URINE samples from hundreds of French children have yielded evidence for a link between autism and exposure to heavy metals. If validated, the findings might mean some cases of autism could be treated with drugs that purge the body of heavy metals.

Samples from children with autism contained abnormally high levels of a family of proteins called porphyrins, which are precursors in the production of haem, the oxygen-carrying component in haemoglobin. Heavy metals block haem production, causing porphyrins to accumulate in urine. Concentrations of one molecule, coproporphyrin, were 2.6 times as high in urine from children with autism as in controls.

Autism is thought to have a number of unknown genetic and environmental causes. Richard Lathe of Pieta Research in Edinburgh, UK, says he has found one of these factors. "It's highly likely that heavy metals are responsible for childhood autistic disorder in a majority of cases," he claims. The study will appear in Toxicology and Applied Pharmacology.

Lathe says these porphyrin metabolites bind to receptors in the brain and have been linked with epilepsy and autism.

The researchers restored porphyrin concentrations to normal in 12 children by treating them with "chelation" drugs that mop up heavy metals and are then excreted. It is not yet known whether the children's symptoms have eased, but Lathe cites anecdotal reports suggesting the drugs might do some good.