By environment correspondent Alex Kirby
Pollution and other environmental threats are harming the intelligence of millions of people across the world, says a United Kingdom review of the available evidence.
The causes are poisons such as lead, PCBs (polychlorinated biphenyls, synthetic compounds used in electrical equipment), and radiation.
A further problem is the loss of micronutrients like iron and iodine through soil erosion, impoverishing food crops. And scientists say it is hard to know the full extent of the problem, because of the difficulty of gathering data.
The author, Dr Chris Williams, a social scientist at the Institute of Education, London University, said one problem could compound another, with iron deficiency in children, for example, able to increase their lead uptake.
"We only have single-substance science, which does not account for compounding effects. So the overall scale of the problem is far greater than previously estimated."
Dr Williams is a fellow of the Global Environmental Change Programme, a £15m social science initiative of the Economic and Social Research Council.
He undertook a global review of science-based research into the impact of environmental factors on intelligence.
One of his most disturbing findings is that epidemiologists have detected a statistically significant increase in the birth of children with Downs Syndrome which is linked to radiation from the explosion of the Chernobyl nuclear reactor.
The increase was dependent on rainfall in the period following the explosion. Excess Downs Syndrome births were recorded in parts of Germany, Scandinavia and the Lothian region of central Scotland nine months after the disaster.
Dr Williams found a study by the Russian Academy of Sciences which said that radiation from a uranium mine had caused mental impairment in 95% of the children of one town in Russia.
In south east Asia 1.5bn people are affected by the iron deficiency of many Green Revolution crops, especially maize, and even more in the poor world are at risk from iodine deficiency.
In parts of the Himalayas and China the problem is exacerbated when deforestation allows rain to wash the soil away, taking with it nutrients which are essential in the human diet.
Growing threat to children
The phenomenon is not new, but is becoming worse with increased logging and growing population pressure.
Lead in the environment is a threat, with the blood-lead level of one child in 10 in the UK high enough for intelligence to be affected.
In some African cities the proportion is nine children in ten. The intelligence of Inuit children in the Arctic is being damaged by PCBs which originate in the tropics and arrive in Canada within a week.
Dr Williams told BBC News Online: "The big feeling I have about this is in the context of evolution.
"The human brain is now at risk from its own behaviour, and nothing else in the ecosystem is harming itself in the same way.
"Even lemmings dont really behave like lemmings. Thats a myth. But we are acting like lemmings.
Need for action
"Ive seen Indian villages where the wells have been poisoned with fluoride, causing a loss of intelligence. The bright people move out, the spiral continues, and you see what can happen to a community.
"Im afraid therell be many more underfed, poisoned people in the poor world unless we recognise what is happening."
The director of the Global Environmental Change Programme, Dr Frans Berkhout, said: "This issue reveals a wider problem that science has when faced with complex and uncertain environmental issues.
"Some of the most difficult environmental challenges are not being adequately addressed simply because of the difficulties of collecting the necessary evidence and establishing cause and effect."