Polycyclic aromatic hydrocarbons are hydrocarbons—organic compounds containing only carbon and hydrogen—that
are composed of multiple aromatic rings. Formally, the class is further defined as lacking further
branching substituents off of these ring structures. Polynuclear aromatic hydrocarbons are a subset
of PAHs that have fused aromatic rings, that is, rings that share one or more sides. Though
poly- in these cases literally means "many", there is precedence in nomenclature for beginning
this class and subclass with the two ring cases, where biphenyl and naphthalene would
therefore be considered simple examples; beginning at three rings, examples include anthracene
and phenanthrene. PAHs are neutral, nonpolar molecules; they
are found in fossil fuels and in tar deposits, and are produced, generally, when insufficient
oxygen or other factors result in incomplete combustion of organic matter. PAHs can also
be found at high levels in cooked foods, e.g., in meat cooked at high temperatures over open
flame. Benzo[a]pyrene is a well-research example of a coal tar PAH whose metabolites are mutagenic
and highly carcinogenic; as a class, benzopyrenes, a ring fusion between monocyclic benzene and
tetracyclic pyrene rings, result from incomplete combustion at temperatures between 300 °C
and 600 °C. PAHs may also be abundant in the universe,
and are conjectured to have formed 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,
and PAHs are discussed as possible starting materials for abiologic syntheses of materials
required by the earliest forms of life. Critically, as noted, PAHs have been identified
as carcinogenic and mutagenic, and are considered pollutants of concern for the potency of potential
adverse health impacts; the same holds true of their presence at significant levels over
time in human diets.
Nomenclature, structure, properties Structure and properties
As noted, poly- literally means "many", but there is precedence in nomenclature for beginning
the PAH class and subclass with the two ring cases; beginning at three fused aromatic rings,
the simplest PAHs are phenanthrene and anthracene. Smaller molecules, such as benzene, are not
PAHs, and PAHs are not generally considered as containing heteroatoms or carry substituents.
Hence, PAHs may contain four-, five-, six- or seven-member rings; those with five or
six are most common. The following are PAH examples with larger numbers of rings:
Examples of PAH compounds
PAHs composed only of six-membered rings are called alternant PAHs, which include benzenoid
PAHs. PAHs containing up to six fused aromatic rings are often known as "small" PAHs, and
those containing more than six aromatic rings are called "large" PAHs. The bulk of research
on PAHs has been on small PAHs, due to their availability. Large PAHs are found as combustion
products, but at lower levels than the small PAHs. In addition, many more isomers are possible
for the larger PAHs, leading to lower frequency of occurrence of individual large PAH structures.
The biological activity and occurrence of the large PAHs appears to be a continuation
of the trends and effects seen for small PAHs. Aqueous solubility of PAHs decreases approximately
one order of magnitude for each additional ring.
Occurrence and pollution
Polycyclic aromatic hydrocarbons are lipophilic, meaning they mix more easily with oil than
water. The larger compounds are less water-soluble and less volatile. Because of these properties,
PAHs in the environment are found primarily in soil, sediment, and oily substances, as
opposed to in water or air. However, they are also a component of concern in particulate
matter suspended in air. Natural crude oil and coal deposits contain
significant amounts of PAHs, arising from chemical conversion of natural product molecules,
such as steroids, to aromatic hydrocarbons. They are also found in processed fossil fuels,
tar and various edible oils. In a study evaluating the genotoxic and carcinogenic risks associated
with the consumption of repeatedly heated coconut oil, one of the commonly consumed
cooking and frying medium, it was concluded that dietary consumption of RCO can cause
a genotoxic and preneoplastic change in the liver.
PAHs are one of the most widespread organic pollutants. In addition to their presence
in fossil fuels they are also formed by incomplete combustion of carbon-containing fuels such
as wood, coal, diesel, fat, tobacco, and incense. Different types of combustion yield different
distributions of PAHs in both relative amounts of individual PAHs and in which isomers are
produced. Thus, coal burning produces a different mixture than motor-fuel combustion or a forest
fire, making the compounds potentially useful as indicators of the burning history. Hydrocarbon
emissions from fossil fuel-burning engines are regulated in developed countries. Certain
species of bacteria are capable of degrading polycyclic hydrocarbons, such as Kordiimonas
gwangyangensis. PAH chemicals are often linked to oil spills.
Following the massive Deepwater Horizon oil spill, scientists found PAH levels to be 40
times higher than before the area was affected. The researchers said that the compounds can
enter the food chain through organisms such as plankton or fish. The mechanism by which
oil spill PAHs poison fish was reported in a 2014 paper in the journal Science, which
found that PAHs kill fish by sending them into cardiac arrest. The study also found
even very low concentrations of PAHs can slow fish heartbeats and disrupt the development
of fish larvae. The work may have implications for mammals and other forms of vertebrate
life. PAHs are also found in cooked foods. Studies
have shown that high levels of PAHs are found, for example, in meat cooked at high temperatures
such as grilling or barbecuing, and in smoked fish.
Human health The toxicity of PAHs is structure-dependent.
Isomers can vary from being nontoxic to extremely toxic. One PAH compound, benzo[a]pyrene, is
notable for being the first chemical carcinogen to be discovered. The EPA has classified seven
PAH compounds as probable human carcinogens: benz[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene,
benzo[k]fluoranthene, chrysene, dibenz(a,h)anthracene, and indeno(1,2,3-cd)pyrene.
PAHs known for their carcinogenic, mutagenic, and teratogenic properties are benz[a]anthracene
and chrysene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene,
coronene, dibenz(a,h)anthracene (C 20H
14), indeno(1,2,3-cd)pyrene (C 22H
12), and ovalene. High prenatal exposure to PAH is associated
with lower IQ and childhood asthma. The Center for Children's Environmental Health reports
studies that demonstrate that exposure to PAH pollution during pregnancy is related
to adverse birth outcomes including low birth weight, premature delivery, and heart malformations.
Cord blood of exposed babies shows DNA damage that has been linked to cancer. Follow-up
studies show a higher level of developmental delays at age three, lower scores on IQ tests
and increased behavioral problems at ages six and eight.
In addition, a 2012 Columbia University study in Environmental Health Perspectives linked
prenatal exposure to pollutants and eventual child behavioral outcomes. The study found
that exposure to higher levels of PAH was associated with a 24% higher score of anxiety
or depression for children ages six to seven than those with low exposure levels. Infants
found to have elevated PAH levels in their umbilical cord blood were 46% more likely
to eventually score highly on the anxiety/depression scale than those with low PAH levels in cord
blood. Agency for Toxic Substances and Disease Registry
listings Although the health effects of individual
PAHs are not exactly alike, the following 18 PAHs are considered as a group in this
profile issued by the Agency for Toxic Substances and Disease Registry:
Additional PAHs: naphthalene
Environmental Protection Agency priority pollutants The United States Environmental Protection
Agency has designated 32 PAH compounds as priority pollutants. The original 16 are listed;
they are naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene,
pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz(ah)anthracene,
benzo[ghi]perylene, and indeno[1,2,3-cd]pyrene. This list of the 16 EPA priority PAHs is often
targeted for measurement in environmental samples.
Detection and optical properties 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. 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. Theoretical aspects
Aromaticity Although PAHs clearly are aromatic compounds,
the degree of aromaticity can be different for each ring segment. According to Clar's
rule for PAHs the resonance structure with the most disjoint aromatic п-sextets—i.e.
benzene-like moieties—is the most important for the characterization of the properties.
For example, in phenanthrene one Clar structure has two sextets at the extremities, while
the other resonance structure has just one central sextet; therefore in this molecule
the outer rings are have greater aromatic character whereas the central ring is less
aromatic and therefore more reactive. In contrast, in anthracene the resonance structures have
one sextet, 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 UV
absorbance spectra of these two isomers; phenanthrene has a highest wavelength absorbance around
290 nm, while anthracene has highest wavelength bands around 380 nm. Three Clar structures
with two sextets each are present in chrysene. Superposition of these structures reveals
that the aromaticity in the outer rings is greater compared to the inner rings. Another
example Clar hydrocarbon is zethrene, which is illustrated in the form with the most sextets.
Four of the rings are highly aromatic, while the others are less-so, and therefore more
reactive. 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, and are associated with new stars and exoplanets.
More than 20% of the carbon in the universe may be associated with PAHs. PAHs are considered
possible starting material for the earliest forms of life. In January 2004, it was reported
that a team led by A. Witt of the University of Toledo, Ohio studied ultraviolet light
emitted by the Red Rectangle nebula and found spectral signatures that suggest the presence
of anthracene and pyrene. 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 core 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. Witt and his team inferred
that PAHs —which may have been vital in the formation of early life on Earth— in
a nebula, by necessity they must originate in nebulae.
More recently, fullerenes, have been detected in other nebulae. 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." In September 2012, NASA
scientists reported results of analog studies in vitro that PAHs, subjected to interstellar
medium 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". 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."
In June 2013, the detection of PAHs in the upper atmosphere of Titan, the largest moon
of the planet Saturn, was reported by scientists at IAA-CSIC.
On February 21, 2014, NASA announced a greatly upgraded database for tracking polycyclic
aromatic hydrocarbons in the universe. According to scientists, more than 20% of the carbon
in the universe may be associated with PAHs, possible starting materials for the formation
of life. PAHs seem to have been formed as early as "a couple of billion years" after
the Big Bang, are widespread throughout the universe, and are associated with new stars
and exoplanets. See also
Chicken wire Graphene
References
External links ATSDR - Toxicity of Polycyclic Aromatic Hydrocarbons
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 National Pollutant Inventory: Polycyclic Aromatic
Hydrocarbon Fact Sheet Understanding Polycyclic Aromatic Hydrocarbons
NASA Spitzer Space Telescope Astrobiology magazine Aromatic World An interview
with Professor Pascale Ehrenfreund on PAH origin of life. Accessed June 2006
Oregon State University Superfund Research Center focused on new technologies and emerging
health risks of Polycyclic Aromatic Hydrocarbons Polycyclic Aromatic Hydrocarbons--EPA Fact
Sheet. U.S. Environmental Protection Agency, Office of Solid Waste, January 2008.