Comment: DDT use down but not out



Posted on 01 August 1998  | 
Gland, Switzerland: I have often wondered just what it is about the human mind that causes it to cling doggedly to dangerous and destructive practices long after less harmful alternatives have become available. On the one hand we are an endlessly creative and inventive species, yet on the other we seem often reluctant fully to exploit the fruits of our invention preferring to continue old ways even though we are well aware of the damage they do.

Of course, money is often at the root of the problem. Change frequently involves cost and the benefits of that short-term expenditure are not always immediately obvious. Moreover, it is often seen to be a case of 'better the devil you know', since new inventions or techniques are likely to need time to establish themselves as an improvement on the tried and tested methods.

Yet such cautious, short-term and short-sighted attitudes ignore the fact that human knowledge is increasing all the time, often on the basis of past mistakes. It is said that during the last hundred years we have learned more than in all the rest of our history put together. We are more capable than ever before of forecasting the likely impact of what we invent.

A case in point is the insecticide DDT (Dichloro-diphenyl-trichloroethane), about which I have written before. For decades this highly toxic chemical was sprayed indiscriminately both for pest control in agriculture an in the battle against mosquitoes and other insects that spread terrible tropical diseases such as malaria, sleeping sickness and river blindness. It certainly seemed to work, and in 1955 the World Health Organization endorsed it as the main agent in a drive to eliminate malaria from the face of the earth.

But early success was not sustained, so that today some 2.5 billion people in more than 90 countries still live with the risk of this nasty disease. Each year up to 500 million clinical cases of malaria are reported, causing some three million deaths more than half among children under the age of five, who die at the rate of approximately 5,000 a day, or four every minute.

These are desperate statistics and we might think that if DDT can cause a significant dent in them, then it is worth persisting with it. However, by the early 1960s it was becoming clear that DDT had alarming side-effects, often far from where it had been used. It has a stable chemical structure that makes it practically insoluble in water and very resistant to biodegradation in soil: in one case, 40 per cent of the DDT sprayed on an American orchard was found to be still present 20 years later.

As a recent WWF report Resolving the DDT Dilemma pointed out, DDT's long life or persistence, once hailed as its key benefit as an insecticide, has led to contamination of every corner of the globe and virtually every living animal and human.

What does this contamination mean? Well, DDT is known to kill fish and invertebrates and cause serious harm to birds. In mammals it can cause cardiac and respiratory failure, brain and nerve damage leading to death. It is suspected of causing cancer in humans and of interfering with fertility. But many of its effects on organisms are more subtle and chronic rather than acute. It can damage the immune system and, in birds, it has been linked to sexual changes. The fact is, with such a long-lasting chemical, no one knows precisely what the future effects might be or even whether they will actually be noticed, since causal links can be difficult to determine.

Banned virtually throughout the world from agricultural use, DDT is now only employed in the continuing fight against tropical disease, with about 30,000 tonnes produced each year. But, vital though the task is, WWF is still calling for a complete ban on the use of DDT by 2007. The evidence against it is overwhelming and more sophisticated, as well as safer alternatives are now available. The World Health Organization has abandoned its malaria eradication drive in favour of a localised system of control and scientific opinion is that, instead of spraying with DDT, the best means is what is known as integrated vector management, or IVM. This technique tackles the sources of infection (that means the mosquitoes and other insects) by a variety of methods that rely less on insecticides than on biology and human ingenuity.

Insect habitats can be permanently or temporarily removed; predators can be introduced; sterilised insects can be released into populations to diminish reproductive capacity. Instead of having homes sprayed with DDT, people can be encouraged to fit insect screens, wear protective clothing, and use mosquito nets. Biological insecticides can be used and (as a last resort) chemicals less toxic than DDT can be employed in carefully limited areas. Apart from its environmental soundness, IVM is an option that can be managed by local communities, as an element in a general public health programme. So while there may be extra start-up costs to provide local funding, the techniques will rapidly become integrated into community life and the continuing cost will be much less than that of the regular purchase and use of DDT. And since pesticide resistance in insects is a factor in the persistent menace of malaria, it is likely that IVM will actually be more effective in controlling the disease.

So why are we still using 30,000 tonnes of DDT a year? Why is IVM not being widely employed against diseases that take such a toll of human life? As I indicated earlier, it is one thing to establish beyond doubt the dangers of some long established and familiar practice, but quite another to persuade interested parties that it should be replaced by a better one.

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