A polycyclic aromatic hydrocarbon (PAH) is a class of
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s that is composed of multiple
aromatic rings. The simplest representative is
naphthalene
Naphthalene is an organic compound with formula . 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 aromat ...
, having two aromatic rings and the three-ring compounds
anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes. Anthracene is co ...
and
phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon (PAH) with formula C14H10, consisting of three fused benzene rings. It is a colorless, crystal-like solid, but can also appear yellow. Phenanthrene is used to make dyes, plastics and pesticides, ...
. PAHs are uncharged, non-polar and planar. Many are colorless. Many of them are found in
coal
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 when ...
and in
oil deposits, and are also produced by the combustion of
organic matter
Organic matter, organic material, or natural organic matter 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 c ...
—for example, in engines and incinerators or when biomass burns in
forest fires.
Polycyclic aromatic hydrocarbons are discussed as possible
starting materials for
abiotic syntheses of
material
Material is a substance or mixture of substances that constitutes an object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical and chemical properties, or on their geolo ...
s required by the
earliest forms of life.
Nomenclature and structure
The terms polyaromatic hydrocarbon or polynuclear aromatic hydrocarbon are also used for this concept.
[
By definition, polycyclic aromatic hydrocarbons have multiple rings, precluding ]benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
from being considered a PAH. Some sources, such as the US EPA and CDC
The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georg ...
, consider naphthalene
Naphthalene is an organic compound with formula . 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 aromat ...
to be the simplest PAH. Other authors consider PAHs to start with the tricyclic species phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon (PAH) with formula C14H10, consisting of three fused benzene rings. It is a colorless, crystal-like solid, but can also appear yellow. Phenanthrene is used to make dyes, plastics and pesticides, ...
and anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes. Anthracene is co ...
. Most authors exclude compounds that include heteroatom
In chemistry, a heteroatom () is, strictly, any atom that is not carbon or hydrogen.
Organic chemistry
In practice, the term is usually used more specifically to indicate that non-carbon atoms have replaced carbon in the backbone of the molecula ...
s in the rings, or carry substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...
s.
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.
The following are examples of PAHs that vary in the number and arrangement of their rings:
File:Naphthalene-2D-Skeletal.svg , Naphthalene
Naphthalene is an organic compound with formula . 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 aromat ...
File:Biphenyl.svg , Biphenyl
Biphenyl (also known as diphenyl, phenylbenzene, 1,1′-biphenyl, lemonene or BP) is an organic compound that forms colorless crystals. Particularly in older literature, compounds containing the functional group consisting of biphenyl less one ...
File:Fluorene.svg , Fluorene
Fluorene , or 9''H''-fluorene is an organic compound with the formula (C6H4)2CH2. It forms white crystals that exhibit a characteristic, aromatic odor similar to that of naphthalene. It has a violet fluorescence, hence its name. For commercial pu ...
File:Anthracene.svg , Anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes. Anthracene is co ...
File:Phenanthrene.svg , Phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon (PAH) with formula C14H10, consisting of three fused benzene rings. It is a colorless, crystal-like solid, but can also appear yellow. Phenanthrene is used to make dyes, plastics and pesticides, ...
File:Phenalene.svg , Phenalene
1''H''-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 ph ...
File:Naftacene.svg , Tetracene
Tetracene, also called naphthacene, is a polycyclic aromatic hydrocarbon. It has the appearance of a pale orange powder. Tetracene is the four-ringed member of the series of acenes.
Tetracene is a molecular organic semiconductor, used in organ ...
File:Chrysene.svg , Chrysene
Chrysene is a polycyclic aromatic hydrocarbon (PAH) with the molecular formula 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 creoso ...
File:Triphenylene.svg , Triphenylene
Triphenylene is an organic compound with the formula (C6H4)3. A flat polycyclic aromatic hydrocarbon (PAH), it consists of four fused benzene rings. Triphenylene has delocalized 18-''π''-electron systems based on a planar structure, correspondi ...
File:Pyrene.svg , Pyrene
Pyrene is a polycyclic aromatic hydrocarbon (PAH) consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This yellow solid is the smallest peri-fused PAH (one where the rings are fused through mo ...
File:Pentacene.svg , Pentacene
File:Perylene.svg , Perylene
Perylene or perilene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12, occurring as a brown solid. It or its derivatives may be carcinogenic, and it is considered to be a hazardous pollutant. In cell membrane cytochemistry, ...
File:Benzo-a-pyrene.svg , Benzo yrene">'a''yrene
File:Corannulene.svg , Corannulene
Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20 H10. The molecule consists of a cyclopentane ring fused with 5 benzene rings, so another name for it is irculene. It is of scientific interest because it is a geodes ...
File:Benzo(ghi)perilene.png , Benzo erylene">'ghi''erylene
File:Coronene.svg , Coronene
File:Ovalene.svg , Ovalene
File:Benzocfluorene.svg , Benzo luorene">'c''luorene
Geometry
Most PAHs, like naphthalene, anthracene, and coronene, are planar. This geometry is a consequence of the fact that the σ-bonds that result from the merger of sp2 hybrid orbitals of adjacent carbons lie on the same plane as the carbon atom. Those compounds are achiral
Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is distinguishable from i ...
, 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
In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ...
of the molecule and the stiffness (in length and angle) of the carbon-carbon bonds. For example, unlike coronene
Coronene (also known as superbenzene and cyclobenzene) is a polycyclic aromatic hydrocarbon (PAH) comprising seven peri-fused benzene rings. Its chemical formula is . It is a yellow material that dissolves in common solvents including benzene, tol ...
, corannulene
Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20 H10. The molecule consists of a cyclopentane ring fused with 5 benzene rings, so another name for it is irculene. It is of scientific interest because it is a geodes ...
adopts a bowl shape in order to reduce the bond stress. The two possible configurations, concave and convex, are separated by a relatively lowenergy barrier (about 11 kcal
The calorie is a unit of energy. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of o ...
/ mol)
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; and, as of 2002, none of them had been synthesized.
Other PAHs that might seem to be planar, considering only the carbon skeleton, may be distorted by repulsion or steric hindrance between the hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
atoms in their periphery. Benzo henantrene, 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. This effect also causes distortion of picene.[Takuya Echigo, Mitsuyoshi Kimata, and Teruyuki Maruoka (2007): "Crystal-chemical and carbon-isotopic characteristics of karpatite (C24H12) from the Picacho Peak Area, San Benito County, California: Evidences for the hydrothermal formation". ''American Mineralogist'', volume 92, issues 8-9, pages 1262–1269.
]
Adding another benzene ring to form dibenzo ,ghenantrene creates steric hindrance
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
between the two extreme hydrogen atoms. Adding two more rings on the same sense yields heptahelicene in which the two extreme rings overlap. These non-planar forms are chiral, and their enantiomer
In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical anti ...
s can be isolated.[Ivan Gutman and Sven J. Cyvin (2012): ''Introduction to the Theory of Benzenoid Hydrocarbons''. 152 pages. ]
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 sp2 structure of benzene. This class is largely a subset of the alternant PAHs, but is considered to include unstable or hypothetical compounds like triangulene or heptacene
Heptacene is an organic compound and a polycyclic aromatic hydrocarbon and the seventh member of the acene or polyacene family of linear fused benzene rings. This compound has long been pursued by chemists because of its potential interest in elec ...
.[
As of 2012, over 300 benzenoid hydrocarbons had been isolated and characterized.][
]
Bonding and aromaticity
The aromaticity
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
varies for PAHs. According to Clar's rule
In organic and physical organic chemistry, Clar's rule is an empirical rule that relates the chemical stability of a molecule with its aromaticity. It was introduced in 1972 by the Austrian organic chemist Erich Clar in his book ''The Aroma ...
, the resonance structure
In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or ' ...
of a PAH that has the largest number of disjoint aromatic pi sextets—i.e. benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
-like moieties—is the most important for the characterization of the properties of that PAH.
File:Phenanthrene Clar rule.svg, Phenanthrene
File:Anthracene Clar rule.svg, Anthracene
File:Chrysene Clar rule.svg, Chrysene
For example, phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon (PAH) with formula C14H10, consisting of three fused benzene rings. It is a colorless, crystal-like solid, but can also appear yellow. Phenanthrene is used to make dyes, plastics and pesticides, ...
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. In contrast, in anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes. Anthracene is co ...
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. 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; the highest-wavelength absorbance of phenanthrene is at 293 nm, while anthracene is at 374 nm. Three Clar structures with two sextets each are present in the four-ring chrysene
Chrysene is a polycyclic aromatic hydrocarbon (PAH) with the molecular formula 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 creoso ...
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. 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 nonpolar
In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end.
Polar molecules must contain one or more polar ...
and lipophilic
Lipophilicity (from Greek λίπος "fat" and φίλος "friendly"), refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such non-polar solvents are themselves lipo ...
. Larger PAHs are generally insoluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
in water, although some smaller PAHs are soluble. The larger members are also poorly soluble in organic solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
s and in lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
s. The larger members, e.g. perylene, are strongly colored.[
]
Redox
Polycyclic aromatic compounds characteristically yield radical
Radical may refer to:
Politics and ideology Politics
* Radical politics, the political intent of fundamental societal change
*Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe an ...
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s upon treatment with alkali metals. The large PAH form dianions as well. The redox potential correlates with the size of the PAH.
:
Sources
Natural
Fossil carbon
Polycyclic aromatic hydrocarbons are primarily found in natural sources such as bitumen
Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...
.
PAHs can also be produced geologically when organic sediments are chemically transformed into fossil fuel
A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ma ...
s such as oil and coal
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 when ...
. The rare minerals idrialite
Idrialite is a rare hydrocarbon mineral with approximate chemical formula C22H14.
Idrialite usually occurs as soft orthorhombic crystals, is usually greenish yellow to light brown in color with bluish fluorescence. It is named after Idrija, town i ...
, curtisite, and carpathite
Carpathite is a very rare hydrocarbon mineral, consisting of exceptionally pure coronene (C24H12), a polycyclic aromatic hydrocarbon. The name has been spelled karpatite and the mineral was improperly renamed pendletonite.
Discovery
The mineral wa ...
consist almost entirely of PAHs that originated from such sediments, that were extracted, processed, separated, and deposited by very hot fluids.[
]
Natural fires
PAHs may result from the incomplete combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...
of organic matter
Organic matter, organic material, or natural organic matter 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 c ...
in natural wildfire
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...
s. 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.
High levels of such PAHs have been detected in the Cretaceous-Tertiary (K-T) boundary, more than 100 times the level in adjacent layers. The spike was attributed to massive fires that consumed about 20% of the terrestrial above-ground biomass in a very short time.
Extraterrestrial
PAHs are prevalent in the interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
(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. PAHs generally trace regions of cold molecular gas, which are optimum environments for the formation of stars.
NASA's Spitzer Space Telescope
The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, ...
and James Webb Space Telescope
The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Sp ...
include instruments for obtaining both images and spectra of light emitted by PAHs associated with . These images can trace the surface of star-forming clouds
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planetary body or similar space. ...
in our own galaxy or identify star forming galaxies in the distant universe.
In June 2013, PAHs were detected in the upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes:
* The mesosphere, which on Earth lies between th ...
of Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
, the largest moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
of the planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
.
Minor sources
Volcanic eruption
Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are oft ...
s may emit PAHs.
Certain PAHs such as perylene
Perylene or perilene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12, occurring as a brown solid. It or its derivatives may be carcinogenic, and it is considered to be a hazardous pollutant. In cell membrane cytochemistry, ...
can also be generated in anaerobic
Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to:
* Anaerobic adhesive, a bonding a ...
sediments from existing organic material, although it remains undetermined whether abiotic or microbial processes drive their production.
Artificial
The dominant sources of PAHs in the environment are thus from human activity: wood-burning and combustion of other biofuels
Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. According to the United States Energy Information Administration (E ...
such as dung or crop residues contribute more than half of annual global PAH emissions, particularly due to biofuel use in India and China. 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.
A year-long sampling campaign in Athens, Greece found a third (31%) of PAH urban air pollution
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...
to be caused by wood-burning, like diesel and oil (33%) and gasoline (29%). It also found that wood-burning is responsible for nearly half (43%) of annual PAH cancer-risk (carcinogenic potential) compared to the other sources and that wintertime PAH levels were 7 times higher than in other seasons, especially if atmospheric dispersion is low.
Lower-temperature combustion, such as tobacco smoking
Tobacco smoking is the practice of burning tobacco and ingesting the resulting smoke. The smoke may be inhaled, as is done with cigarettes, or simply released from the mouth, as is generally done with pipes and cigars. The practice is believed ...
or wood-burning, tends to generate low molecular weight PAHs, whereas high-temperature industrial processes typically generate PAHs with higher molecular weights.
PAHs are typically found as complex mixtures.
Distribution in the environment
Aquatic environments
Most PAHs are insoluble in water, which limits their mobility in the environment, although PAHs sorb to fine-grained organic-rich sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s. Aqueous solubility of PAHs decreases approximately logarithmically
In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 o ...
as molecular mass
The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...
increases.
Two-ringed PAHs, and to a lesser extent three-ringed PAHs, dissolve in water, making them more available for biological uptake and degradation
Degradation may refer to:
Science
* Degradation (geology), lowering of a fluvial surface by erosion
* Degradation (telecommunications), of an electronic signal
* Biodegradation of organic substances by living organisms
* Environmental degradatio ...
. 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. In contrast, compounds with five or more rings have low solubility in water and low volatility; they are therefore predominantly in solid state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* ''Our S ...
, bound to particulate
Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The te ...
air pollution
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...
, soils
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former term ...
, or sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s. In solid state, these compounds are less accessible for biological uptake or degradation, increasing their persistence in the environment.
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. 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.
Surgical smoke plumes have been proven to contain PAHs in several independent research studies.
Burning solid fuels such as coal
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 when ...
and biofuel
Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. According to the United States Energy Information Administration (E ...
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 particulate air pollution containing PAHs, particularly for women and children who spend more time in the home or cooking.
In industrial countries, people who smoke tobacco products, or who are exposed to second-hand smoke
Passive smoking is the inhalation of tobacco smoke, called secondhand smoke (SHS), or environmental tobacco smoke (ETS), by persons other than the intended "active" smoker. It occurs when tobacco smoke enters an environment, causing its inhalat ...
, are among the most highly exposed groups; tobacco smoke
Tobacco smoke is a sooty aerosol produced by the incomplete combustion of tobacco during the tobacco smoking, smoking of cigarettes and other tobacco products. Temperatures in burning cigarettes range from about 400 °C between puffs to abo ...
contributes to 90% of indoor PAH levels in the homes of smokers. 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. PAHs are typically at low concentrations in drinking water.
Emissions from vehicles such as cars and trucks can be a substantial outdoor source of PAHs in particulate air pollution. Geographically, major roadways are thus sources of PAHs, which may distribute in the atmosphere or deposit nearby. Catalytic converter
A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usually ...
s are estimated to reduce PAH emissions from gasoline-fired vehicles by 25-fold.
People can also be occupationally exposed during work that involves fossil fuels or their derivatives, wood-burning, carbon electrode
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on larg ...
s, or exposure to diesel exhaust
Diesel exhaust is the gaseous exhaust produced by a diesel type of internal combustion engine, plus any contained particulates. Its composition may vary with the fuel type or rate of consumption, or speed of engine operation (e.g., idling or at ...
. Industrial activity that can produce and distribute PAHs includes aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
, iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
, and steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
manufacturing; coal gasification, tar
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. "a dark brown or black bit ...
distillation, shale oil extraction
Shale oil extraction is an industrial process for unconventional oil production. This process converts kerogen in oil shale into shale oil by pyrolysis, hydrogenation, or thermal dissolution. The resultant shale oil is used as fuel oil or up ...
; production of coke, creosote
Creosote is a category of carbonaceous chemicals formed by the distillation of various tars and pyrolysis of plant-derived material, such as wood or fossil fuel. They are typically used as preservatives or antiseptics.
Some creosote types were ...
, carbon black
Carbon black (subtypes are acetylene black, channel black, furnace black, lamp black and thermal black) is a material produced by the incomplete combustion of coal and coal tar, vegetable matter, or petroleum products, including fuel oil, fluid ...
, and calcium carbide
Calcium carbide, also known as calcium acetylide, is a chemical compound with the chemical formula of Ca C2. Its main use industrially is in the production of acetylene and calcium cyanamide.
The pure material is colorless, while pieces of tec ...
; road paving and asphalt
Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...
manufacturing; rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
tire
A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineering), t ...
production; manufacturing or use of metal working
Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale ...
fluids; and activity of coal or natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
power station
A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid.
Many p ...
s.
Environmental pollution and degradation
PAHs typically disperse from urban
Urban means "related to a city". In that sense, the term may refer to:
* Urban area, geographical area distinct from rural areas
* Urban culture, the culture of towns and cities
Urban may also refer to:
General
* Urban (name), a list of people ...
and suburban
A suburb (more broadly suburban area) is an area within a metropolitan area, which may include commercial and mixed-use, that is primarily a residential area. A suburb can exist either as part of a larger city/urban area or as a separate ...
non-point sources
Nonpoint source (NPS) pollution refers to diffuse contamination (or pollution) of water or air that does not originate from a single discrete source. This type of pollution is often the cumulative effect of small amounts of contaminants gathered ...
through road runoff
Runoff, run-off or RUNOFF may refer to:
* RUNOFF, the first computer text-formatting program
* Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed
* Runoff or run-off, a stock market ...
, sewage
Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system. Sewage consists of wastewater discharged from residenc ...
, and atmospheric circulation
Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from year to year, but t ...
and subsequent deposition of particulate air pollution. Soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
and river sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
near industrial sites such as creosote manufacturing facilities can be highly contaminated with PAHs. Oil spill
An oil spill is the release of a liquid petroleum hydrocarbon into the environment, especially the marine ecosystem, due to human activity, and is a form of pollution. The term is usually given to marine oil spills, where oil is released into th ...
s, creosote, coal mining
Coal mining is the process of extracting coal from the ground. Coal is valued for its energy content and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from ...
dust, and other fossil fuel sources can also distribute PAHs in the environment.
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
A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
. PAHs have a strong affinity for organic carbon
Total organic carbon (TOC) is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic c ...
, and thus highly organic sediments in river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...
s, lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
s, and the ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
can be a substantial sink for PAHs.
Algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
and some invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s such as protozoans
Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
, mollusks
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
, and many polychaete
Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...
s have limited ability to metabolize
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
PAHs and bioaccumulate
Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance at a rate faster than that at which the substance is lost or eliminated ...
disproportionate concentrations of PAHs in their tissues; however, PAH metabolism can vary substantially across invertebrate species. Most vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
s metabolize and excrete PAHs relatively rapidly. Tissue concentrations of PAHs do not increase (biomagnify
Biomagnification, also known as bioamplification or biological magnification, is any concentration of a toxin, such as pesticides, in the tissues of tolerant organisms at successively higher levels in a food chain. This increase can occur as a ...
) from the lowest to highest levels of food chains.
PAHs transform slowly to a wide range of degradation products. Biological degradation by microbe
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s is a dominant form of PAH transformation in the environment. Soil-consuming invertebrates such as earthworm
An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. Th ...
s speed PAH degradation, either through direct metabolism or by improving the conditions for microbial transformations. 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.
Urban soils
The British Geological Survey
The British Geological Survey (BGS) is a partly publicly funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research.
The BGS h ...
reported the amount and distribution of PAH compounds including parent and alkylated forms in urban soils at 76 locations in Greater London
Greater may refer to:
*Greatness, the state of being great
*Greater than, in inequality (mathematics), inequality
*Greater (film), ''Greater'' (film), a 2016 American film
*Greater (flamingo), the oldest flamingo on record
*Greater (song), "Greate ...
. 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
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 ...
and pyrene
Pyrene is a polycyclic aromatic hydrocarbon (PAH) consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This yellow solid is the smallest peri-fused PAH (one where the rings are fused through mor ...
were generally the most abundant PAHs.[ Benzo yrene">'a''yrene (B''a''P), the most toxic of the parent PAHs, is widely considered a key marker PAH for environmental assessments; the normal background concentration of B''a''P in the London urban sites was 6.9 mg/kg (dry soil weight).][ ]London
London is the capital and largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary down to the North Sea, and has been a majo ...
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
Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...
.
Peatlands
Managed burning
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...
of moorland
Moorland or moor is a type of habitat found in upland areas in temperate grasslands, savannas, and shrublands and montane grasslands and shrublands biomes, characterised by low-growing vegetation on acidic soils. Moorland, nowadays, generally ...
vegetation in the UK has been shown to generate PAHs which become incorporated into the peat
Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficien ...
surface. 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
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
PAHs are attenuated by biotic decay and photodegradation Photodegradation is the alteration of materials by light. Commonly, the term is used loosely to refer to the combined action of sunlight and air, which cause oxidation and hydrolysis. Often photodegradation is intentionally avoided, since it destro ...
.[ 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).][
]
Rivers, estuarine and coastal sediments
Concentrations of PAHs in river and estuarine sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
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. Consequently, the concentrations of pollutants
A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...
in estuaries tends to decrease at the river mouth. Understanding of sediment hosted PAHs in estuaries is important for the protection of commercial fisheries
Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, both ...
(such as mussel
Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...
s) and general environmental habitat conservation because PAHs can impact the health of suspension and sediment feeding organism. 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.[ Typical PAH concentrations in UK estuaries range from about 19 to 16,163 µg/kg (dry sediment weight) in the ]River Clyde
The River Clyde ( gd, Abhainn Chluaidh, , sco, Clyde Watter, or ) is a river that flows into the Firth of Clyde in Scotland. It is the ninth-longest river in the United Kingdom, and the third-longest in Scotland. It runs through the major cit ...
and 626 to 3,766 µg/kg in the River Mersey
The River Mersey () is in North West England. Its name derives from Old English and means "boundary river", possibly referring to its having been a border between the ancient kingdoms of Mercia and Northumbria. For centuries it has formed part ...
. In general estuarine sediments with a higher natural total organic carbon
Total organic carbon (TOC) is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic c ...
content (TOC) tend to accumulate PAHs due to high sorption
Sorption is a physical and chemical process by which one substance becomes attached to another. Specific cases of sorption are treated in the following articles:
; Absorption: "the incorporation of a substance in one state into another of a d ...
capacity of organic matter.[ A similar correspondence between PAHs and TOC has also been observed in the sediments of tropical ]mangrove
A mangrove is a shrub or tree that grows in coastal saline water, saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evoluti ...
s located on the coast of southern China.
Human health
Cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
is a primary human health risk of exposure to PAHs. Exposure to PAHs has also been linked with cardiovascular disease and poor fetal development.
Cancer
PAHs have been linked to skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
, lung
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
, bladder
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
, liver
The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
, and stomach
The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...
cancers in well-established animal model studies. Specific compounds classified by various agencies as possible or probable human carcinogens are identified in the section " Regulation and Oversight" below.
History
Historically, PAHs contributed substantially to our understanding of adverse health effects from exposures to environmental contaminants, including chemical carcinogenesis
Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abno ...
. In 1775, Percivall Pott
Percivall Pott (6 January 1714, in London – 22 December 1788) was an English surgeon, one of the founders of orthopaedics, and the first scientist to demonstrate that a cancer may be caused by an environmental carcinogen.
Career
He was the ...
, 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
Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed ...
. A century later, Richard von Volkmann
Richard von Volkmann (17 August 1830 – 28 November 1889) was a prominent German surgeon and author of poetry and fiction. Some of his works were illustrated by his son, Hans, a well known artist.
Biography
He was born in Leipzig on 17 August ...
reported increased skin cancers in workers of the coal tar
Coal tar is a thick dark liquid which is a by-product of the production of coke and coal gas from coal. It is a type of creosote. It has both medical and industrial uses. Medicinally it is a topical medication applied to skin to treat psoriasi ...
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.
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
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
pattern that was similar but not identical to benz nthracene">'a''nthracene, a PAH that was subsequently demonstrated to cause tumor
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
s. Cook, Hewett and Hieger then linked the specific spectroscopic fluorescent profile of benzo yrene">'a''yrene to that of the carcinogenic component of coal tar, 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
Sir William Richard Shaboe Doll (28 October 1912 – 24 July 2005) was a British physician who became an epidemiologist in the mid-20th century and made important contributions to that discipline. He was a pioneer in research linking smoking ...
and various others, reported greater rates of death from lung cancer
Lung cancer, also known as lung carcinoma (since about 98–99% of all lung cancers are carcinomas), is a malignant lung tumor characterized by uncontrolled cell growth in tissue (biology), tissues of the lung. Lung carcinomas derive from tran ...
following occupational exposure to PAH-rich environments among workers in coke oven
Coke is a grey, hard, and porous coal-based fuel with a high carbon content and few impurities, made by heating coal or oil in the absence of air—a destructive distillation process. It is an important industrial product, used mainly in iron ...
s and coal carbonization
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 when de ...
and gasification
Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (). This is achieved by reacting ...
processes.
Mechanisms of carcinogenesis
The structure of a PAH influences whether and how the individual compound is carcinogenic. Some carcinogenic PAHs are genotoxic Genotoxicity is the property of chemical agents that damage the genetic information within a cell causing mutations, which may lead to cancer. While genotoxicity is often confused with mutagenicity, all mutagens are genotoxic, but some genotoxic s ...
and induce mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
s that initiate cancer; others are not genotoxic and instead affect cancer promotion or progression.
PAHs that affect cancer initiation are typically first chemically modified by enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
s 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''yrene, 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. Mutagenic metabolites of PAHs include diol epoxides, quinones
The quinones are a class of organic compounds that are formally "derived from aromatic compounds uch as benzene or naphthalene">benzene.html" ;"title="uch as benzene">uch as benzene or naphthalene] by conversion of an even number of –CH= group ...
, and radical (chemistry), radical PAH cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s. These metabolites can bind to DNA at specific sites, forming bulky complexes called DNA adduct
In molecular genetics, a DNA adduct is a segment of DNA bound to a cancer-causing chemical. This process could lead to the development of cancerous cells, or carcinogenesis. DNA adducts in scientific experiments are used as biomarkers of exposur ...
s that can be stable or unstable. Stable adducts may lead to DNA replication
In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritanc ...
errors, while unstable adducts react with the DNA strand, removing a purine
Purine is a heterocyclic compound, heterocyclic aromatic organic compound that consists of two rings (pyrimidine and imidazole) fused together. It is water-soluble. Purine also gives its name to the wider class of molecules, purines, which includ ...
base (either adenine
Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivati ...
or guanine
Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is called ...
). Such mutations, if they are not repaired, can transform genes encoding for normal cell signaling
In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
proteins into cancer-causing oncogene
An oncogene is a gene that has the potential to cause cancer. In tumor cells, these genes are often mutated, or expressed at high levels. s. Quinones can also repeatedly generate reactive oxygen species
In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
The reduction of molecular oxygen () p ...
that may independently damage DNA.
Enzymes in the cytochrome
Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of bin ...
family (CYP1A1
Cytochrome P450, family 1, subfamily A, polypeptide 1 is a protein
that in humans is encoded by the ''CYP1A1'' gene.
The protein is a member of the cytochrome P450 superfamily of enzymes.
Function
Metabolism of xenobiotics and drugs
CYP1A1 ...
, CYP1A2
Cytochrome P450 1A2 (abbreviated CYP1A2), a member of the cytochrome P450 mixed-function oxidase system, is involved in the metabolism of xenobiotics in the human body. In humans, the CYP1A2 enzyme is encoded by the ''CYP1A2'' gene.
Function
...
, CYP1B1
Cytochrome P450 1B1 is an enzyme that in humans is encoded by the ''CYP1B1'' gene.
Function
CYP1B1 belongs to the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved i ...
) metabolize PAHs to diol epoxides. PAH exposure can increase production of the cytochrome enzymes, allowing the enzymes to convert PAHs into mutagenic diol epoxides at greater rates. In this pathway, PAH molecules bind to the aryl hydrocarbon receptor
The aryl hydrocarbon receptor (also known as AhR, AHR, ahr, ahR, or dioxin receptor) is a protein that in humans is encoded by the AHR gene. The aryl hydrocarbon receptor is a transcription factor that regulates gene expression. It was originall ...
(AhR) and activate it as a transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...
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.
Low molecular weight PAHs, with two to four aromatic hydrocarbon rings, are more potent as co-carcinogen
A co-carcinogen is a chemical that promotes the effects of a carcinogen in the production of cancer. Usually, the term is used to refer to chemicals that are not carcinogenic on their own, such that an equivalent amount of the chemical is insuf ...
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. 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. 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
Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction (commonly known as a heart attack). Other CVDs include stroke, h ...
. PAHs are among the complex suite of contaminants in tobacco smoke
Tobacco smoke is a sooty aerosol produced by the incomplete combustion of tobacco during the tobacco smoking, smoking of cigarettes and other tobacco products. Temperatures in burning cigarettes range from about 400 °C between puffs to abo ...
and particulate air pollution
Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The te ...
and may contribute to cardiovascular disease resulting from such exposures.
In laboratory experiments, animals exposed to certain PAHs have shown increased development of plaques (atherogenesis
Atherosclerosis is a pattern of the disease arteriosclerosis in which the wall of the artery develops abnormalities, called lesions. These lesions may lead to narrowing due to the buildup of atheromatous plaque. At onset there are usually no sy ...
) within arteries. Potential mechanisms for the pathogenesis
Pathogenesis is the process by which a disease or disorder develops. It can include factors which contribute not only to the onset of the disease or disorder, but also to its progression and maintenance. The word comes from Greek πάθος ''pat ...
and development of atherosclerotic plaques may be similar to the mechanisms involved in the carcinogenic and mutagenic properties of PAHs. A leading hypothesis is that PAHs may activate the cytochrome enzyme CYP1B1
Cytochrome P450 1B1 is an enzyme that in humans is encoded by the ''CYP1B1'' gene.
Function
CYP1B1 belongs to the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved i ...
in vascular smooth muscle
Vascular smooth muscle is the type of smooth muscle that makes up most of the walls of blood vessels.
Structure
Vascular smooth muscle refers to the particular type of smooth muscle found within, and composing the majority of the wall of blood ...
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
Plaque may refer to:
Commemorations or awards
* Commemorative plaque, a plate or tablet fixed to a wall to mark an event, person, etc.
* Memorial Plaque (medallion), issued to next-of-kin of dead British military personnel after World War I
* Pla ...
formation. These quinone metabolites also generate reactive oxygen species
In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
The reduction of molecular oxygen () p ...
that may alter the activity of genes that affect plaque formation.
Oxidative stress
Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily Detoxification, detoxify the reactive intermediates or to repair the resulting damage. Disturbances ...
following PAH exposure could also result in cardiovascular disease by causing inflammation
Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...
, which has been recognized as an important factor in the development of atherosclerosis and cardiovascular disease. Biomarker
In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, ...
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.
Developmental impacts
Multiple epidemiological
Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population.
It is a cornerstone of public health, and shapes policy decisions and evidenc ...
studies of people living in Europe, the United States, and China have linked in utero
''In Utero'' is the third and final studio album by American rock band Nirvana. It was released on September 21, 1993, by DGC Records. After breaking into the mainstream with their second album, ''Nevermind'' (1991), Nirvana hired Steve Albin ...
exposure to PAHs, through air pollution or parental occupational exposure, with poor fetal growth, reduced immune function, and poorer neurological
Neurology (from el, νεῦρον (neûron), "string, nerve" and the suffix -logia, "study of") is the branch of medicine dealing with the diagnosis and treatment of all categories of conditions and disease involving the brain, the spinal ...
development, including lower IQ.
Regulation and oversight
Some governmental bodies, including the European Union
The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been des ...
as well as NIOSH
The National Institute for Occupational Safety and Health (NIOSH, ) is the United States federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness. NIOSH is part of the C ...
and the United States Environmental Protection Agency
The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...
(EPA), regulate concentrations of PAHs in air, water, and soil.[ The ]European Commission
The European Commission (EC) is the executive of the European Union (EU). It operates as a cabinet government, with 27 members of the Commission (informally known as "Commissioners") headed by a President. It includes an administrative body o ...
has restricted concentrations of 8 carcinogenic PAHs in consumer products that contact the skin or mouth.
Priority polycyclic aromatic hydrocarbons identified by the US EPA, the US Agency for Toxic Substances and Disease Registry
The Agency for Toxic Substances and Disease Registry (ATSDR) is a federal public health agency within the United States Department of Health and Human Services. The agency focuses on minimizing human health risks associated with exposure to haz ...
(ATSDR), and the European Food Safety Authority
The European Food Safety Authority (EFSA) is the agency of the European Union (EU) that provides independent scientific advice and communicates on existing and emerging risks associated with the food chain. EFSA was established in February 2002, ...
(EFSA) due to their carcinogenicity or genotoxicity and/or ability to be monitored are the following:
::Considered probable or possible human carcinogens by the US EPA, the European Union, and/or the International Agency for Research on Cancer
The International Agency for Research on Cancer (IARC; french: Centre International de Recherche sur le Cancer, CIRC) is an intergovernmental agency forming part of the World Health Organization of the United Nations.
Its role is to conduct and ...
(IARC).
Detection and optical properties
A spectral database exists for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe
The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...
. Detection of PAHs in materials is often done using gas chromatography-mass spectrometry
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
or liquid chromatography
In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system (a ...
with ultraviolet-visible or fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
spectroscopic methods or by using rapid test PAH indicator strips. Structures of PAHs have been analyzed using infrared spectroscopy.
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 isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.
Iso ...
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
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
, 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
PAHs may be abundant in the universe. They seem to have been formed as early as a couple of billion years after the Big Bang
The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
, and are associated with new stars and exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
s. More than 20% of the carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
in the universe may be associated with PAHs. PAHs are considered possible starting material for the earliest forms of life.
Light emitted by the Red Rectangle nebula
The Red Rectangle Nebula, so called because of its red color and unique rectangular shape, is a protoplanetary nebula in the Monoceros constellation. Also known as HD 44179, the nebula was discovered in 1973 during a rocket flight associated w ...
possesses spectral signatures that suggest the presence of anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes. Anthracene is co ...
and pyrene
Pyrene is a polycyclic aromatic hydrocarbon (PAH) consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This yellow solid is the smallest peri-fused PAH (one where the rings are fused through mor ...
. 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[ that PAHs—which may have been vital in the formation of early life on Earth—can only originate in nebulae.][
]
PAHs, subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation
Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a Catalysis, catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or S ...
, oxygenation, and hydroxylation
In chemistry, hydroxylation can refer to:
*(i) most commonly, hydroxylation describes a chemical process that introduces a hydroxyl group () into an organic compound.
*(ii) the ''degree of hydroxylation'' refers to the number of OH groups in a ...
, to more complex organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s—"a step along the path toward amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s and nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...
s, the raw materials of protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s and DNA, respectively". 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 Interstellar ice consists of grains of volatiles in the ice phase that form in the interstellar medium. Ice and dust grains form the primary material out of which the Solar System was formed. Grains of ice are found in the dense regions of molecular ...
grains
A grain is a small, hard, dry fruit (caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legumes ...
, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disk
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...
s."
Low-temperature chemical pathways from simple organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s to complex PAHs are of interest. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
's moon Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
, and may be significant pathways, in terms of the 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 ...
, in producing precursors to biochemicals related to life as we know it.
See also
* Chicken wire (chemistry)
In chemistry the term chicken wire is used in different contexts. Most of them relate to the similarity of the regular hexagonal (honeycomb-like) patterns found in certain chemical compounds to the mesh structure commonly seen in real chicken wi ...
* F number
In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill ...
* Graphene
Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.
* Total petroleum hydrocarbon Total petroleum hydrocarbons (TPH) is a term used for any mixture of hydrocarbons that are found in crude oil. There are several hundred of these compounds, but not all occur in any one sample. Crude oil is used to make petroleum products, which can ...
References
External links
ATSDR - Toxicity of Polycyclic Aromatic Hydrocarbons (PAHs)
U.S. Department of Health and Human Services
Fused Ring and Bridged Fused Ring Nomenclature
Database of PAH structures
Cagliari PAH Theoretical Database
NASA Ames PAH IR Spectroscopic Database
* ttps://web.archive.org/web/20060623051810/http://www.spitzer.caltech.edu/features/articles/20050627.shtml Understanding Polycyclic Aromatic HydrocarbonsNASA Spitzer Space Telescope
"The Aromatic World: An Interview with Professor Pascale Ehrenfreund"
from ''Astrobiology Magazine
''Astrobiology Magazine (exploring the solar system and beyond)'', or ''Astrobiology Mag'', was an American, formerly NASA-sponsored, international online popular science magazine that contained popular science content, which referred to articl ...
''
Oregon State University Superfund Research Center
focused on new technologies and emerging health risks of Polycyclic Aromatic Hydrocarbons (PAHs)
Polycyclic Aromatic Hydrocarbons (PAHs)--EPA Fact Sheet
U.S. Environmental Protection Agency, Office of Solid Waste, January 2008.
{{DEFAULTSORT:Polycyclic Aromatic Hydrocarbon
Carcinogens
Origin of life
Astrochemistry
Persistent organic pollutants under the Convention on Long-Range Transboundary Air Pollution