ADHD “is a complex disorder so there are likely many risk factors. We are still far from understanding its etiology and relationship to environmental chemicals,” she said.
Attention-deficit symptoms were three times more frequent among the Inuit children with high mercury exposure than among their classmates with the lowest exposures, according to the study.
Nunavik children are highly exposed to mercury mostly from eating beluga whale meat. Methylmercury accumulates in large fish and marine mammals near the top of food chains.
Such high mercury levels are rare among non-Arctic populations
“We think that there’s not likely to be a significant proportion of the U.S. or Canada population exposed to the mercury levels where we’ve seen these effects,” Muckle said. But she cautioned that some segments of the population, particularly Asian communities, who eat large quantities of albacore tuna, swordfish and other large predator fish could be equally exposed.
In contrast, the lead levels associated with ADHD symptoms in the Nunavik children are considered low. They are roughly the same as average levels in U.S. children.
Previous studies of U.S. children have linked lead exposure to ADHD.
“Regardless of the population, there are effects [from lead],” Muckle said. “This is another confirmation that really low exposure to lead is associated with greater behavioral difficulties in schools.”
Lead shot used by hunters was identified through isotopes as the source in the Inuit kids. In U.S. kids, lead is mostly from old peeling paint and contaminated soil.
“It almost doesn’t matter what the source is,” Lanphear said. “It looks like lead is associated with certain effects whether it’s from paint or it’s from lead shot.”
The amounts of lead linked to hyperactivity symptoms were far below the advisory level (5 micrograms per deciliter in blood) recently set by the CDC. About half a million U.S. children between the ages of 1 and 5 exceed that level.
“A notable result of this study is that ADHD-type symptoms were found in kids with lead exposures far below the current health-based guidelines. It really reinforces that we need to rethink the guidelines if we’re finding effects at levels that are one-third or half of the current guidelines,” said Glenys Webster, a postdoctoral fellow at Simon Fraser University who studies prenatal exposure to contaminants. She was not involved in the study.
The research uncovered some important differences between lead and mercury.
For lead, the ADHD effects were linked to post-natal exposures – what the children themselves were exposed to. For mercury, it was their exposures via their pregnant mothers, detected in cord blood collected at birth.
One possible reason that lead was connected to hyperactivity and mercury to attention deficits may be that mercury exposure comes during rapid brain growth in the womb, while lead exposures come when the brain is more robust, Muckle said.
Lanphear said the scientists “did a nice job” of factoring in other influences on brain development, including nutrients from fish and smoking. One weakness of the study, he said, is that there was no information on the parents’ rates of ADHD, so the study could not factor in a potential genetic role.
The children in the study were not actually diagnosed by psychiatrists but Lanphear said teacher surveys are a “validated approach” to check for ADHD symptoms. A child is typically diagnosed with ADHD using the surveys at both school and at home.
Funded by the U.S. and Canada governments, the study is part of a two-decade investigation of children in Nunavik, who are exposed to some of the world’s highest levels of mercury and industrial chemicals called PCBs or polychlorinated biphenyls. The children have been studied for the effects of contaminants since they were born between 1993 and 1996.