Was lead pollution behind the baby boomer crime wave? Studies have already shown exposure to lead can cause permanent damage to areas of the brain responsible impulse control, judgment, and regulating emotions. The idea is extraordinary because if proven it could eclipse previous assumptions about the causes of crime - such as social and economic factors - and how to tackle it. It is the latest theory to try to explain why crime in US cities soared in the middle decades of the twentieth century before before peaking and declining in the early 1990s. A similar trend was seen in other Western populations. Other explanations include zero-tolerance policing, the tailing-off of the 1980s crack epidemic, and even the legalisation of abortion in the 1970s - so that with fewer unwanted babies there were fewer disturbed and violent young men 20 years later. But now added prominence has been given to the role of lead after Kevin Drum, a blogger on the US website Mother Jones, picked up on previously little-publicised studies into its effects, carried out at local, national, and international level. One recent paper looked at six US cities that had good data for crime and lead pollution levels going back to the 1950s, and found a correlation each time. Howard Mielke, of Tulane University, said he had studied concentrations of lead at a neighbourhood level in New Orleans and shared his maps with local police. He told the blogger: "When they overlay them with crime maps, they realise they match up." Lead spewed from car exhaust pipes had apparently continued to cause a pollution problem in places where it had settled in soil. Professor Mielke told BBC Radio 4"s Today programme that the starting point of his study had been to look at the relationship between lead in the environment and childhood achievement in school. "The group that had this high level were having difficulties in school, they were having behavioural problems, they started having criminal issues," he said. Alistair Hay, professor of environmental toxicology at Leeds University said the theory of the link between lead levels and crime should be taken seriously. He told Today: "I have done a lot of work on it in the past and have been concerned about its direct effects on the nervous system and elsewhere in the body." Professor Hay said he was "very impressed" with the studies that had been done and that they appeared to have been "carefully controlled" in relation to isolating the chemical's effects from other factors thought to impact on crime levels. "The thing that stands out is, associated with a rise and fall in crime is a rise and fall in lead levels," he said. He said that although further studies were needed he found the work - which was not saying changes in lead levels were responsible for all falls in crime - "quite convincing."