NEW YORK - Children exposed to secondhand smoke at home may be more likely to have learning and behavioral problems, according to a US study.

Of more than 55,000 US children younger than 12 years, 6 percent lived with a smoker -- and those children were more likely to have ADHD compared to children in smoke-free homes, the study, published in Pediatrics, found.

Even after accounting for a number of possible explanations, such as parents' income and education levels, secondhand smoke was still tied to a higher risk of behavioral problems, said Hillel Alpert at the Harvard School of Public Health, one of the researchers.

The findings don't prove a smoke-filled home is to blame, because there are other factors that the study didn't look at that may also be to blame -- but it may give parents yet another reason to keep their homes smoke-free.

Health experts already recommend that children be shielded from secondhand smoke for health reasons, since it can increase their risk of respiratory infections, 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.

One other factor to consider is that children exposed to secondhand smoke often had intra-uterine exposure as well, which has been linked to increased risks of learning and behavioral problems.

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

The results are based on a 2007 national survey of parents of 55,358 children younger than 12. The finding that 6 percent lived with a smoker translates into nearly 5 million US children exposed to secondhand smoke at home, according to the research team.

About 20 percent of parents in smoking households said their child had at least one type of conduct disorder, versus less than 9 percent of parents in non-smoking homes.

When Alpert's team accounted for poverty, race, mothers' education levels and other factors, secondhand smoke was tied to a 51 percent increase in a child's risk of having one of the three disorders.

The researchers said that it's unclear exactly how secondhand smoke would contribute to learning and behavioral problems. Some research has speculated that the smoke may affect certain chemicals in children's developing brains.

A second study in Pediatrics suggested that children's reactions to their parents' secondhand smoke may also play some role in their own likelihood of taking up the habit.

Among 165 low-income preteens from smoking households, those who thought secondhand smoke was "unpleasant or gross" were 78 percent less likely than other children to be at a high risk of smoking.

Alpert said that whatever the reasons for the current findings, they underscore the need for children to be kept away from smoke.

"We still have 5 million children exposed to secondhand smoke at home," he said.

"A lot of progress has been made in reducing that number, but there's still a lot left to be done." \