The organochlorines were the first group of synthetic insecticides and without them the dramatic decrease in malaria observed in the 1950s would have been impossible. The best known of this class is DDT (dichlorodiphenyltrichloroethane) but others include 2,4 DD, hexachlorbenzene, and lin-dane. Of these only lindane (7-hexachlorocyclohex-ane, see Table 1) is still in use in the developed world.

These compounds are very slow to break down in the environment and one result of this persistance was the decline in bird numbers graphically described by Rachel Carson in the book Silent Spring. The problem was that DDT was concentrated through the food chain and predator birds in particular were failing to raise chicks. Since the organochlorine pesticides and other sources of orga-nochlorines in the environment have been largely phased out, numbers of many species of birds are rising again. It is recognized that pesticides are still having an adverse influence on numbers of some birds that inhabit farmland. However, this is not a straightforward effect. In the case of the grey partridge, for example, it is because herbicides have reduced the number of weeds, which in turn has reduced the number of insects that feed on the weeds, resulting in fewer insects for the chicks to eat.

The mechanism of action of the organochlorines is not known in detail although they appear to act on the central nervous system. In humans the orga-nochlorine compounds tend to accumulate in the body fat and in mothers' milk. While there is no direct evidence that they cause mutations or cancers, there is concern that lindane may be a carcinogen and its role in breast cancer is still under review. However, in contrast, DDT and 7-HCH have both been shown to inhibit tumors in mice initiated by aflatoxin B1.

Although organochlorine pesticides have largely been phased out in Europe, analysis for them continues and low levels of lindane are still being detected in milk in the UK (typically at 0.005mgkg-1 compared with the maximum residue limit (see below) of 0.008mgkg-1 and an acceptable daily intake (see below) of 0.05 mg per kg body weight).

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