DDT


The Most Famous (and Infamous) Insecticide

DDT is the abbreviation for

The Malaria-buster

DDT first came into the public's attention in WW2 in 1943, when it was used by the US Army in Naples to stop a typhus epidemic. This is quite remarkable, when you realise that typhus killed 2.5 million Russians during WW2, and the use of this chemical had stopped it cold! It was then discovered that DDT could kill the mosquitoes which spread malaria, and DDT was then sprayed all over the world in malarial areas - at one time consuming 60,000 tons of DDT every year. This eradication program was remarkably successful - in India alone the number of deaths dropped from 500,000 in 1960 to only 1,000 by the 1970s. Worldwide, the number of malaria deaths has been reduced from 6,000,000 per year to less than 2,000,000 per year, entirely due to the success of DDT and other insecticides.

Concentration Problems

By the mid 1960s, however, concern was being expressed that DDT might not be as harmless as first believed. Attention focused on the plight of wildlife, particularly birds, in areas where DDT had been used. These birds were producing eggs with very thin shells which broke before hatching was due, and deaths among sparrowhawks and falcons increased rapidly. Investigation showed that some of these birds had extremely high concentration of DDT in their bodies, up to several thousand ppm!

This was a result of two of the properties of the DDT molecule. First it was virtually insoluble in water, but very soluble in organic solvents and fats. Secondly, it is very stable, which is ironic since this stability was one of the advantages DDT had over other pesticides since it meant that the chemical did not have to be frequently reapplied. In the environment, therefore, DDT is slowly washed into rivers and streams, where it then enters the foodchain. It passes up the foodchain, concentrating in the fatty tissues as it does so. At every step up the foodchain, the relative concentration increases (see table).

Figures from the Lake Michigan area of the USA, an important fruit-growing area where DDT was sprayed extensively in the 1960s.
DDT concentration
(ppm of body weight)
Lake Water0.00002
Muds at the bottom of lakes0.014
Amphipods0.410
Trout (and other fish)6
Herring Gulls99
Peregrine Falcons5,000

DDT has no appreciable effect at concentrations below those seen in fish, but by the time it gets to the birds, particularly the falcons, the concentrations are now so high that even the relatively safe DDT molecule becomes toxic. This leads to serious health problems or even death. Because of these problems, and also the fact that DDT-resistant evolved, DDT was banned in the USA in 1973, and has since been banned in many other countries throughout the world. Alternative insecticides, made from organophosphates, are now used instead, but these cost up to 3 times as much as DDT.

A Balanced Viewpoint

It is important to remain objective on issues like these, since any emotional judgement of the environmental problems that DDT caused must be balanced by consideration of the great benefits which it brought to mankind, and the millions of lives it saved in the malaria eradication campaigns of the 1950s and 1960s. On balance, DDT has been of immense benefit to the world, however it still serves as a powerful reminder that any chemical introduced into the environment on a large scale should first be studied for any potential problems which may result.

References: