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As children, they swallowed or breathed in high levels of lead. Now they are adults, and sophisticated forms of brain imaging are allowing scientists to see how that early exposure damaged the wiring of their brains in significant, permanent ways.

Brain imaging is emerging as an important new tool to explore how environmental contaminants affect the developing brain, both in the womb and after birth. Now, the science is offering a physical explanation - or at least a partial one - for the diminished intelligence, poor academic performance, higher rates of attention deficit hyperactivity disorder and other behavioural problems associated with exposure to lead early in life.

The work is in the early stages, but is already raising troubling questions about links between poverty and subtle but significant brain damage - reductions in volume, missing connections - that may be caused by contaminants in the environment. It adds to the growing evidence that reducing exposure to some kinds of pollution could help poor kids do better in school.

For years, studies in Canada and the United States have documented that children in poor neighbourhoods are more likely to be exposed to lead, higher levels of air pollution and some pesticides. Some studies also link those exposures to cognitive deficits and behavioural problems.

But there has always been controversy over whether attention should be directed at specific contaminants, or whether other factors associated with poverty, such as poor nutrition, low-quality schools or lack of stimulation in early childhood, were the culprits.

"For too long, we have blamed familial transmission of poverty to poor parenting or genetics. This emerging research helps confirm that reducing exposures to environmental pollutants - which are more common in poor children - will reduce higher rates of learning problems and behavioural problems in poor children," said Simon Fraser University's Bruce Lanphear, who is also affiliated with BC Children's Hospital. He worked with Cincinnati Children's Hospital neuroscientist Kim Cecil on studies that offer new evidence about how lead, known to be a neurotoxin for more than a century, permanently damages children's brains.

"Being able to see a change in brain structure or function is compelling to people in a new way that traditional assessment tools are not," said Harvard University's David Bellinger, an expert on the impact of lead.

All volunteers in the Cincinnati study grew up in impoverished inner-city homes, but some were exposed to much more lead than others. Specialized MRI scans that microscopically examined the size, architecture and activity of their brains found significant differences in the adults who were exposed to higher levels of the heavy metal.

Put together, the imaging studies are beginning to paint a picture for researchers, Dr. Cecil said. "Some circuits don't work right … for others, the connections aren't very efficient. It explains the lower IQ, the cognitive problems, some of the irritability and aggressive tendencies."

Researchers in the United States and Canada are also starting to use sophisticated forms of magnetic resonance imaging to study the neurodevelopmental effects of pesticides, air pollution, tobacco smoke and bisphenol A, or BPA, a chemical found in some plastics.

However, high exposure to a contaminant is not always linked to poverty. A recent study in Quebec found that children with high levels of manganese in their drinking water performed significantly worse on tests that measured intellectual functioning than children who had lower levels of the metal in their water. But the study found no relation between family income and levels of exposure, the University of Montreal's Maryse Bouchard said.

Bradley Peterson at Columbia University, has been scanning the brains of children in New York who were exposed to high levels of polycyclic aromatic hydrocarbons, or PAHS, in the womb.

He is working with his colleague at Columbia, Frederica Perera, part of a team that has been following the children since before they were born. Their mothers were recruited from Washington Heights, Harlem and the South Bronx. During their pregnancies, they carried backpacks with equipment to measure PAHS, a component of air pollution.

Dr. Peterson and his colleagues are also assessing how the pesticide chlorpyrifos and secondhand smoke affect brain development.

"We believe that different toxins have different effects on different regions and pathways," he said, but there may be some overlap.

The 250 people in the Cincinnati lead study are now in their mid 20s, and researchers at the University of Cincinnati and the children's hospital have been tracking them since they were in the womb. As children, all had lead in their blood, but the concentrations ranged from five to 37 micrograms per deciliter.

There is no established safe level for lead exposure, said Dr. Bellinger, but 10 micrograms per deciliter is considered too high.

The participants were tested four times a year until the age of five and twice a year after that until they were 6 1/2. Those who had higher levels between the ages of three and six tended to have less grey matter as adults. The difference was most striking in the frontal lobes of the young men.

There is also intriguing evidence from functional MRI scans of how the brains of the children exposed to higher levels of lead have attempted to compensate for early damage. "These parts of the brain develop early," Dr. Cecil said, "and it keeps trying to repair them."

Blood lead levels are measured in micrograms per decilitre of blood.

10 This many micrograms of lead per decilitre of blood is considered to be too high.

27% Amount of Canadians who were over this threshold three decades ago.

1.34 This many micrograms per decilitre is the average concentration today, with older adults having higher levels than children.

70 This many micrograms of lead per decilitre can cause the brain to swell and have serious consequences.