Researchers from the Peninsula Medical School, the University of Cambridge and the University of Michigan have published the results of the first large-scale study to indicate that second-hand smoke exposure could lead to dementia and other neurological problems. The results were published by the BMJ online on Friday 13th February 2009.

The research highlighted a 44% increase in risk of cognitive impairment when exposed to high levels of second-hand smoke.

Research has already identified possible links between active smoking and cognitive impairment, and previous findings have suggested exposure to second-hand smoke is linked to poor cognitive performance in children and adolescents. However, this is the first study of its kind to link second-hand smoke exposure to cognitive impairment in adult non-smokers.

The research, led by Dr. Llewellyn of the University of Cambridge, used saliva samples from nearly 5000 non-smoking adults over the age of 50. By measuring levels of cotinine (a by-product of nicotine) in their saliva and taking a detailed smoking history, the researchers were able to assess levels of exposure to second-hand smoke.

Established neuropsychological tests were used to assess brain function and cognitive impairment. These focused on memory function, numeracy and verbal fluency. The test results were added together to provide a global score for cognitive function. Those whose scores were in the lowest 10 per cent were identified as suffering from cognitive impairment.

From their results they concluded that exposure to second-hand smoke may be linked to an increased chance of developing cognitive impairment, including dementia. The authors proposed a number of possible explanations for why exposure to second-hand smoke may increase the odds of dementia, including an increased risk of heart disease and stroke which are known to increase the risk of cognitive impairment and dementia.

Dr. Llewellyn commented on the research, "We have conducted the first study to examine the association between second-hand smoke exposure and cognitive impairment in elderly non-smokers.

"Our results suggest that inhaling other people's smoke may damage the brain, impair cognitive functions such as memory, and make dementia more likely. 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. Our findings also support calls to ban smoking in public places."

Dr. Iain Lang from the Peninsula Medical School, who worked on the study, commented: "This is the first time that anyone has used biological measures of exposure to second-hand smoke to show that passive smoking is bad for the human brain. While the ban on smoking in public places has gone some way to mitigate this problem, there is still a risk from smoking at home. We hope that our findings will encourage smokers to change their behavior in order to reduce the risk to others."

He added, "These findings are of enormous public health importance for two reasons. First, a lot of people smoke cigarettes and it's important that they are fully aware of the harm they do to themselves, and others, by smoking. Second, the rapidly growing numbers of older people in the population, and the escalating cost of caring for people with dementia and related problems, mean that it's crucial to identify and take action on factors that can reduce the risk of developing cognitive problems. Encouraging people to quit smoking - or not to take it up in the first place - may be one way to achieve this."