Children may be at an increased risk of exposure to secondhand smoke by living in apartment complexes rather than in houses, a new study finds.

Using data from a national survey, researchers compared  levels of cotinine — a substance formed by the breakdown of nicotine that is used to test for tobacco exposure — in the blood of more than 5,000 youngsters ages 6 to 18. Seventy-three percent of the kids had been exposed to tobacco smoke, but the researchers found that the levels of cotinine were higher in children living in apartments than in those living in houses.

Smoke that creeps through ventilation systems or migrates through the walls of multi-unit housing may be the cause, the researchers said, but they couldn't be sure. None of the parents of the children in the original study said they smoked inside the home.

"Parents try so hard to protect their children from dangers, such as tobacco smoke. It's surprising to see these results and realize that too many parents have no control over whether their children are exposed to secondhand smoke in their own homes," said study researcher Dr. Karen Wilson, an assistant professor of pediatrics at the University of Rochester Medical Center.

The researchers noted they couldn't distinguish between the contributions of smoke exposure in and out of the home. Some of the cotinine levels in the blood could be from smoky air at relatives' houses or at day care facilities, they said.

Even if parents smoked outside and residue on their clothes found its way inside, this was unlikely to explain the large number of children exposed to tobacco smoke, Wilson said. She added that only 20 percent of adults smoke.

Indoor smoke

Wilson and her colleagues examined information from a national survey administered between 2001 and 2006, known as the National Health and Nutrition Examination Survey. They found about 84 percent of kids living in apartments had recently been exposed to tobacco smoke, compared with about 80 percent of those living in attached houses (such as duplexes) and 70 percent living in detached houses.

After the researchers accounted for factors that might influence tobacco exposure, such as soicioeconomic status and age, they found that children living in apartments had cotinine levels 45 percent higher than those living in detached houses.

This increased exposure was found for black and white children, but not for those of other ethnicities.

Smoking ban

Exposure to tobacco is known to increase the risk of respiratory illnesses, such as asthma, and is associated with sudden infant death syndrome, Wilson said.

"Even at the very lowest levels of exposure, children have increased risk of cognitive deficits," she told MyHealthNewsDaily.

The findings support the idea of banning smoking in multi-unit housing, the researchers said.

"In general, people who smoke are very respectful of not exposing children and non-smokers to tobacco smoke in indoor environments," said study author Dr. Jonathan Winickoff, of the MassGeneral Hospital for Children. "This research will help promote the notion that it is never acceptable to smoke indoors, even in your own unit, because the smoke gets into the bodies of children in other units."

The study will be published in the Janurary 2011 issue of the journal Pediatrics.

Pass it on: Kids who live in multi-unit appartment buildings may be exposed to more smoke than those who live in houses.

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This article was provided by MyHealthNewsDaily, a sister site to LiveScience.