Secondhand Smoke Exposure May Lower Children’s IQ by 2 to 5 Points

Newswise — Children exposed to environmental tobacco smoke (ETS)—commonly known as "secondhand smoke" —had mildly to moderately depressed scores on tests of math, reading, and visuospatial skills as compared to children who lacked such exposure, according to a study published today in the January issue of the peer-reviewed journal Environmental Health Perspectives (EHP). In the largest-ever study of its type, 4,399 children aged 6-16 years who participated in the Third National Health and Nutrition Examination Survey (NHANES III) provided blood samples and took tests of reading, math, and memory. Analysis of the NHANES III data revealed that even extremely low-level exposure to ETS may be neurotoxic.

"The range of decrement in scores is very roughly equivalent to the loss of two to five IQ points at varying levels of exposure," Kimberly Yolton, the lead author on the study, told EHP for an accompanying article about the study.

This is the first study to use a biological marker—blood levels of cotinine, a breakdown product of nicotine—to measure exposure to ETS rather than relying on data from interviews or questionnaires. Cotinine is often used to measure tobacco smoke exposure in smokers and nonsmokers alike. Children who reported using tobacco products themselves within five days of testing, or whose cotinine levels indicated they were probably active smokers, were excluded from the study.

Although children with the highest levels of cotinine scored significantly lower on tests of math, reading, and visuospatial reasoning than children with the lowest levels, the greatest cognitive deficits proportionally speaking occurred for those whose overall exposure to ETS was low. However, tests of short-term memory showed no deficits in the group exposed to ETS relative to those not exposed.

Concentrations of cotinine in the blood were significantly higher among African Americans (compared to Hispanics and non-Hispanic whites), among children of parents with a lower household income or lower educational achievement, among children living in the U.S. Midwest, and among those with higher blood lead concentrations. Consistent with earlier research, this study found that cotinine levels were significantly higher among children who lived in a home with at least one smoker. The levels increased as the number of smokers in a household increase and as the number of cigarette packs smoked per day in a household increase.

"The findings of this study confirm previous research indicating an inverse relationship with ETS exposure and cognitive outcomes. We also provide new information indicating that ETS is neurotoxic at extremely low levels," the study authors write. "We estimated that over 21.9 million children are at risk for ETS-related reading deficits. While further research is necessary to confirm these findings, this analysis along with other studies provides adequate evidence to support policy to further reduce childhood exposure to ETS."

Despite the fact that many earlier studies have found an increased risk of harm to children from ETS, more than 40% of U.S. children are still exposed in their homes.

The findings in this study are further reason to keep children away from secondhand smoke, according to Dr. Jim Burkhart, science editor for EHP. "Children who are exposed to ETS are more likely to be held back in school, do worse on tests, have more ear infections, and die from sudden infant death syndrome. Today's study is just the latest in a long list of reasons to keep children away from any sort of tobacco smoke," he said.

Yolton is with the University of Cincinnati College of Medicine and Cincinnati Children's Hospital Medical Center. Other authors include Kim Dietrich, Peggy Auinger, Bruce P. Lanphear, and Richard Hornung. The article is available free of charge at http://ehp.niehs.nih.gov/members/2004/7210/7210.html.

EHP is published by the National Institute of Environmental Health Sciences, part of the U.S. Department of Health and Human Services. EHP is an Open Access journal. More information is available online at http://www.ehponline.org/.