Secondhand Smoke Bad for Brain?

Feb. 12, 2009 -- People who are exposed to secondhand smoke may be more likely than their peers to have cognitive impairment, a new study shows.

The study, published in the advance online edition of BMJ (formerly called the British Medical Journal), doesn't prove that secondhand smoke exposure causes cognitive impairment. But it does show that cognitive impairment was more common among nonsmokers and former smokers with high levels of cotinine, a nicotine-related chemical, in their saliva samples.

The study included 4,809 adults 50 and older in England. They provided saliva samples and took various tests of mental skills, including memory and attention, between 1998 and 2002. Participants were considered cognitively impaired if their overall test score was in the bottom 10% of the group.

Never smokers with the highest salivary levels of cotinine were 70% more likely to be cognitively impaired than never smokers with the lowest salivary levels of cotinine. Former smokers with the highest salivary cotinine levels were 32% more likely to have cognitive impairment than former smokers with the lowest salivary cotinine levels.

The findings held despite other factors, including participants' age, sex, education, diabetes, high blood pressure, and heart disease. But the researchers -- who included David Llewellyn, PhD, of England's University of Cambridge -- note that the study also has some limits.

For instance, the researchers couldn't control for every possible factor that may have influenced the results. Also, participants weren't followed over time, so it's not clear which came first -- cognitive impairment or secondhand smoke exposure. And although cognitive impairment can take decades to develop, cotinine doesn't linger in the body very long, so it's not a marker of long-term exposure to secondhand smoke.

"This study raises the strong possibility that secondhand smoke causes cognitive decline, but further research is needed to establish a causal effect," Mark Eisner, MD, MPH, an associate professor of medicine at the University of California, San Francisco, writes in an editorial published with the study.