TUESDAY, Jan. 4, 2005 (HealthDayNews) -- Even small amounts of secondhand smoke can sabotage a child's performance on reading, math, reasoning and logic tests, a new study finds.

Scores on standardized exams to measure cognitive function decrease in what is called a dose-response relationship found that, the more tobacco smoke a child is exposed to, the worse he or she does on the test, according to the report in the January issue of Environmental Health Perspectives.

"We were looking for links between environmental tobacco smoke and performance on cognitive testing," said study author Kimberly Yolton, an assistant professor of pediatrics at the Children's Environmental Health Center at Cincinnati Children's Hospital Medical Center.

Yolton and her colleagues collected data on 4,399 children, aged 6 to 16, who were part of the third National Health and Nutrition Examination Survey (NHANES-III). The study was conducted from 1988 to 1994 by the National Center for Health Statistics at the U.S. Centers for Disease Control and Prevention.

To determine the amount of exposure to tobacco smoke, Yolton's team measured levels of cotinine in the children's bodies. Cotinine is a substance produced when nicotine is metabolized. Cognitive and academic ability were measured using parts of standardized intelligence and achievement tests.

"We found that there was a clear, strong association between exposure to tobacco smoke and declining scores in reading, math and problem-solving. Children who had more exposure had a greater detriment in their scores," Yolton said.

"This effect remained significant even at very low levels of exposure," Yolton added. In general, there was about a three-point drop in reading scores and about a two-point drop in math scores among those exposed to the highest levels of secondhand smoke, she noted.

"A lot of people would say, 'What's three points on a reading test that has a mean of 100?' For the individual child, it may not seem like it's a big effect that we need to be concerned about," Yolton said. "But if you think about it, we estimated that somewhere around 33 million children are exposed to tobacco smoke. So if you think about 33 million children across the country losing three points, then it has a greater societal impact."

Whether these effects of secondhand smoke on cognitive skills remain throughout the child's lifetime is not clear. Yolton is not sure if passive smoke causes permanent changes in mental function by altering brain development, or if the changes are temporary.

"Children are getting exposed to tobacco smoke in places other than in their homes," Yolton said. "Parents need to pay attention to where their children are, and what they are surrounded by." While only 43 percent of the children reported being exposed to smoking at home, 84 percent had some level of exposure, she noted.

"The study does suffer from one serious limitation: There was no measure of parental intelligence," said Dr. David L. Katz, director of the Prevention Research Center at Yale University School of Medicine.

It is well-established that, other things being equal, more intelligent parents tend to have more intelligent children, and vice versa, Katz said.

"It may just be that somewhat less intelligent parents are the ones who expose their children to tobacco smoke; after all, based on evidence available long before this study, doing so simply isn't very bright. If that is the case, serum cotinine in children might simply be a surrogate marker for less intelligence in the parents."

While this possibility should be considered and studied further, the facts of the study are simple, Katz noted.

"As if we didn't already know this, children should not be exposed to tobacco smoke, period," he said. "To expose children to tobacco smoke is, for all intents and purposes, to poison them." The study has "simply clarified another of this insidious poison's likely consequences."

More information

The U.S. National Cancer Institute explains secondhand smoke and its dangers.

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