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.
Parents in smoking households were more likely to say their child had been diagnosed with ADHD, a learning problem or "conduct disorder" -- where a child is often aggressive and antisocial.
About 20 percent said their child had at least one of those disorders, versus less than nine 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 point out the study's limitations, like relying on parents' reports.
It's also unclear exactly how secondhand smoke, itself, would contribute to learning and behavioral problems. Some researchers have speculated that the smoke may affect certain chemicals in children's developing brains.
Whatever the reasons for the current findings, Alpert said they underscore the need to keep kids away from secondhand 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."
A second study in the same journal suggests that kids' reactions to their parents' secondhand smoke may also play some role in their own likelihood of taking up the habit.
Researchers found that among 165 low-income preteens from smoking households, those who thought secondhand smoke was "unpleasant or gross" were 78 percent less likely than other kids to be at high-risk for smoking.
The study did not look at whether kids actually did take up the habit, but asked them whether they thought they might try it in the future.
Still, the researchers write, the findings suggest that children's sensitivity to secondhand smoke -- or lack thereof -- may help predict which ones are at greatest risk of trying cigarettes.