Persistent organic pollutants (POPs) are the organic compounds which can persist for longer time in the environment. They are toxic, persistent, and bioaccumulative in human and animal tissue and biomagnify in food chains and tend to long-range transport. They have long half-lives in soils, sediments, air or biota. The half lives generally depend on the given media and it can be of years or decades in soil, sediments and having several days in the atmosphere. POPs are resistant to environmental degradation through chemical, biological and photolytic processes and they show the potential significant impacts on human health and the environment. One of the major problems is their worldwide distribution in the environment, in food chains and even in humans.
POPs can enter the gas phase under environmental temperatures. Hence they may volatilize from water bodies, soils, vegetation and biota into the atmosphere. As they are resistant to the breakdown processes, they travel long distances before being re-deposited. As a result, the POPs can accumulate in an area far away from where they were emitted.
Persistent Organic Pollutants have low water solubility i.e. they are hydrophobic and lipophilic, semi-volatility and high molecular masses. POPs with lower molecular weight are less toxic, less persistent in the environment, and have more reversible effects than those with higher molecular masses. The important factor of their lipid solubility results in the ability to pass through biological phospholipid membranes and thus they can bioaccumulate in the fatty tissues of living organisms.
Earlier the twelve POPs were known as ‘dirty dozen’ by UN Environmental Programme Governing Council-UNEP which include aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, PCBs, polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans andtoxaphene. The groups of compounds under POPs are also classed as PBTs (Persistent Bioaccumulative and Toxic) or TOMPs (Toxic Organic Micro Pollutants)
Then, additional group of substances are covered by the United Nations/Economic Council for Europe Protocol on POPs (UNECE, 1998) (see lower part of Table 1) to distinguish between chemicals that are intentionally produced and chemicals that are formed as accidental by-products of various combustion processes. Amongst the important classes of POPS, compounds are from many families like chlorinated and brominated aromatics including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and-furans (PCDD/Fs), polybrominated diphenyl ethers (PBDEs) and different organochlorine pesticides and also the polycyclic aromatic hydrocarbons(PAHs) are included. (Farraret al., 2004). PAHs are only recognized as POPs under the Aarhus Protocol (UNECE, 1998). The intentionally produced POPs are chlorinated paraffins, hexachlorobutadiene, pentachlorobenzene, ”penta” and ”octa” BDEs, perfluorinatedoctyl sulfonamides and sulfonates (PFOS and related chemicals).
POPs can be classified as flyers, multi or single hoppers and swimmers (Lohmann et al., 2007). This is based on the modes of chemical transport behavior on a global scale according to their specific partitioning property combinations by (Wania., 2003). Most of the POPs like, chlorobenzenes and organochlorine pesticides (OCPs), polychlorinatedbiphenyls (PCBs), DDT can be classified as multi-hoppers (Wania, 2003, 2006). In contrast to these multi-media chemicals, the hexachlorocyclohexanes (HCHs) are in the ”swimmers” category. Till today, more attention was given to the multipleand single hoppers, whereas many ”emerging POPs” are polar compounds or ”swimmers” such as PFOS, brominated cyclohexanes. Hoppers are relatively easy to determine by gas chromatography (GC) analysis of air, soil, vegetation, and sediments samples and for polar and non-volatile compounds analysis is carried out with liquid chromatography, particularly for ionic chemicals (Lohmann et al., 2004). Table 1 Listed best available estimates of the global historical production or consumption of intentionally produced POPs, or annual global emissions in the case of PCDD/Fs, PAHs and HCB (Lohmann et al., 2007).
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