It’s well known that children are more susceptible to the harmful effects of chemical exposure than adults. For one, their bodies are still developing rapidly, so chemicals can interfere with the normal growth of their brains and other organs, or disrupt their hormones at key developmental periods. For another, doses are likely to be disproportionately large, because their bodies are so much smaller than adults’ and because their breathing rates are so much more rapid. Finally, their habits — crawling on the floor, testing the world with their mouths and frequently putting their hands into their mouths dirty — mean they are exposed to more toxic chemicals on the ground.

A new study suggests yet more reason kids are more susceptible to the harmful effects of harsh chemicals: Their bodies have yet to produce significant quantities of a detoxifying enzyme that helps adults rid their bodies of organophosphate chemicals, a class of chemicals that includes many pesticides.

The research, by University of California-Berkeley scientists, published in Environmental Health Perspectives and publicized by Beyond Pesticides, didn’t discover the importance of this enzyme. But it showed that children have low levels of paraoxonase 1 — one-third or less that of their mothers — far longer into childhood than previously thought. Whereas it was thought the levels of enzyme approached adult levels by age 2, the new research suggests children remain uniquely susceptible until age 7. (The enzyme may also be important in warding off other types of disease, from asthma to obesity and cardiovascular disease, the study’s authors said.)

The study’s authors said that the Environmental Protection Agency should take heed and adjust its calculations for childhood health risk to pesticide exposure. The EPA has only in the past decade or so even considered the unique vulnerabilities of children when approving chemicals for use.

“Current EPA standards of exposure for some pesticides assume children are three to five times more susceptible than adults, and for other pesticides the standards assume no difference,” Nina Holland, a lead author of the paper, was quoted as saying by ScienceDaily and others. “Our study is the first to show quantitatively that young children may be more susceptible to certain organophosphate pesticides up to age seven. Our results suggest that the EPA standards need to be re-examined to determine if they are adequately protecting the most vulnerable members of the population.”

Organophosphate pesticides are insecticides that attack bugs’ nervous systems. They are used on farms, and some are labeled for home use to kill or repel mosquitoes, ants, cockroaches and other household and garden pests. Dozens of brands (pdf) use organophosphates in insecticides, according to the National Pesticide Information Center.

Like many harsh chemicals, they may cause serious health problems with humans, as well. Related chemicals were originally developed as nerve gases during World War I, and may affect normal brain, reproductive and other body development (some chemicals may even make you fat). Further, the EPA can often be slow to remove profitable pesticides from the market.

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