Fifty years ago this month Winston Churchill spoke of a new chemical, describing it as 'an excellent powder . . . which yields astonishing results, and which will be used on a great scale by the British forces in Burma'. The chemical eventually saved an estimated 50 million lives. In a war-torn world this was good news indeed, but until then the molecule had been a military secret, code-named G4. Earlier that year the Allies had used it to stop a typhus outbreak in Naples.
G4 was not a new wonder drug, but the insecticide DDT. The initials stand for dichloro-diphenyl-trichloro- ethane. The molecule had first been made as long ago as 1874, by a chemistry student, Othmer Zeidler. He had taken chloral (popularly known as 'knock-out drops' or Mickey Finn) and mixed it with chlorobenezene in sulphuric acid. The result was a white precipitate of DDT crystals. Zeidler reported his new molecule, and that was that.
DDT was rediscovered in 1939 at the Geigy company in Switzerland by Paul Hermann Muller, who was searching for new insecticides. He tested the powder and was amazed how it killed all kinds of insects at very low doses. It was soon in production and in the next 30 years more than 2 million tons of DDT were manufactured. Muller received the 1948 Nobel Prize in Medicine and Physiology.
His 'excellent powder' saved millions of lives by eradicating disease-bearing insects such as lice, which spread typhus, fleas which disseminate plague, and mosquitos which are the vectors of malaria and yellow fever.
DDT also destroyed crop pests such as the Colorado potato beetle, and it was much safer than the insecticides then in use which were based on the poisonous elements arsenic, lead and mercury. Today many people regard DDT as an equally dangerous poison.
However, in the Fifties its achievements were impressive. A campaign to eradicate malaria from Ceylon (Sri Lanka) began in 1948 when there were 2.5 million cases annually. Every home on the island was sprayed regularly with DDT and by 1962 there were only 31 reported cases.
Not everyone approved of chemical insecticides, and that same year Rachael Carson's anti-chemical text, Silent Spring, was published. She referred to DDT as the 'elixir of death'. The emergent environmentalist lobby wanted it banned, claiming it was killing wildlife, especially birds, causing cancer in humans, and building up in the environment because it was not biodegradable. Moreover, analytical chemists were able to detect this pesticide in tiny amounts and they had revealed that it was everywhere: in soil, in water, in our food, and even in human tissue.
A stronger reason for phasing it out was the appearance of DDT-resistant strains of insects. Today there are almost 500 species resistant to DDT - mute testimony to its over-use. It is still employed as an insecticide in some tropical countries such as India, although restricted to 10,000 tonnes per year. The United States banned DDT in 1972, as did Britain and many other developed countries. The spraying of homes in Ceylon had ceased in 1964, and within five years there were again 2.5 million cases of malaria on the island.
DDT kills an insect by interfering with its nerve cells. The molecule unlocks a channel through the cell membrane which allows sodium atoms to flow in unchecked, causing the nerve to trigger repeatedly until the insect dies of exhaustion. Animal nerve cells are not affected in this way. Insects which become resistant produce an enzyme that detoxifies DDT by removing a chlorine atom from the molecule.
In her recent book Toxic Terror, Elizabeth Whelan, president of the American Council on Science and Health, debates the pros and cons of DDT and questions the wisdom of banning this cheap and effective insecticide. She points out that DDT has saved more lives than any other chemical. She also challenges many of the misconceptions about it, and says that there is no evidence from human studies that it causes cancer.
The original report that DDT persisted in the environment was based on applying it to a plot of soil at 10 times the normal level, and then keeping the soil dry and in the dark. The DDT did not degrade. However, in normal soil DDT is digested by microbes and its activity persists for only about two weeks. They also deactivate it by removing a chlorine atom; the same thing happens in sea water, where 90 per cent of DDT disappears within a month.
Nevertheless DDT did accumulate in humans, and at the time it was banned the average person had about 7 parts per million in their body. This came from their diet, because most food in the Sixties contained about 0.2 parts per million of the insecticide. The DDT concentrated in fatty tissue and was excreted only slowly; the half-life of DDT in the body is 16 weeks (this is the time it takes for half of a given amount to be excreted).
Such levels never posed a threat to our health. The World Health Organisation's guidelines for a safe level of DDT intake is 255mg per year - about 10 times the amount that consumers were exposed to when DDT use was at its height in the late Sixties. We know from accidents and suicide attempts that people have drunk a glassful of insecticide fluid, containing about 4,000mg of DDT, without harmful effects. The fatal dose for a human is thought to be about an ounce (28,000mg) of the pure material.
Some insect species can tolerate even higher doses, such as the Brazilian bee Eufriesia purpurata which is found in the Amazon. Some bees were found to have more than 4 per cent of their weight as DDT - the equivalent of a human having 6lb of the insecticide in their body.
John Emsley's book 'The Consumer's Good Chemical Guide - A jargon-free guide to the chemicals of everyday life' will be published by WH Freeman in October.
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