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Public Radio's Environmental News Magazine (follow us on Google News)

Pollutants Implicated in ADHD

Air Date: Week of

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Dr. Gina Muckle

About one in every ten children in the United States has been diagnosed with ADHD. Now scientists at Laval University in Quebec say some environmental pollutants increase the incidence of attention problems. Dr. Gina Muckle tells host Steve Curwood about her study demonstrating an association between ADHD in children and exposure to lead and mercury.


CURWOOD: From the Jennifer and Ted Stanley Studios in Boston, this is Living on Earth. I'm Steve Curwood. At least one in every ten school-aged children in the United States has been diagnosed with ADHD, and over the last decade, prescriptions for attention deficit medications are up nearly 50 percent.

There are many theories about the cause of the increase in ADHD, but scientists from Laval University in Quebec have new evidence implicating two notorious toxins. In a study just published in Environmental Health Perspectives they found an association between ADHD in children and exposure to mercury and lead.

Joining me from Quebec City is one of the study's authors, Dr. Gina Muckle, who's a professor of psychology at Laval University - welcome to Living on Earth!

MUCKLE: Thank you!

CURWOOD: So, you conducted this study up in the Canadian Arctic. Tell us a little bit about that region and why you chose to do your research there.

MUCKLE: In the northern part of the province of Quebec, there’s about 11,000 Inuit living in the north. And we’ve discovered in the beginning of the 80s that this population was the most highly exposed population in the world with regards to PCBs and mercury. So we decided that time that we would follow a cohort of babies by picking cord blood samples at their birth and follow them up to school age.

CURWOOD: Could you please explain the specifics of your study to me?

MUCKLE: Yes. About 300 newborns were born between 1993 and 1996, provided cord blood sample for us so that we could assess their neonatal exposure to environmental contaminants such as mercury, lead, and PCBs. While they were at school age which is 11 years of age we saw this group of children again and we did an interview with their mother to document different cognitive growth and behavioral effects that we could see in association with their prenatal exposure. At that age, we asked their schoolteacher to fill in forms to document ADHD behavior.

CURWOOD: Now, how exactly did you measure attention deficit disorder?

MUCKLE: Well, there are questionnaires that we can use that are well used all around the world aimed to assess the manifestations of ADHD, which are inattention difficulties in school, aggressive behaviors, non-respect of rules and hyperactivity symptoms. So the teachers were asked to assess those behaviors for all of the children.

CURWOOD: And what were your findings?

MUCKLE: The first and most important result was that prenatal mercury exposure was associated with greater attention problems and with an increased risk of teacher-reported symptoms consistent with ADHD. To be more specific, what we observed is that children with higher cord mercury concentrations at birth were about 3 times more likely to be identified by their teacher in the classroom as exhibiting behaviors characteristic of the inattentive type of ADHD.

CURWOOD: And what did you find for lead?

MUCKLE: In lead what we saw was that childhood exposure from blood samples we took while the child was 11 years of age, so this exposure was related to greater aggressive and mood-breaking behaviors, and with behaviors consistent with ADHD. Actually, 11 year-old children who were exposed to lead were about four to five times more likely to be classified with hyperactive impulsivity by their teacher.

CURWOOD: Now, how is it that mercury and lead get into these Inuit children?

MUCKLE: Well, the main exposure source of mercury of the Inuit population is through consumption of beluga meat. And the main source of exposure of lead is through the use of lead-containing ammunition for hunting.

CURWOOD: How relevant are these results for non-Inuit children? Those, say, living in the United States, or elsewhere in Canada?

MUCKLE: For lead, the effects that we are reporting are seen at very low levels of exposure. We can estimate from the Canadian Health survey in Canada that there’s about 10 percent of Canadian children between six to 11 years who might be exposed to levels greater than the ones where we saw negative effects.

According to the CDC in the US, there’s about half a million of US children aged between one and five with blood lead levels above five micrograms per deciliter, which corresponds to about 2.5 percent of the US population. And the effects that we are reporting are below this five micrograms per deciliter. So, there’s certainly, at least in the US, more than five percent of the population who are exposed to those levels.

CURWOOD: What are the primary sources of lead and mercury throughout North America?

MUCKLE: Mercury travels between countries through air currents - it’s reached the Arctic as well, and mercury emissions are the primary source of human exposure. Mercury emissions are primarily due to coal combustion for production of electricity, and in Canada, it’s probably about the same as it was in the United States – China was identified as the largest source of mercury deposition. They are responsible for about 45 percent of the entire mercury deposition in Canada.

CURWOOD: What do you hope will come out of your research?

MUCKLE: We do need to have much more information in what are the determinants of child behavior. Especially ADHD. And so far, the scientific research has been conducted to look at many different determinants of ADHD and I think we are just starting to look at the environment - the physical environment and the chemical environment - as a potential determinant of such kinds of difficulties in children. So, what we hope is to be able to contribute to the explanation of behavior difficulties in children.

CURWOOD: Dr. Gina Muckle is Professor of psychology at Laval University in Quebec City. Thank you so much.

MUCKLE: Thank you very much, it was a pleasure!



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