By Alex Kirby
BBC News Online environment correspondent
The brains of children in many parts of Europe are suffering greater damage from environmental risks than previously recognised, scientists say.
The WHO says prevention is the way forward
The WHO claims lead continues to be a menace - up to 30% of urban children show high blood levels in some places.
It says the emphasis from now on should be on the precautionary principle, putting safety first.
The WHO says "the vested interests of industry and free trade" have worked against this approach so far.
Call for caution
Its call for caution came at a meeting in Malta of European delegates preparing for a ministerial conference on environment and health, The Future For Our Children, being held in Budapest, the Hungarian capital, in June.
The Malta meeting has been given preliminary results from a comprehensive study on environmental threats to children's health, being conducted by the WHO and the University of Udine, Italy.
The full report is to be published at the Budapest conference. The findings suggest lead is the single most important damaging chemical for children. In 2001, the estimated percentage of European children in urban areas with elevated blood levels (above 10 micrograms per decilitre) ranged from 0.1% to 30.2%.
Globally, the WHO says, 15 to 18 million children in developing countries suffer permanent brain damage from lead poisoning. Other threats to children's health include methylmercury, dioxins, furans, PCBs, pesticides, nitrites and nitrates, and benzene.
Dr Marc Danzon, WHO regional director for Europe, said: "Evidence shows that reducing exposure to lead protects a child's intellectual potential. We should take action to make sure that our children are all protected from this and other environmental hazards."
In a statement the WHO says: "While effective prevention is the key to addressing known health threats, the precautionary principle needs to be applied when facing uncertain risks...If applied earlier, it could have saved millions of lives, but convincing proof of harm was awaited before action was taken.
"This controversy has been fuelled by pressure from the vested interests of industry and free trade that consider the precautionary principle a hindrance."
Dr Roberto Bertollini of WHO said: "For too long, policy-makers have retrospectively pleaded, 'If only we had known earlier what we know now'.
"I believe that what we do know now must guide us in our review and approval processes, and should become the basis of a bold new precautionary approach that puts the burden of evidence on safety first."