In July 1998, representatives from 120 nations met to discuss a United Nations treaty to ban the insecticide DDT and 11 other chemicals worldwide by the year 2000 (1). The U.S. and other industrialized nations favor the ban because these persistent organic chemicals, as they are called, accumulate in the environment and can harm wildlife populations and enter our food chain. But many nations oppose the ban of DDT worldwide because it is used to control the mosquitoes that spread malaria. Malaria occurs in 90 countries throughout the world, and is a major cause of death, especially in equatorial Africa. The World Health Organization estimates that some 2.5 million people die yearly from malaria and that it is making a comeback in many countries (2). But because DDT is so effective in controlling malaria-carrying mosquitoes, many experts think that the insecticide may have saved more lives than any other chemical (3). DDT first went into large-scale production in 1939, after chemist Paul Herman Moller discovered that low dosages could kill all kinds of insects by interfering with their nervous systems. At the time, DDT was considered a cheap and safe alternative to insecticides based on arsenic or mercury compounds (3). Unfortunately, no one anticipated the damage widespread use of DDT would do to the environment.

As a persistent organic compound, DDT does not easily break down, or degrade into simpler substances. Once it enters the food chain, it has a half-life of up to eight years, meaning it takes eight years for half of the DDT to break down. When ingested by animals, it accumulates in fat tissues and in the liver. Because the concentration of DDT increases as it moves up the food chain, predatory animals suffered the greatest harm. Populations of bald eagles and peregrine falcons plummeted because DDT caused the bird to lay thin-shelled eggs that couldnú’ withstand incubation. Sea lions off the coast of California miscarried fetuses after eating contaminated fish (4).As you can see in the diagram, DDT (dichlorodiphenyltrichloroethane) is a chlorinated hydrocarbon. Each of the hexagons in the structure is a phenyl group (C6H5-) that has a chlorine (Cl) atom replacing one of the hydrogens. However, a slight change in molecular structure may make chlorinated hydrocarbons chemically active. By manipulating DDT molecules in this way, chemists hope to develop more effective insecticides that break down rapidly and therefore do not harm the environment. At the same time, researchers are investigating other ways to control mosquito populations. One is the use of a hormonelike compound that causes mosquitoes to starve to death, reducing their population and the resulting spread of malaria.

As a persistent organic compound, DDT does not easily break down, or degrade into simpler substances. Once it enters the food chain, it has a half-life of up to eight years, meaning it takes eight years for half of the DDT to break down. When ingested by animals, it accumulates in fat tissues and in the liver. Because the concentration of DDT increases as it moves up the food chain, predatory animals suffered the greatest harm. Populations of bald eagles and peregrine falcons plummeted because DDT caused the bird to lay thin-shelled eggs that couldnú’ withstand incubation. Sea lions off the coast of California miscarried fetuses after eating contaminated fish (4).

As you can see in the diagram, DDT (dichlorodiphenyltrichloroethane) is a chlorinated hydrocarbon. Each of the hexagons in the structure is a phenyl group (C₆H₅^-) that has a chlorine (Cl) atom replacing one of the hydrogens. However, a slight change in molecular structure may make chlorinated hydrocarbons chemically active.

By manipulating DDT molecules in this way, chemists hope to develop more effective insecticides that break down rapidly and therefore do not harm the environment. At the same time, researchers are investigating other ways to control mosquito populations. One is the use of a hormonelike compound that causes mosquitoes to starve to death, reducing their population and the resulting spread of malaria.

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