Polycyclic Aromatic Hydrocarbons and Neurological Disorders: From Exposure to Preventive Interventions

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental toxic chemicals which include more than 100 chemicals, mainly produced as a result of improper combustion of organic substances like wood, coal, petrol and oil. These pollutants are released into the environment due to various activities including open air burning, natural losses, leakage of various chemicals, accidental fire and many more. The most common sources of PAHs production are house hold heating systems, plants using coal for gasification and liquefaction, various industries and factories manufacturing different livelihood products, petroleum refineries and automobile exhaust. PAHs metabolites (especially 1-OHP) in urine, and PAH–DNA adducts with DNA, RNA and proteins in WBCs and other tissues, which are part of PAH toxic mechanisms, are used to measure PAH exposure in humans. Despite of different biological barriers such as placenta and blood–brain barrier, brain still has unique susceptibility to environmental risk factors during its developmental period compared with the mature nervous system. Benzo(a)pyrene is one of the most toxic PAHs known which is often used as an indicator of PAH exposure in various epidemiological studies. PAHs combat and interfere with the functions of cellular membrane and its enzyme systems to cause cytotoxicity. Chronic exposure to even low concentration of these chemicals cause ever-lasting damages including infertility, cancer and neurotoxicity to humans as well as wild life. Epigenetic effects, oxidative stress and endocrine disruptions are some of the mechanisms investigated for PAHs neurotoxicity. As brain is the most vulnerable organ to oxidative damage due to low oxygen level which may cause alteration in gene expression, impairment in cellular signaling, membrane integrity disruption, altered neurotransmission and ultimately neuronal cell death. The oxidative stress (due to ROS) produced in the CNS causes reduction in the antioxidant enzymes activities which are crucial for the behavioral effects induced by PAHs especially B(a)P. Adsorption, volatilization, photolysis and chemical degradation are some of the important processes in the removal of PAHs from both the atmosphere and environment. Among these, microbial degradation is considered to be the major PAHs degradation process.