Organic Chemical Environmental Pollutants - By Atir Naeem Qurashi

Polycyclic aromatic hydrocarbons (PAHs) as Organic Chemical Pollutants 

This is a public awareness blog about hazardous chemical pollutants spread around us.

INTRODUCTION

•    PAHs 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.
•      In the two ring cases where biphenyl and naphthalene are simple examples; beginning at three rings, examples include anthracene and phenanthrene.
•     PAHs are neutral, nonpolar molecules. Critically, as noted, PAHs have been identified as carcinogenic and mutagenic (as well as teratogenic), 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.

OCCURRENCE 

•       They are found in fossil fuels (oil and coal) and in tar deposits, and are produced, generally, when insufficient oxygen or other factors result in incomplete combustion of organic matter (e.g., in engines and incinerators, when biomass burns in forest fires, etc.).
•       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 (see image below) whose metabolites are mutagenic and highly carcinogenic.

                                                                     Benzo-a-pyrene 

•         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.
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               Two extremely bright stars illuminate a mist of PAHs in this Spitzer image
  
  

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 e.g., biphenyl and naphthalene 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. 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 following are PAH examples with larger numbers of rings.

The bulk of research on PAHs has been on small PAHs, due to their availability. Aqueous solubility of PAHs decreases approximately one order of magnitude for each additional ring.

Structures of few PAHs are shown in the image bellow.

PAHs as pollutants

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.

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.

Health Hazards To Fishes 

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.[9][10][11] 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.

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).

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.

Polycyclic aromatic hydrocarbons affect organisms through various toxic actions. The mechanism of toxicity is considered to be interference with function of cellular membranes as well as with enzyme systems which are associated with the membrane. They have been shown to cause carcinogenic and mutagenic effects and are potent immunosuppressants.

PAHs known for their carcinogenic, mutagenic, and teratogenic properties

EPA has classified following PAH compounds as probable human carcinogens:

1. benz[a]anthracene
2. chrysene
3. benzo[b]fluoranthene
4. benzo[j]fluoranthene
5. benzo[k]fluoranthene
6. benzo[a]pyrene
7. benzo[ghi]perylene
8. coronene
9. dibenz(a,h)
10. anthracene (C20H14)
11. indeno(1,2,3-cd)
12. pyrene (C22H12)
13. ovalene

 Detection of PAHs

                                         gas chromatograph mass spectrometer systems

Detection of PAHs in materials is often done using gas chromatography-mass spectrometry (GC-MS) or liquid chromatography with ultraviolet-visible or fluorescence spectroscopic methods or by using rapid test PAH indicator strips.