Polycyclic aromatic hydrocarbons (PAHs)


Structure

Polycyclic aromatic hydrocarbons (also known as polynuclear aromatic hydrocarbons) are composed of two or more aromatic (benzene) rings which are fused together when a pair of carbon atoms is shared between them (Figure 1). The resulting structure is a molecule where all carbon and hydrogen atoms lie in one plane. Naphthalene (C10H8; MW = 128.16 g), formed from two benzene rings fused together, has the lowest molecular weight of all PAHs. The environmentally significant PAHs are those molecules which contain two (e.g., naphthalene) to seven benzene rings (e.g., coronene with a chemical formula C24H12; MW = 300.36 g). In this range, there are a large number of PAHs which differ in number of aromatic rings, position at which aromatic rings are fused to one another, and number, chemistry, and position of substituents on the basic ring system.

Physicochemical properties
Physical and chemical characteristics of PAHs vary with molecular weight (Table 1). For instance, PAH resistance to oxidation, reduction, and vapourization increases with increasing molecular weight, whereas the aqueous solubility of these compounds decreases. As a result, PAHs differ in their behaviour, distribution in the environment, and their effects on biological systems. PAHs can be divided into two groups based on their physical, chemical, and biological characteristics. The lower molecular weight PAHs (e.g., 2 to 3 ring group of PAHs such as naphthalenes, fluorenes, phenanthrenes, and anthracenes) have significant acute toxicity to aquatic organisms, whereas the high molecular weight PAHs, 4 to 7 ring (from chrysenes to coronenes) do not. However, several members of the high molecular weight PAHs have been known to be carcinogenic.



Occurrence and pollution
Because many PAHs are neither soluble in water nor volatile, PAHs in the environment are found primarily in soil, sediment, oily substances, and 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 (RCO), 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 incompletecombustion 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.

Human health
The toxicity of PAHs is structure-dependent. Isomers (PAHs with the same formula and number of rings) 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 (and is one of many carcinogens found in cigarette smoke).
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 (C20H14), indeno(1,2,3-cd)pyrene (C22H12), and ovalene.
High prenatal exposure to PAH is associated with lower IQ and childhood asthma.[24] 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.

References
http://www.env.gov.bc.ca/wat/wq/BCguidelines/pahs/pahs-01.htm

Recent Publications
Atmospheric polycyclic aromatic hydrocarbons in Mumbai, India.
Indoor Polycyclic Aromatic Hydrocarbon Concentration in Central India.
Assessing hazardous risks of indoor airborne polycyclic aromatic hydrocarbons in the kitchen and its association with lung functions and urinary PAH metabolites in kitchen workers.
Exposure to multiple sources of polycyclic aromatic hydrocarbons and breast cancer incidence.
Dietary intake polycyclic aromatic hydrocarbons (PAHs) and associated cancer risk in a cohort of Chinese urban adults:Inter- and intra-individual variability.