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DDT — BOON OR BANE?

IN A MONTH'S time, the Stockholm Convention on Persistent Organic Pollutants (POP) comes into force. The Convention seeks to eliminate toxic chemicals that resist degradation and therefore linger in the environment for many years. The 12 chemicals covered by the Convention's initial list are readily absorbed and retained in the fatty tissue of fish, birds, and mammals (including humans). In the bodies of these animals, POPs can reach concentrations many thousand times greater than the background levels of those chemicals. Most of the 12 chemicals will be banned immediately. But one chemical is still needed in the fight against insects carrying deadly diseases. Chemists call it dichloro-diphenyl-trichloroethane but it is better known as DDT. For discovering its strong insecticidal action on contact, Paul Muller won the Nobel Prize for Physiology or Medicine in 1948. During the Second World War, the Allied forces turned to this chemical to halt a dangerous outbreak of typhus in Naples. In the post-war years, DDT was deployed to such effect that malaria was eradicated or greatly reduced in many countries around the world. In India, there were 75 million cases of malaria and 800,000 deaths when the National Malaria Control Programme was launched in 1953. The number of malaria cases fell to 100,000 in 1965-66, there has since been a resurgence of malaria.

Rachel Carson's book, Silent Spring, published in 1962 was largely instrumental in turning the tide of public opinion against DDT. It chronicled how the chemical entered the food chain and then steadily accumulated in the bodies of animals and humans. It recorded the chemical's toxic effects, especially on fish and birds. When the Environment Protection Agency was established in the United States in 1970, DDT was the first chemical banned. Now there are worries that the pendulum has swung too far against any use of this chemical and that public health in poor nations, where insect-borne diseases are endemic, could suffer as a result. DDT is cheap: its sprays are half the cost of alternative pesticides, according to a recent Indian estimate. Used as indoor sprays, as the World Health Organisation recommends, it can be effective, without being too harmful to people or the environment. In 1995, when South Africa switched from DDT to pyrethroids (a different class of insecticides), which seemed at least as effective in short-term trials, cases of malaria increased fourfold. DDT sprays were then reintroduced, with dramatic results. India produces it. However, The New York Times reported recently that African countries needing DDT could not get it because aid agencies refused to fund its purchase; the report also noted that companies selling more expensive insecticides have been lobbying strongly against it.

The Stockholm Convention commits nations to "reducing and ultimately eliminating the use of DDT." However, it would be foolish to do away with it completely unless there are alternative ways of controlling mosquitoes and other disease-carrying insects. But preventing the excessive and indiscriminate use of this chemical is vital. Although the Indian Government banned its use in agriculture some years ago, its illicit application for such purposes is known to continue. Alternatives to DDT, such as other pesticides, bed-nets soaked in insecticides, bio-insecticides, herbal formulations, and biological control methods (such as fish that eat mosquito larvae) have their advantages as well as limitations. Employing these methods effectively depends substantially on local conditions, such as the breeding and feeding habits of mosquitoes. The moral of the story is that while a top-down bureaucratic approach was followed, with considerable success but with risk to the environment, for DDT spraying, the challenge today is to encourage local level decision-making in choosing the most appropriate insect control techniques.

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