Autism symptoms correspond to the level of toxic metal in the blood, with exposure to lead particularly harmful, according to a US study.

Autism sufferers carried 74% more lead in their systems than control subjects, and 115% more tin. Sufferers with more severe symptoms carried higher concentrations of metal, with researchers at Arizona State University finding that toxic metal was the “strongest factor” in determining severity.

“We knew that exposure to lead makes people lose IQ points, and clearly it can induce autism”, lead researcher Professor James Adams told Metro. “The study also showed that people with the highest levels are least able to excrete them”.

Autism was previously thought be largely a hereditary condition but environmental and chemical reasons are now being accepted. “Studies such as this improve our understanding”, said Caroline Hattersley, head of information and advice at the UK-based National Autism Society, calling for “more research…across a greater geographical area.”

It is the latest lead concern, after Metro reported research in January that revealed exposure could induce aggression, violence and brain damage. “Anything to do with the nervous system will be badly affected, we’re only starting to see the full damage”, Professor Howard Mielke, who studied the toxin’s effects in 14 countries, told Metro. “There are huge repercussions for learning ability from just a small change in exposure.”

Lead remains widespread across the world, most commonly in soil and paint. But neither government nor the petrol companies that produced leaded gasoline have taken action to clear it, with the Ethyl Corporation suing environmental groups that attempted to hold it responsible.

In civil courts, hundreds of millions of dollars have been recovered by the victims of lead poisoning, and is a rapidly growing area for compensation claims. The largest award to date is the $320 million paid out to victims of exposure at a St. Louis smelting plant in 2011.

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