Children exposed to secondhand smoke in their homes face a higher risk of developing attention- deficit/hyperactivity disorder (ADHD), other behavioural problems and learning disorders.

It's difficult to confirm whether secondhand smoke causes children's health problems because it would be unethical to expose children to smoke and watch what happens to them, Health News reported.

Instead, researchers often must look backward, as they did in this study, and try to find the cause by probable causes linking smoke exposure and illness.

Researchers examined the results of a 2007 telephone survey of American families that included 55,358 children under the age of 12 years. Six percent of them were exposed to secondhand smoke in the home.

The researchers found that about 8 percent of the children had learning disabilities, 6 percent had attention-deficit/hyperactivity disorder, and almost 4 percent had behavioral and conduct disorders.

It was found that those who lived in homes with smokers were more likely to have at least two of the conditions, even after the researchers accounted 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.

Children with smoke exposure at home were also more likely to receive behavioural counselling or treatment, which greatly increases health care costs.

Parents should consider banning smoking from their homes. Not 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.

This study 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.