The name of this insecticide is to many people synonymous with environmental pollution, and it is therefore an important example in this chapter but the story is not simple. DDT is an abbreviation for the chemical name of the insecticide dichloro-diphenyl-trichloroethane. It contains five chlorine atoms and is hence called an organochlorine compound. It was first made in 1874 but not found to be an insecticide until 1939 by Paul Müller. It was used extensively in the Second World War for the control of insects such as lice and mosquitoes, carriers of the diseases typhus and malaria. It was very effective in controlling these pests, and the diseases they carried, and undoubtedly thousands of soldiers' lives were saved. Since then, the lives of millions of people throughout the world have also been saved by this insecticide both as a result of the reduction of these and other diseases and as a result of the improvement in crop yields which has reduced starvation. Indeed, in 1953 it was estimated that the use of DDT for malaria eradication had saved 50 million lives and averted more
than i billion human illnesses. In 1971 the World Health Organization has estimated that more than i billion people had been freed from the risk of malaria, in the previous twenty-five years, mostly as a result of DDT use.1 The soldiers who were dusted with DDT powder, wore underclothes treated with DDT, or sprayed or were contaminated with DDT did not suffer ill effects. Volunteers who were subjected to exposure to DDT reported no ill effects either. Indeed, some volunteers were induced to take half a milligram of DDT by mouth daily for over one year and yet showed no signs of toxic effects. In the UK there has been little evidence of harm to either humans or wildlife.
So why is DDT perceived as such a dangerous chemical? In the USA DDT was used extensively after the war for the control of various insect pests. It was used in some cases in much greater quantities than it had been previously, and possibly such quantities were unnecessary. For example, i lb of DDT per acre had been sufficient to eradicate the malarial mosquito from swamps, yet in the control of the bark beetles, which carry Dutch Elm disease, 25 lb per acre were used in the USA for spraying the trees in some cases. The disease was devastating the trees, and perhaps some overkill was understandable, but this use of DDT led to effects on wildlife, with birds being noticeably affected. For example, the population of the American robin plummeted and was even wiped out completely in some areas. The reason for this was that earthworms and caterpillars became contaminated and in turn contaminated the birds that fed on them. Fish were also found to be especially susceptible and died in some areas of DDT use. Before 1954 there had been little complaint about DDT, but such events started to attract comment. The reason for the concern and for the appearance of effects on wildlife was that DDT accumulates and is persistent in the environment (see below).
This meant that even the use of lower levels than the excess sometimes applied could, with time, lead to levels that were toxic or had particular effects on certain susceptible animal species such as predatory birds.
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