Pollution and other environmental factors may be threatening the intelligence of millions of people around the world, according to a new research review conducted by Chris Williams of London University. Environmental problems that can lead to lower IQs include lead, PCBs, and radiation. Epidemiologists have detected an increase in the number of children born with Down’s syndrome that correlates with the explosion of the Chernobyl nuclear reactor in 1986. In Britain, one in 10 children has high enough levels of lead in their blood to affect intelligence, and in some African cities the ratio is nine children out of 10.