Children and teenagers exposed to secondhand smoke at home may get poorer grades than their peers from smoke-free homes, a study of Hong Kong students suggests.

Secondhand smoke is a well-known health threat to children, being linked to increased risks of asthma, as well as bronchitis, pneumonia and other respiratory infections. Studies have also found a connection between smoking during pregnancy and higher risks of childhood behavior problems and attention-deficit/hyperactivity disorder.

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. But whether secondhand smoke itself is to blame remains unclear.

In the new study, researchers found that among 23,000 11- to 20-year-old non-smoking students, the one-third who lived with at least one smoker were more likely to describe their own school performance as “poor.”

Of students who said they were exposed to smoking at home at least five days a week, 23 percent said their school performance was poor compared with their classmates’. That rate was 20 percent among kids who had less frequent secondhand-smoke exposure at home, and 17 percent among those from smoke-free homes.

The researchers were able to account for certain other factors, like parents’ education levels and the type of housing - both markers of socioeconomic status. They found that students’ exposure to secondhand smoke, itself, was linked to a 14 percent to 28 percent greater risk of poor school performance, depending on how frequent the exposure was.

Dr. Sai-Yin Ho and colleagues at the University of Hong Kong report the results in the Journal of Pediatrics.

The findings do not prove that secondhand smoke was the reason for the poorer grades.

The study had a number of limitations, including its reliance on students’ ratings of their own academic performance and exposure to tobacco smoke. Future studies should include objective measures of secondhand-smoke exposure, using urine samples, as well as official school records, Ho’s team writes.

The researchers also could not account for the full range of factors that might be related to both secondhand-smoke exposure and children’s school performance. They lacked information, for example, on whether students were exposed to smoking in the womb.

Still, Ho’s team notes, it is biologically plausible that the many toxic compounds in tobacco smoke - including lead, arsenic, ammonia and hydrogen cyanide - could affect children’s cognitive abilities.

Regardless of whether secondhand smoke does hurt kids’ school performance, there are many established reasons for parents to quit smoking and limit their children’s exposure to smokers.

These findings, the researchers write, offer another potential reason for parents to “eliminate smoking at home” and warn their children to avoid secondhand exposure.

SOURCE: Journal of Pediatrics, online July 20, 2010.