May 17, 2010 -- Relatively low-level exposure to common pesticides -- probably from residues on foods -- doubles kids' risk of ADHD, Harvard researchers find.
The findings come from a nationally representative sample of 1,139 U.S. kids aged 8 to 15 who were tested for ADHD (attention deficit hyperactivity disorder) and had urine samples tested for signs of exposure to various organophosphate pesticides such as malathion.
Kids with higher-than-average levels of pesticide metabolites were about twice as likely to have ADHD as kids with undetectable levels of pesticide metabolites, find Marc C. Weisskopf, PhD, ScD, associate professor of environmental health and epidemiology at Harvard School of Public Health, and colleagues.
"This raises concerns that ubiquitous pesticides may be contributing to the national burden of ADHD, which already is quite high," Weisskopf tells WebMD.
It's not just kids who live on farms or otherwise get extremely frequent or high-dose exposure to pesticides. The metabolites detected in the Weisskopf study indicate that these kids have ongoing, low-level exposure to pesticides at levels that may affect their development.
"What I think is so important is this is not a select group of people with unusually high pesticide exposure," Weisskopf says. "This is a general population sample. If this link with ADHD is proved true, there is a big chunk of people this is going to be relevant for."
Weisskopf notes that his study is designed to detect a possible risk but is not able to prove that one thing caused another. For example, the data could be taken to mean that kids with ADHD somehow behave in ways that increases their exposure to pesticides. While that appears counterintuitive, further studies are needed to test whether pesticides truly contribute to ADHD.
Alarmingly, the Weisskopf study complements an earlier study by Virginia A. Rauh, ScD, MSW, professor of family health at Columbia University's Mailman School of Public Health and co-deputy director of the Columbia Center for Children's Environmental Health.
In their 2006 study, Rauh and colleagues found that kids with the most exposure to a household organophosphate pesticide had significantly delayed mental and motor development. These effects increased over time. And kids who were exposed while still in their mothers' wombs were more likely than other kids to have ADHD.
"We were quite sure the exposure came from residential pesticide use and from food," Rauh tells WebMD. "What happens is a whole variety of commonly used foods are sprayed routinely with organophosphates to eliminate pests. That is where the food residue comes from."
Unlike cigarette smoke, a health-harming pollutant that one can do much to avoid, pesticides are hard to avoid. They're everywhere -- even in foods we generally consider healthy.
"Here is a situation where the average consumer isn't buying the wrong kind of food or breathing the wrong kind of air. There is not a whole lot the average person can do," Rauh says. "And that is where we need the EPA [Environmental Protection Agency] to take a good look at all these studies and see if the risk warrants dropping the safety limit for these chemicals and tightening their regulation."