The study by the Cincinnati Children’s Hospital Medical Center is the largest of its kind. It looked at 4,399 children ages 6 to 16, from homes where people smoked and homes where they didn’t.

The youths took tests to measure thinking and skills in such areas as reading and math. Researchers also tested their logic and reasoning abilities. The result? Children who were repeatedly exposed to tobacco smoke – even at low levels – appeared to have impaired thinking and reasoning skills.

The study focused on kids who had the most exposure to environmental tobacco smoke but who didn’t smoke themselves. To do so, researchers measured blood levels of the chemical cotinine, created when the body breaks down nicotine.

They found that children with the greatest exposure to secondhand smoke did the poorest on tests. The higher the cotinine levels in a child’s bloodstream, the lower the scores on math, reading and reasoning tests. Even low levels of exposure seemed to impair mental performance.

Forty-three percent of U.S. children are exposed to secondhand smoke at home. The greatest exposure to tobacco smoke often comes from children’s mothers, who typically are the main caregivers.

Secondhand smoke is particularly dangerous for children because their bodies and organs are still developing. That makes them more vulnerable to toxins. Doctors have long known, for instance, that respiratory problems are more likely among children who inhale a lot of secondhand smoke.

Pound for pound, children breathe in more air than adults, so their exposure is greater. In addition, a child’s body can’t get rid of toxins as efficiently as an adult’s.