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 will be published by the BMJ online on Friday 13th February 2009.
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 team examined saliva samples from almost 5000 non-smoking adults over the age of 50, using data from the 1998, 1999 and 2001 waves of the Health Survey and England. The participants subsequently took part in the English Longitudinal Study of Ageing.
The saliva samples were tested for cotinine, a product of nicotine that remains in the saliva for about 25 hours after exposure to second-hand smoke. Those who took part in the study also provided a detailed smoking history, and those who had never smoked, or who were previous smokers, were assessed separately.
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.
The researchers believe that the link between second-hand smoke and cognitive impairment could be explained by the fact that heart disease increases the risk of developing dementia, and that exposure to second-hand smoke is known to cause heart disease.
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 behaviour 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."