DDT: Careful research and calculations vital

And because we are part of it, and do not want to rot in our own waste, we need to be aware of the effects of each activity, be able to assess the risks and advantages of that activity and work out ways that maximise advantages and minimise risks.

Total bans on some activities may be necessary, but generally far fewer are needed than First World armchair environmentalists, sitting comfortably in houses heated with electricity derived from a nuclear power station, might lead us to believe.

The recommendation to use the insecticide DDT in certain forms of malaria control is a good example. There is always the knee-jerk reaction that DDT is totally evil.

In fact, when this insecticide first became available it was regarded as a safe and extremely useful product. Compared to the highly polluting but rather inefficient chemicals that it replaced, this was a good assessment. DDT has minimal effects on vertebrates. A human can eat it by the teaspoon without anything dramatic happening.

Those fighting malaria and agricultural pests were especially excited with DDT. Whole states were just about bathed in the stuff. Considering that previous malaria control had largely consisted of pouring oil on lakes this was a considerable advance in minimising environmental damage.

It was then found that DDT, in high concentrations in birds, caused them to lay eggs with very thin shells. There was a very real risk that predatory birds at the top of a food chain, and especially fish-eating birds, could be wiped out. So, in a very few years, DDT was banned.

But this does not mean that it must never be used. It is still a very safe, very effective broad-spectrum insecticide. It is also a persistent insecticide, with a half life of 11 years; that is every 11 years half of what is in the environment has broken down to safe by-products. So, because of the risks and because of the persistence, it needs to be controlled and its use monitored.

There are two areas where DDT will probably have to be used for many decades. The first is to control, in a one-off treatment, sudden plagues of certain types of insect. The one-off use of DDT over a smallish area will cause far less environmental damage than allowing the plague to spread and the use of far larger quantities of alternative insecticides as a result.

The second is in malaria control, but only in specific circumstances. Bathing a county or province in the stuff has to remain banned. The risks far outweigh the advantages. But spraying houses and huts seems to pose insignificant risks and gives many advantages, such as saving the lives of tens of thousands of children a year.

For a start not much DDT is used in spraying. Secondly almost all that is washed off the walls – and careful spraying will ensure that most sinks into the wall – ends up in the ground outside a house and spreads no further. So environmental contamination is minimal.

The persistence of the insecticide is an advantage in this case, since spraying does not have to be repeated very often.

The recommendation to use DDT carefully, very selectively on house walls, and by trained spraying staff is thus one of those sensible suggestions made by people who can calculate the low risks and can see the very high advantages.

We do this calculation in many other areas. Millions of motor cars are driven in the world every day. It was found that certain pollutants emitted in small quantities by each car could, when enough were concentrated in a small area, cause havoc. So lead was banned as an additive and catalytic converters were fitted, by law, in most industrialised countries.

Then it was found that greenhouse gases were a problem. So car manufacturers are now being forced to rapidly upgrade efficiency of engines.

Nuclear energy has many advantages, but no one is allowed to dump spent fuel rods or other waste on the local tip.

We have to stop looking at every environmental issue emotionally. What is needed is careful research followed by a careful calculation of risk versus advantage. That is what the use of DDT on house walls is all about.

September 2006
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