Chlorinated hydrocarbons

Extensive use of synthetic pesticides derived from chlorinated hydrocarbons in pest control has had disastrous environmental side effects. These organochlorine pesticides are highly persistent and resist biological degradation. Relatively insoluble in water, they cling to plant tissues and accumulate in soils, the bottom mud of streams and ponds, and the atmosphere. Once volatilized, the pesticides are distributed worldwide, contaminating wilderness areas far removed from agricultural regions, and even the Antarctic and Arctic zones.

Although these synthetic chemicals are not found in nature, they nevertheless enter the food chain. The pesticides are either taken in by plant eaters or absorbed directly through the skin by such aquatic organisms as fish and various invertebrates. The pesticide is further concentrated as it passes from herbivores (plant eaters) to carnivores (meat eaters), it becomes highly concentrated in the tissues of animals at the end of the food chain, such as the peregrine falcon, bald eagle, and osprey.

Because of the dangers of pesticides to wildlife and to humans, and because insects have acquired resistance to them, the use of halogenated hydrocarbons such as DDT is declining rapidly in the Western world, although rather large quantities are still used in developing countries. In the early 1980s, the halogenated pesticide EDB, or Ethylene Dibromide, also aroused great concern as a potential carcinogen, and was eventually banned.

Closely related to DDT is another group of compounds: the polychlorinated biphenyls, or PCBs. For years these compounds have been used in industrial production, and eventually they found their way into the environment. Their impact on humans and wildlife has been similar to that of pesticides. Because of their extreme toxicity, the use of PCBs is now restricted to insulators in electrical transformers and capacitors.

PCDD is the most toxic of another related group of highly toxic compounds, the dioxins, or polychlorinated dibenzo-para-dioxins. The extent of toxicity

Of these carcinogenic compounds in humans has not yet been proved. PCDD may be found as an impurity in wood and paper preservatives and in herbicides. Agent Orange, a defoliant widely used in jungle warfare, contains minute traces of dioxin.

Other toxic substances

Toxic substances are chemicals and mixtures of chemicals whose manufacturing, processing, distribution, use, and disposal present an unreasonable risk to human health and the environment. Most of these toxic substances are synthetic chemicals that enter the environment and persist there for long periods of time. Major concentrations of toxic substances occur in chemical dump sites. If they seep into soil and water, the chemicals can contaminate water supplies, air, crops, and domestic animals, and have been associated with human birth defects, miscarriages, and organic diseases. Despite known dangers, the problem is not lessening. In the recent decades more than 4 million new synthetic chemicals were manufactured, and new ones are being created at the rate of 500 to 1,000 each year.