Kids and teenagers exposed to even trace amounts of secondhand smoke score lower on tests of reading and reasoning, according to new research.

Overall, up to 33 million children and teenagers in the U.S. may be exposed to enough secondhand tobacco smoke to affect their reading ability, making this a huge public health issue, study author Dr. Kimberly Yolton of the Cincinnati Children’s Hospital Medical Center in Ohio told AMN Health.

“That’s an enormous amount of children,” she said in an interview. “We really need to do a better job of making sure they have clean air to breathe, so they can reach their fullest potential.”

During the study, Yolton and her colleagues asked 4,399 kids between the ages of 6 and 16 to complete reading, math and reasoning tests. The researchers also checked their blood for cotinine, a substance created when the body breaks down nicotine, thereby serving as a marker for exposure to tobacco smoke.

Cotinine scores were typically higher in African-American kids, and in those who shared a home with at least one smoker.

The researchers found that children with more cotinine in their blood tended to score lower on the reading, math and reasoning tests. And the higher the cotinine levels, the lower were their scores, the authors note in the journal Environmental Health Perspectives.

Moreover, even trace amounts of cotinine in blood appeared to lower kids’ test scores in reading and reasoning.

Yolton explained that the amount of secondhand smoke kids inhale depends on many variables, including ventilation and how close they are to smokers, making it difficult to equate cotinine levels with a specific number of cigarettes smoked.

However, she said that she and her colleagues saw decreases in reading scores from cotinine levels associated with living in a house with a smoker who consumes less than one pack per day.

Just why tobacco smoke may influence kids’ test scores is also unclear, she added. Research in animals suggests that smoke can alter the structure of the nervous system, Yolton said. It also makes sense that breathing tobacco smoke may deprive kids of oxygen, she noted.

“When we breathe (cigarette smoke) in, it takes the place of oxygen that we need to let our brains function well,” Yolton said.

Although kids are mostly exposed to cigarette smoke at home, older kids spend more time outside of the home, and parents who want to limit kids’ exposure to secondhand smoke should find out if their friends smoke, or if they hang out in smoky places, the researcher noted.

“When they leave the house, we have to think about ways to protect them,” she said.

SOURCE: Environmental Health Perspectives, January 2005.