As well as the health risks associated with these pesticides, there’s growing evidence that they may not be as effective as the manufacturers claim. Like any organism, head lice can develop resistance to toxins.

As the strength of the toxins begin to fade (some days after the last treatment), any lice which manage to survive a small dose – or have hatched soon after the last treatment – can pass on resistance to following generations.

One of the first studies anywhere in the world which showed insects could become resistant to insecticides concerned head lice.

For many years DDT had been used in the US to combat head lice. But it was banned in 1949 when it was found that lice had developed an immunity to the toxin.

In January 2002 The US Journal of Dermatology carried an extensive report into increased levels of head lice resistance.

Its conclusion reads: “The data suggests head lice resistance is present in many parts of  US to over-the-counter products containing synthetic insecticides.”

It also added: “Resistance is starting to develop to carbaryl – and extensive use of this product would lead to significant resistance.”

Carbaryl is the most powerful head lice insecticide currently prescribed in the US. The next most potent head lice toxin is melathion. And reports are now beginning to surface showing the same pattern of resistance to this chemical.