Ddt (Dichlorodiphenyl Trichloroethane)
DDT, dichlorodiphenyl trichloroethane, was synthesized in 1874, but its insecticidal properties were first identified in 1939 by P.H. Mueller. He received the Nobel Prize for his discovery, which coincided with the outbreak of World War II, when DDT was used extensively to keep soldiers free of head and body lice. DDT also proved very effective against mosquitoes, which transmit a serious global human disease, malaria, as well as yellow fever. After the war, DDT was developed extensively as an agricultural pesticide.
DDT has an extremely low volatility and may be the least soluble chemical known, which makes it extremely persistent in soils and aquatic sediments. It has relatively low acute mammalian toxicity and is toxic to a wide range of insects. It kills insects by affecting the transmission of nerve impulses, probably by influencing the delicate balance of sodium and potassium within the neuron.
More than four billion pounds of DDT have been used throughout the world since 1940. Production in the United States peaked in 1961 when 160 million pounds were manufactured. Large economic benefits have resulted from the control of many serious agricultural and forestry pests, including Colorado potato beetle, cotton boll weevil, and pests of fruit, vegetables, corn, and tobacco. In forestry, its greatest success occurred in combatingspruce budworm and gypsy moth. However, its major impact lay in the control of mosquitoes that transmit malaria, as well as body lice and fleas; many millions of lives have been saved through these uses.
A comparison of a normal Peregrine falcon eggshell and one thinned by exposure to the pesticide DDT. (©Galen Rowell/Corbis. Reproduced by permission.)DDT's potentially adverse environmental effects were brought to public attention by Rachel Carson in her book Silent Spring (1963). Carson emphasized the great persistence of DDT in soils and river sediments and focused on the bioconcentration of DDT through the trophic levels of food chains. One result of the bioaccumulation of DDT was the thinning of the eggshells of predatory birds such as bald eagles, peregrine falcons, golden eagles, hawks, and pelicans, resulting in embryonic death and decreasing populations of these species. DDT bioconcentrates because it has low water solubility and high fat solubility, that is, a high lipid-to-water partition coefficient (e.g., it can concentrate into fatty tissues from water). In the 1960s large DDT residues in human tissues and human milk began to be reported, probably from the consumption of food containing traces of DDT. DDT in body fat was reported to cause convulsions in laboratory rats; it also reached human fetuses by crossing the placenta. However, few serious effects on human health were officially recorded.
Many pests began to develop resistance to DDT, necessitating the progressive use of more of the pesticide to control such pests. In 1972 the use of DDT in the United States was banned on environmental grounds, including the widespread contamination of the environment with DDT, its ability to bioconcentrate, and its effects on endangered bird species.
Suitable alternatives to DDT were found in the United States and other industrialized countries that also banned its use in the 1970s. However, tropical developing countries that used inexpensive DDT extensively to control malaria and other pests faced a significant dilemma. Moreover, although the United States no longer used DDT, it continued to manufacture and export very large quantities to developing countries and how much DDT is still used. It is difficult to say with accuracy exactly which countries still use DDT. Some countries use it illegally, others only in small quantities. And information is often impossible to obtain because questionnaires from an organization like the World Health Organization (WHO) generally have only a 50 to 60 percent response rate. Nonetheless, it is known that poorer countries in Central and South America, Africa, and Asia, as well as the large nation of China, continue to utilize sizable quantities of DDT.
Internet Resource
"World Wildlife Federation DDT Report." Available from http://www.worldwildlife.org/toxics.
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