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Posted: February 24, 2009

Second-Hand Smoke May Cause Dementia

Breathing second-hand smoke can increase the risk of developing dementia and other forms of cognitive impairment in adults by as much as 44%, according to a large-scale international study published online by the British Medical Journal.

While previous studies have concluded that active smoking is linked to cognitive impairment and that second-hand smoke could be linked to poor cognitive performance in children, this latest research is the first sizeable study to connect second-hand smoke with dementia and other neurological problems in non-smoking adults.


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"Our results suggest that inhaling other people's smoke may damage the brain, impair cognitive functions such as memory, and make dementia more likely,” said Dr. David Llewellyn, of the University of Cambridge in England, who led the research project with colleagues from the University of Michigan and Peninsula Medical School in the UK.


“Given that passive smoking is also linked to other serious health problems such as heart disease and stroke, smokers should avoid lighting up near non-smokers,” Llewellyn reported. “Our findings also support calls to ban smoking in public places."


Llewellyn’s team examined saliva samples from nearly 5,000 non-smoking adults over the age of 50 using data from the 1998, 1999 and 2001 reports of the Health Survey for England (HSE). Each participant had also taken part in the English Longitudinal Study of Aging (ELSA).


The samples were tested for cotinine, a product of nicotine that can be found in saliva for more than 24 hours after exposure to second-hand smoke. Participants in the study also provided a detailed smoking history. Those who had never smoked and previous smokers were assessed separately.


The researchers used established neuropsychological tests to assess brain function and cognitive impairment. These focused on memory function, skills involving numbers, and verbal fluency (for example, naming as many animals as possible in one minute). The results of the tests were combined to provide a global cognitive function score.


Participants whose scores were in the lowest 10% were defined as suffering from some level of cognitive impairment, including dementia.


The researchers contend that the link between second-hand smoke and cognitive impairment might be explained by the fact that heart disease increases the risk of developing dementia and second-hand smoke exposure is known to cause heart disease.


In an accompanying British Medical Journal editorial, Dr. Mark Eisner, from the University of California, writes that while the serious negative health effects of second-hand smoke, such as cancer and premature death, have been established beyond doubt, there is still a lot to learn about the scale of illness caused by second-hand smoke.


"Emerging evidence suggests that parental smoking may impair childhood cognitive development,” Eisner wrote. “Later in life, second-hand smoke may cause cardiovascular disease and stroke, which are themselves linked to cognitive decline.


“Until now, however, the suspicion that passive smoking is bad for the adult brain had not been scientifically confirmed."


Eisner added that greater public awareness about the dangers of second-hand smoke, particularly awareness about a much-feared disease like dementia, "would eventually translate into political action aimed at passing smoke-free legislation in regions of the world where public smoking is still permitted."

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