Cigarette smoke has been shown to contain numerous
The effect of environmental pollution can best be illustrated by describing the phenomenon of bioconcentration and its impact on the food chain. Widely used insecticides, which metabolically degrade very slowly and are highly soluble in fat, are a case in point. After being applied on land for agricultural uses, such compounds are washed by rains into streams, rivers, and lakes, where they are ingested by microscopic life forms, which serve as food for fish that are, in turn, the major food source for larger fish and aquatic birds. Widely used insecticides, which metabolically degrade very slowly and are highly soluble in fat, are a case in point. After being applied on land for agricultural uses, such compounds are washed by rains into streams, rivers, and lakes, where they are ingested by microscopic life forms, which serve as food for fish that are, in turn, the major food source for larger fish and aquatic birds. Since these compounds are soluble in fat, after being ingested by an animal they are stored and concentrated in the animal's body fat. Repeated feeding eventually leads to high concentrations of the compounds in the animal's body, so that its subsequent ingestion by a larger predator, perhaps by humans, presents the predator with a significant level of the compound. This can pose a health hazard so serious that, for example, fishermen on a contaminated lake are warned not to eat the fish they catch. The magnitude of the problem is illustrated by the insecticide DDT, high levels of which persist in the environment and in the bodies of humans, despite the fact that its use was restricted worldwide by the early 1970s. Food additives are another source of environmental chemicals that has caused concern. Although these have been the object of dispute and have given rise to the 'natural food' fad, there is no evidence that food additives cause human cancer. In fact, some food additives, especially those that protect foods from becoming rancid, have actually been shown to prevent chemically induced cancer in experimental animals. An additive that remains a matter of concern, however, is sodium nitrite, which is widely used to preserve processed meats. It has been shown that nitrite can react in the stomach with amines, which arise from the digestion of meat, to form ratrosamines, a group of compounds that are potent carcinogens for certain laboratory animals. These compounds are formed in such minute amounts in the stomach that some researchers doubt that they pose a significant carcinogenic hazard for humans. Similarly, the demonstration that certain compounds formed by the burning of meat are carcinogenic for animals must be placed in proper perspective. Carcinogenesis experiments in animals usually require continuous exposure to high levels of chemicals to obtain statistically valid results in their relatively short life span of a year or two; extrapolacals of these results to the effect on human health should be approached most carefully.