The first defence of any chemical company being sued for damages is that its product is safe if used in the recommended way. The recommended doses are usually determined by laboratory tests of single products on animals, rather than on humans working and living in the chemical cocktail that is the real world.

This distinction is crucial, as new work from Dr. Goran Jamal showed. He cited research by Mohammed Abou-Donia, professor of neurobiology and neurotoxicology at Duke University in North Carolina (US). Dr. Abou-Donia established the safe levels of three different chemicals for his research subjects (battery hens). He also established the lethal dose for one of the chemicals, the organophosphate pesticide chlorpyrifos (manufactured as Dursban by Dow Chemicals). When the hens were given safe levels of each chemical separately, no ill effects occurred. When the three chemicals were given in combination, harmful effects equivalent to the lethal effect of chlorpyrifos occurred. The toxicity of the combination had increased to the equivalent of several hundred times the dose of chlorpyrifos given.

The three chemicals used were typical of combinations found commonly in the outside world: an organophosphate pesticide, a synthetic pyrethroid pesticide and an organochlorine (OP) pesticide. Such combinations are frequently used by livestock farmers, and were used by UK and US troops during the Gulf War. Dr Jamal explained that such a huge increase in toxicity occured because some chemicals work by binding to and blocking the action of protective enzymes, thus leaving the body undefended against the other chemicals present.

This effect was also shown by Israeli scientists in 1998. They showed how a combination of chemicals undermined the effectiveness of animals' blood-brain barriers, permitting 100-fold higher levels of toxic substances into the central nervous system. It has also been shown that skin exposed to a combination becomes increasingly sensitive.