Persistent organic pollutants (POPs) are ''chemical substances that persist in the environment, bioaccumulate through the food web, and pose a risk of causing adverse effects to human health and the environment'' (United Nations Environment Programme, UNEP).
As with other chemicals, these compounds can be harmful, even if environmental contamination levels are low. Furthermore, prolonged exposure to the toxic substance increases the risk of damage.
POPs are, by definition, persistent, and this property is generally correlated to their chemical stability. In contrast to highly reactive substances which are relatively short-lived, persistent pollutants are highly resistant to biological, photolytic, and chemical degradation. This characteristic makes POPs persist in the environment for an extended period of time (i.e., with half-lives (ti/2) greater than 6 months), thus representing a risk of a longtime exposure. The persistence of a chemical depends on the difficulty with which it is broken down and degraded into other less hazardous substances. This is connected with some structural characteristics of the molecule, such as the presence of aromatic systems and the substitution of one or more hydrogen atoms in the aromatic structure by one or more halogens. The carbon-halogen bond is in fact very stable and resistant to hydrolysis, and the greater the number of halogen atoms, the greater is the resistance to biological and photolytic degradation. From a technical point of view, the stability obtained by halogenation is a desirable characteristic. The fire resistance of poly-chlorinated biphenyls (PCBs), for instance, is increased by chlorination, while in the case of 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT) it ensures longer-lasting effects on insect pests. On the other hand, the more stable the substances are the longer they resist degradation and the longer they remain in the environment. Furthermore, POPs can be found in every part of the world, even in areas where human activities are almost completely absent, for example, in the Antarctic and the Arctic. In fact, despite the fact that these molecules are present preferentially in soils, sediment, and living organisms, under normal conditions they can evaporate. In particular, pollutants with relatively low volatility do not remain long in the atmosphere, but are adsorbed onto the surface of airborne particles, which return them to the ground. However, more volatile POPs stay in the atmosphere for longer, even weeks, before settling back down to the Earth's surface. This means that winds can carry them thousands of kilometers away from their sources, to be found in similar concentration all over the world. In some cases, the concentrations of POPs in waters and sediments of Arctic and Antarctic can be higher than elsewhere. This phenomenon seems to be connected with the decrease of substance volatility with decreasing temperature, where pollutants that vaporize at warm latitudes are carried by winds to polar regions where they condense and concentrate. POPs are also characterized by their ability to bioaccumulate. The extent by which they bioac-cumulate depends on the solubility of the substance in lipids. Highly lipophilic substances are substantially insoluble in water, as commonly shown by the high values of »-octanol-water partition coefficient (Kow). Thus, in the aquatic enviroment, they display strong affinity for suspended particles, sediments, and living organism, where they reach concentrations many times higher than in the water itself. Halogenated organic compounds are less soluble in water and more soluble in lipids respect to their corresponding nonhalogenated compounds. This strengthens the tendency of these substances to be concentrated in the fatty tissue of living organism. PCB concentrations in fish are, for example, tens to hundreds of thousand times higher than in the water in which the fish live. Usually, the ability of halogenated organic compounds to bioaccumulate increases with the degree of halogenation, although in some cases, molecules with maximum degree of halogenation show a smaller tendency to bioaccumulate, due to their greater difficulty of passing through the cell membranes of living organisms.
As a result of bioaccumulation, some organic persistent substances are subject to biomagnification process, so these molecules are found at higher concentrations in animals at the highest levels of the food chain. This is connected with the fact that predatory animals eat hundreds of time their own weight in their prey and thus concentrations of persistent chemicals are far higher in the predators than in their prey.
In conclusion, the conventional POPs are highly chlorinated organic compounds with a molecular weight of 200-500 Da and a vapor pressure lower than 1000 Pa. Table 1 lists the most significant chemical-physical properties of the 12 POPs considered to be of toxicological interest to the scientific community. It includes orga-nochloride insecticides, such as dieldrin, DDT, toxaphene, and chlordane, and several industrial products or byproducts such as PCBs, polychlorinated dibenzo-/>-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs).
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