Children exposed to secondhand smoke at home may be more likely than their peers to have learning and behavioral problems, according to a new study.

Researchers found that of more than 55,000 U.S. children younger than 12 years, six percent lived with a smoker. And those kids were more likely to have attention-deficit hyperactivity disorder (ADHD), a learning disability or “conduct disorder” than children in smoke-free homes.

Even after accounting for a number of possible explanations - like parents’ incomes and education levels - secondhand smoke was still tied to a higher risk of behavioral problems, said Hillel R. Alpert of the Harvard School of Public Health, one of the researchers on the work.

Still, the findings don’t prove a smoke-filled home is to blame, said Alpert, because other factors the study didn’t look at could be at play.

For instance, children exposed to secondhand smoke are often exposed to smoke while they are still in the womb. And mothers’ smoking during pregnancy has been linked to increased risks of learning and behavioral problems.

Alpert’s team, whose results appear in the journal Pediatrics, had no information on mothers’ smoking during pregnancy.

It’s also possible that parents who smoke have, themselves, a greater history of learning or behavior problems compared with non-smoking parents.

Health experts already recommend that kids should be shielded from secondhand smoke, which can increase their risk of respiratory infections like bronchitis and pneumonia, severe asthma and sudden infant death syndrome.

“The key message for parents is to protect their children from exposure to secondhand smoke,” Alpert told Reuters Health.

These latest findings, he said, may give them yet another reason to do so.

The results are based on a 2007 national survey of parents of 55,358 children younger than 12. Six percent of parents said someone in their household smoked - translating into nearly 5 million U.S. children exposed to secondhand smoke at home.