Children exposed to higher levels of mercury or lead are three to five times more likely to be identified by teachers as having problems associated with Attention Deficit Hyperactivity Disorder, according to a scientific study published today.
The study – of Inuit children in Arctic Quebec – is the first to find a high rate of attention-deficit symptoms in children highly exposed to mercury in the womb. In addition, the Inuit children more often had hyperactivity symptoms if they were exposed to the same low levels of lead commonly found in young U.S. children.
In the United States, one of every 10 children has been diagnosed with ADHD, according to the U.S. Centers for Disease Control and Prevention. It is one of the most common brain disorders of childhood.
Researchers from Laval University in Quebec surveyed teachers of 279 children in Nunavik between the ages of 8 and 14, using standardized questionnaires developed by psychiatrists for diagnosing ADHD.
Developmental psychologist Gina Muckle, the study’s senior author, said the findings are important because they show for the first time that mercury’s effects on children are not just subtle, but are actually noticeable to teachers.
The effects from exposure in the womb “may be clinically significant and may interfere with learning and performance in the classroom,” says the study, published online in the journal Environmental Health Perspectives.
For lead, the Inuit children with the highest levels were four to five times more likely to have teacher-reported hyperactivity than their classmates with low lead levels. “We are seeing those effects at very low blood lead levels,” Muckle said.
Although the findings came from a study of Arctic children, the results likely are universal, Muckle said. “At similar levels of exposure without regard of the source of exposure, the effect should be similar,” she said.
Dr. Bruce Lanphear, a professor at Simon Fraser University in Vancouver, said evidence is mounting that a variety of toxic compounds are “shifting children’s behavior.”
“There seem to be a whole host of different toxicants that are associated with ADHD,” said Lanphear, who studies childhood effects of lead, mercury and other contaminants but didn’t participate in the Inuit study. That actually “makes sense” biologically, he said, because ADHD is a syndrome of 23 different behaviors. Each toxic chemical could be altering different parts of the brain during different times of its development.
“To me, what this [new study] confirms is that the pre-frontal cortex appears to be particularly vulnerable to environmental toxicants,” he said. That part of the brain controls not just hyperactivity and attention but also learning disorders and anti-social and criminal behaviors.
Whale and fish consumed by their mothers were the sources of the Inuit children’s exposure to mercury. The lead came from foods the children ate that contained lead shot from hunting.
One of the most intriguing findings was that mercury was linked to attention deficits while lead was associated with hyperactivity. The difference may be the timing of the exposures: in the womb for mercury and during childhood for lead.
Harvard School of Public Health epidemiologist Joe Braun said the findings “suggest the brain may be sensitive to different environmental chemicals at different times in development.”
“Future research will need to confirm this finding and examine the effect of joint exposure to both prenatal mercury and childhood lead,” said Braun, who was not involved in the study.
Mercury and lead exposures had a stronger effect on ADHD symptoms than mothers who smoked during pregnancy, a link reported by other scientists, Muckle said.