There is growing acceptance of the idea that chemicals in the environment may combine to cause damage far greater than the sum of their parts. A year ago US scientists realised that their toxicity may be further strngthened by another environmental factor - ultraviolet light from the sun.

John Battle of Oklahoma University (US) discovered that exposing methoprene - a pesticide used against mosquitoes - to ultraviolet light broke it down into two acids which caused deformities in frogspawn within 96 hours, at 100 times the rate of deformities caused by unexposed methoprene. Another common pesticide, carbaryl, behaved similarly. Exposed to UV light it broke down into a much more toxic substance which seemed to harm tadpoles.

Waves of deformed frogs and other amphibians have been recently noted across North America.