals and personal care products, pesticides and herbicide, in land, fresh water and marine environments. The byproducts of the remediation can be valuable materials themselves, such as enzymes, edible or medicinal mushrooms, making the remediation process even profitable.

Fluoranthene chemical compound

Fluoranthene is a polycyclic aromatic hydrocarbon (PAH). The molecule can be viewed as the fusion of naphthalene and benzene unit connected by a five-membered ring. Although samples are often pale yellow, the compound is colorless. It is soluble in nonpolar organic solvents. It is a member of the class of PAHs known as non-alternant PAHs because it has rings other than those with six carbon atoms. It is a structural isomer of the alternant PAH pyrene. It is not as thermodynamically stable as pyrene. Its name is derived from its fluorescence under UV light.

Occupational lung diseases are occupational, or work-related, lung conditions that have been caused or made worse by the materials a person is exposed to within the workplace. It includes a broad group of diseases, including occupational asthma, industrial bronchitis, chronic obstructive pulmonary disease (COPD), bronchiolitis obliterans, inhalation injury, interstitial lung diseases, infections, lung cancer and mesothelioma. These diseases can be caused directly or due to immunological response to a exposure to a variety of dusts, chemicals, proteins or organisms.

Chrysene Chemical compound

Chrysene is a polycyclic aromatic hydrocarbon (PAH) with the molecular formula C
18
H
12
that consists of four fused benzene rings. It is a natural constituent of coal tar, from which it was first isolated and characterized. It is also found in creosote at levels of 0.5-6 mg/kg.

Phenalene chemical compound

1H-Phenalene, often called simply phenalene is a polycyclic aromatic hydrocarbon (PAH). Like many PAHs, it is an atmospheric pollutant formed during the combustion of fossil fuels. It is the parent compound for the phosphorus-containing phosphaphenalenes.

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.

Benzopyrene family of isomeric compounds

A benzopyrene is an organic compound with the formula C20H12. 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.

Benz(<i>a</i>)anthracene chemical compound

Benz[a]anthracene or benzo[a]anthracene is a polycyclic aromatic hydrocarbon with the chemical formula C18H12.

Benzo(<i>c</i>)phenanthrene chemical compound

Benzo[c]phenanthrene is a polycyclic aromatic hydrocarbon with the chemical formula C18H12. It is a white solid that is soluble in nonpolar organic solvents. It is a nonplanar molecule consisting of the fusion of four fused benzene rings. The compound is of mainly theoretical interest but it is environmentally occurring and weakly carcinogenic.

Benzo(<i>j</i>)fluoranthene chemical compound

Benzo[j]fluoranthene (BjF) is an organic compound with the chemical formula C20H12. Classified as a polycyclic aromatic hydrocarbon (PAH), it is a colourless solid that is poorly soluble in most solvents. Impure samples can appear off white. Closely related isomeric compounds include benzo[a]fluoranthene (BaF), bendo[b]fluoranthene (BbF), benzo[e]fluoranthene (BeF), and benzo[k]fluoranthene (BkF). BjF is present in fossil fuels and is released during incomplete combustion of organic matter. It has been traced in the smoke of cigarettes, exhaust from gasoline engines, emissions from the combustion of various types of coal and emissions from oil heating, as well as an impurity in some oils such as soybean oil.

Dibenzopyrenes group of chemical compounds

Dibenzopyrenes are a group of high molecular weight polycyclic aromatic hydrocarbons with the molecular formula C24H14. There are five isomers of dibenzopyrene which differ by the arrangement of aromatic rings: dibenzo[a,e]pyrene, dibenzo[a,h]pyrene, dibenzo[a,i]pyrene, dibenzo[a,l]pyrene, and dibenzo[e,l]pyrene.

Benzo(<i>c</i>)fluorene chemical compound

Benzo[c]fluorene is a polycyclic aromatic hydrocarbon (PAH) with mutagenic activity. It is a component of coal tar, cigarette smoke and smog and thought to be a major contributor to its carcinogenic properties. The mutagenicity of benzo[c]fluorene is mainly attributed to formation of metabolites that are reactive and capable of forming DNA adducts. According to the KEGG it is a group 3 carcinogen. Other names for benzo[c]fluorene are 7H-benzo[c]fluorene, 3,4-benzofluorene, and NSC 89264.

(+)-Benzo(<i>a</i>)pyrene-7,8-dihydrodiol-9,10-epoxide Cancer-causing agent derived from tobacco smoke

(+)-Benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide is an organic compound with molecular formula C20H14O3. It is a metabolite and derivative of benzo[a]pyrene (found in tobacco smoke) as a result of oxidation to include hydroxyl and epoxide functionalities. (+)-Benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide binds to the N2 atom of a guanine nucleobase in DNA, distorting the double helix structure by intercalation of the pyrene moiety between base pairs through π-stacking. The carcinogenic properties of tobacco smoking are attributed in part to this compound binding and inactivating the tumor suppression ability of certain genes, leading to genetic mutations and potentially to cancer.

Gordonia sp. nov. Q8 is a bacterium in the phylum of Actinobacteria. It was discovered in 2017 as one of eighteen new species isolated from the Jiangsu Wei5 oilfield in East China with the potential for bioremediation. Strain Q8 is rod-shaped and gram-positive with dimensions 1.0–4.0 μm × 0.5–1.2 μm and an optimal growth temperature of 40°C. Phylogenetically, it is most closely related to Gordonia paraffinivorans and Gordonia alkaliphila, both of which are known bioremediators. Q8 was assigned as a novel species based on a <70% ratio of DNA homology with other Gordonia bacteria.

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