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.

SOURCE: Pediatrics, online July 11, 2011