Persistent organic pollutants (POPs) are those chemicals that are not materially broken down over a reasonable period of time, usually measured in decades or more. The POPs of most concern are those that build up in the environment or are bioaccumulated and/or biomagnified in the food chain. The realization and importance of persistent environmental chemicals was first identified in the early 1960s with the publication of Rachel Carson's seminal work, Silent Spring. Carson wrote of the buildup of pesticides in birds and hypothesized that this came from direct and indirect (food chain) exposure. The magnitude of effect from Carson's work can be appreciated when one considers the breadth of environmental health sciences today and the international environmental regulations that have been promulgated.
The chemical characteristics of POPs are relatively similar. Many are polyhalogenated aromatic hydrocarbons (PHAHs), or other polycyclic aromatic hydrocarbons (PAHs) that are very slowly metabolized or otherwise degraded. The chemicals are lipid soluble; hence they are stored in the fatty tissue of all animals, and they build up in the food chain. Some classic examples of POPs are the pesticides DDT, Dieldrin, Aldrin, Heptachlor, Mirex, and Kepone. Another group of POPs are the chlorodibenzodioxins, dibenzofurans, and some PCBs. The pesticides were widely used for several years but eventually discontinued for toxicological and ecological reasons. Because of their lipid solubility, the chlorinated compounds are retained and accumulated in the lipids of insects and other invertebrates that are part of the food chain of higher-order predators, and they can eventually end up in the diets of humans and feed animals. Several of these compounds can be sequestered in soil and sediment, such as the PCBs in the Hudson River bottom sediment, where they can exist for decades.
The health effects of these chemicals, as neat compounds, have been very well studied. However, low-dose, lifetime exposure studies are lacking. Many of the organochlorine pesticides cited above are carcinogenic, teratogenic, and neurotoxic. The dioxins and benzofurans are highly toxic and are extremely persistent in the human body as well as the environment. Several of the POPs, including DDT and its metabolites, PCBs, dioxins, and some chlorobenzene, can be detected in human body fat and serum years after any known exposures. Lindane (hexachlorocyclohexane), which was used for the treatment of body lice and as a broad-spectrum insecticide, resulted in very high tissue levels, and in many cases caused acute deaths when used improperly. Lindane and some of its isomers have been identified in market-basket surveys and in human fat samples.
International efforts to minimize exposure to these compounds include the banning of their use except in emergency situations where it has been determined that no other chemical is efficacious. With the exception of DDT, few, if any, of these compounds have been authorized for use. PCBs, which were widely used in capacitors, transformers, and lubricating oils, have not been manufactured for several decades but linger in the environment. Chlorinated dibenzodioxins and dibenzofurans were never products per se, but are byproducts of products made from chlorophenols. The processes by which these final products are manufactured have been altered to minimize the unwanted dioxins. The other source of dioxins is the chlorine bleaching of paper pulp. This bleaching process has been altered to eliminate chlorine, and thereby to eliminate the possibility of dioxins. Several combustion processes also result in the formation of dioxins and benzofurans. Municipal and chemical waste incinerators can be sources of these unwanted by-products. Engineering controls have been put in place in modern facilities to minimize production. However, older and less controlled processes may continue to contaminate the environment.
Polyaromatic hydrocarbons (PAHs) are found in petroleum and petroleum derivatives. The PAHs are also found in the environment as by-products of coal gasification plants. These compounds, though usually less toxic than their chlorinated cousins, are irritants and some are carcinogenic in skin-painting studies in rodents. These compounds break down very slowly and are contaminants in soils of urban and suburban communities. PAHs will bioaccumulate and are found in fat samples of feral animals and humans.
As a broad class, the POPs are inducers of the cytochromes P450 (the so-called drug and chemical metabolizing enzymes), and in many cases the chemicals are carcinogenic. The approach being taken is to identify contaminated sites, isolate the site, remove the contaminated soil, and if possible destroy the contaminants by combustion or other means.
(SEE ALSO: Brownfields; Carson, Rachel; Dioxins; Environmental Movement; Environmental Protection Agency; PCBs; Pollution; Toxicology)
— MICHAEL GALLO
