Chlorinated polycyclic aromatic hydrocarbon

Chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) are a group of compounds comprising polycyclic aromatic hydrocarbons with two or more aromatic rings and one or more chlorine atoms attached to the ring system. Cl-PAHs can be divided into two groups: chloro-substituted PAHs, which have one or more hydrogen atoms substituted by a chlorine atom, and chloro-added Cl-PAHs, which have two or more chlorine atoms added to the molecule. They are products of incomplete combustion of organic materials. They have many congeners, and the occurrences and toxicities of the congeners differ. Cl-PAHs are hydrophobic compounds and their persistence within ecosystems is due to their low water solubility. They are structurally similar to other halogenated hydrocarbons such as polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs). Cl-PAHs in the environment are strongly susceptible to the effects of gas/particle partitioning, seasonal sources, and climatic conditions.

Toxicity
Some Cl-PAHs have structural similarities to dioxins, they are suspected of having similar toxicities. These types of compounds are known to be carcinogenic, mutagenic, and teratogenic. Toxicological studies have shown that some Cl-PAHs possess greater mutagenicity, aryl-hydorcarbon receptor activity, and dioxin-like toxicity than the corresponding parent PAHs.

The relative potency of three ring Cl-PAHs was found to increase with increasing degree of chlorination as well as with increasing degree of chlorination. However, the relative potencies of the most toxic Cl-PAHs assessed up to now have been found to be 100,000-fold lower than the relative potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Even though Cl-PAHs aren’t as toxic as TCDD, it has been determined using recombinant bacterial cells that the toxicities of exposure to Cl-PAHs based on AhR activity were approximately 30-50 times higher than that of dioxins. Cl-PAHs demonstrate a high enough toxicity to be a potential health risk to human populations that come into contact with them.

DNA interaction
One of the well-established mechanisms by which chlorinated polycyclic aromatic hydrocarbons can exert their toxic effects is via the function of the aryl hydrocarbon receptor (AhR). The AhR-mediated activities of Cl-PAHs have been determined by using yeast assay systems. Aryl Hydrocarbon Receptor (AhR) is a cytosolic, ligand-activated transcription receptor. Cl-PAHs have the ability to bind to and activate the AhR. The biological pathway involves translocation of the activated AhR to the nucleus. In the nucleus, the AhR binds with the AhR nuclear translator protein to form a heterodimer. This process leads to transcriptional modulation of genes, causing adverse changes in cellular processes and function.

Several Cl-PAHs have been determined to be AhR-active. One such Cl-PAH, 6-chlorochrysene, has been shown to have a high affinity for the Ah receptor and to be a potent AHH inducer. Therefore, Cl-PAHs may be toxic to humans, and it is important to better understand their behavior in the environment.

Several Cl-PAHs have also been found to exhibit mutagenic activity toward Salmonella typhimurium in the Ames assay.