THURSDAY, Feb. 12 (HealthDay News) -- People exposed to secondhand smoke may face as much as a 44 percent increased risk of developing dementia, a new study suggests.

While previous research has established a connection between smoking and increased risk for dementia and Alzheimer's disease, this new study is the largest review to date showing a link between secondhand smoke and the threat of dementia, the authors said.

"There is an association between cognitive function, which is often but not necessarily a precursor of dementia, and exposure to passive smoking," said lead researcher Iain Lang, a research fellow in the Public Health and Epidemiology Group at Peninsula Medical School in Exeter, England.

What's more, Lang said, the risk of impaired cognitive function increases with the amount of exposure to secondhand smoke, the findings suggest. "For people at the highest levels of exposure, the risk is probably higher," he said.

The study was published online Feb. 13 in the journal BMJ.com.

For the study, Lang's team collected data on more than 4,800 nonsmokers who were over 50 years old. The researchers tested saliva samples from these people for levels of cotinine, a product of nicotine that can be found in saliva for about 25 hours after exposure to smoke.

The study participants also took neuropsychological tests to assess brain function and cognitive impairment. These tests evaluated memory, math and verbal skills. People whose scores were in the lowest 10 percent were classified as having some level of cognitive impairment.

The researchers found that people with the highest cotinine levels had a 44 percent increased risk of cognitive impairment, compared with people with the lowest cotinine levels. And, while the risk of impairment was lower in people with lower cotinine levels, the risk was still significant.

"We know that active smoking is bad -- being a smoker is bad for your health and increases your risk of Alzheimer's. This study suggests that this is the same for passive smoking," Lang said. "We know that passive smoking is associated with an increased risk of stroke and heart disease. This is just another reason to avoid exposing other people to your smoke, and if you are not a smoker to stay away from smoking places."

Maria Carrillo, director of medical and scientific relations for the Alzheimer's Association, said this study offers more evidence of the dangers of secondhand smoke and the risk for dementia. Smoking is already recognized as a risk factor for Alzheimer's, and the risk can be extended to exposure to secondhand smoke, she said.

"There are findings that secondhand smoke can be just as detrimental as smoking itself," Carrillo said. "We recommend that people do not smoke and try to reduce their exposure to secondhand smoke as well."

Dr. Mark Eisner, an associate professor of medicine at the University of California, San Francisco, and author of an accompanying editorial in the journal, said, "This study should provide further motivation for public policy aimed at making all public spaces smoke-free."

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