Kids Exposed to Mercury, Lead at Risk for ADHD

Medically Reviewed by Louise Chang, MD on September 20, 2012
From the WebMD Archives

Sept. 21, 2012 -- Young children exposed to certain heavy metals are at higher risk for problems with attention and behavior later in life, a new study shows.

The study followed nearly 300 Inuit children who were born in northern Quebec, Canada. One of the main sources of protein in the Inuit diet is beluga whale meat, which can be high in mercury. Inuit children are exposed to lead when they eat shot pellets that are used to kill geese and ducks.

Lead and mercury are potent toxins, and the developing brains of young children are vulnerable to their effects. Studies of kids with mercury poisoning show they have trouble with language skills, attention, and coordination, as well as other problems. Lead affects learning and memory.

Researchers tested a sample of Inuit children's umbilical cord blood at birth for a range of environmental contaminants and nutrients. Years later, when the children were between the ages of 8 and 14, researchers asked their teachers to complete questionnaires about their behavior.

Roughly 14% of the children in the study had inattentive behaviors of attention deficit hyperactivity disorder (ADHD). A similar percentage of the children had hyperactive-impulsive behaviors of ADHD.

Mercury and Lead Linked to More Symptoms of ADHD

Children with the highest concentrations of mercury in their cord blood had more trouble paying attention than those with lower levels. They were also about three times more likely to be flagged by their teachers as having these symptoms of ADHD. That was true even after researchers accounted for things linked to ADHD, like low birth weight and whether or not the mother used tobacco during pregnancy.

Researchers say the mercury levels seen in the study were extremely high. Most women of childbearing age in the U.S. have blood levels of mercury that are about one-third as high, according to the CDC. Certain groups of people, like Asian-Americans born in China, who eat traditional diets rich in large fish like shark, tuna, and swordfish, have been found to have blood levels of mercury that are in the same extreme range as found in this study, says researcher Gina Muckle, PhD, of Laval University in Quebec, Canada.

In contrast, Inuit children with even low to moderate blood levels of lead -- closer to levels measured in some U.S. children -- were more than four times more likely to have problems with hyperactivity than kids with lower lead levels.

"The effects we are seeing are at very low levels of exposure. In [the] U.S. and Canada, for example, we estimate that 10% of children would be exposed to [these] blood lead levels," says Muckle. U.S. children can be exposed to lead when they eat tiny chips of lead-based paint, which can be found in homes built prior to 1978.

The study is published in Environmental Health Perspectives. It was paid for by government grants from the U.S. and Canada.

Mercury Exposure Before Birth

Other studies that have looked for a link between mercury exposure and ADHD have had mixed results. A study of children in the Faroe Islands, where marine mammals like whales are dietary staples, also found attention problems in school-aged children exposed to high levels of the heavy metal before birth.

But another study, of children in the Seychelles Islands, found no clear link between mercury exposure before birth and behavior problems.

"It's quite an interesting sort of finding, really. In the Seychelles they found that the more mercury exposure before birth, the higher the child's IQ. In the Faroe Islands, the more mercury, the worse off the kids were," says Justin Williams, MD, a psychiatrist at the University of Aberdeen in Scotland. Williams was not involved in the study.

Williams says the differences may come down to the source of the mercury. In the Seychelles, pregnant mothers were getting mercury through diets high in fish. Fish is also high in essential fatty acids that may protect developing brains. In the Faroe Islands, mothers got mercury by eating marine mammals, which lack the protective fatty acids found in fish.

"As always, you have to look at these studies and be very cautious, really. What we may find is that mercury is a marker for someone else. It's not necessarily a causal relationship," he says.

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Boucher, O. Environmental Health Sciences, Sept. 21, 2012.

Gina Muckle, PhD, professor of psychology, Laval University, Quebec, Canada.

Justin Williams, MD, psychiatrist, University of Aberdeen, Aberdeen, Scotland.

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