MONDAY, July 11 (HealthDay News) -- Children exposed to secondhand smoke in their homes face a higher risk of developing attention-deficit/hyperactivity disorder, other behavioral problems and learning disorders, a new study finds.

The research doesn't definitively prove that tobacco smoke can harm children's brains, and it doesn't say how much smoke is too much. However, it does add to the evidence that children may be especially vulnerable to the effects of smoke exposure.

"They're in a developmental stage and their body is growing," potentially putting them at greater risk of disruptions to their brains than adults, said study co-author Hillel R. Alpert, a research scientist at Harvard School of Public Health's Center for Global Tobacco Control in Boston.

It's difficult to confirm whether secondhand smoke causes children's health problems because it would be unethical to expose kids to smoke and watch what happens to them. Instead, researchers often must look backward, as they did in this study, and try to eliminate all explanations but one for a link between smoke exposure and illness.

For their study, published online July 11 in the journal Pediatrics, researchers examined the results of a 2007 U.S. telephone survey of families that included 55,358 children under the age of 12. Six percent of them were exposed to secondhand smoke in the home.

After adjusting their numbers to improve their validity from a statistical point of view, the researchers found that about 8 percent of the kids had learning disabilities, 6 percent had attention-deficit/hyperactivity disorder, and almost 4 percent had behavioral and conduct disorders, such as oppositional defiant disorder.

Those who lived in homes with smokers were more likely to have at least two of the conditions, even after the researchers adjusted their statistics to account for such factors as income and education levels of parents.

The researchers estimated that secondhand smoke may be responsible for 274,100 extra cases of the three types of disorders examined.

Older children, particularly those between 9 and 11 years old, boys and poor children were most at risk of developing the disorders as a result of smoke exposure, the researchers found.

Children with smoke exposure at home were also more likely to receive behavioral counseling or treatment, which greatly increases health care costs, the survey found.

"Parents should consider banning smoking from their homes," Alpert said.

No only are children vulnerable because of their physiology, "they're also vulnerable because they do not necessarily have the choice about being exposed to smoke or not," he added.

According to the U.S. Environmental Protection Agency, secondhand smoke has been linked to increased severity of asthma in 200,000 to 1 million children and 150,000-300,000 lower respiratory tract infections in babies. Secondhand smoke is also linked to increased risk of sudden infant death syndrome.

Based on the survey results, the researchers concluded that about 4.8 million U.S. children under the age of 12 live in homes with a smoker, which is slightly less than previous estimates.

More information

For more on secondhand smoke, visit the U.S. Environmental Protection Agency.