It's a well-known fact that secondhand smoke is a health threat to children. It has been linked to increased risks of asthma, as well as bronchitis, pneumonia and other respiratory infections. A study released earlier this year in Finland found that children as young as 13 who have evidence of secondhand smoke in their blood also have visibly thicker arteries, a condition which could lead to heart disease at an early age.

Studies have also found a connection between smoking during pregnancy and higher risks of childhood developmental and behavior problems, such as ADHD. Some research has also found that children exposed to cigarette smoke in the womb or at home may trail their peers when it comes to cognitive abilities like reasoning and remembering.

A new study of Hong Kong students confirms these findings, linking exposure to secondhand smoke with poorer grades. In the study, researchers found that among 23,000 11- to 20-year-old nonsmoking students, the one-third who lived with at least one smoker were more likely to describe their own school performance as "poor."  

The study's authors adjusted for factors, like parents' education levels and the type of housing — two markers of socioeconomic status that could also affect a kid's grades. Still, they found that students' exposure to secondhand smoke was linked to a 14 percent to 28 percent greater risk of poor school performance, depending on how frequent the exposure was.

Of course these findings do not prove that secondhand smoke in and of itself is hurting kids' grades. But they do show that kids who are exposed to secondhand smoke believe that the exposure is affecting their cognitive abilities. And the studies certainly give parents another great reason to quit. 

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