Exposure to second-hand smoke could increase the risk of developing dementia and other forms of cognitive impairment, according to research published today on bmj.com.

A possible link between active smoking and cognitive impairment has already been established and previous findings also suggest that second-hand smoke exposure could be linked to poor cognitive performance in children and adolescents. However, this is the first large-scale study to conclude that second-hand smoke exposure could lead to dementia and other neurological problems.

The authors, Dr David Llewellyn and his research team from the University of Cambridge, Peninsula Medical School and the University of Michigan, 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 for England (HSE). The participants had also taken part in the English Longitudinal Study of Ageing (ELSA).

The samples were tested for cotinine - a product of nicotine that can be found in saliva for about 25 hours after exposure to second-hand smoke. Participants in the study also provided a detailed smoking history. Never smokers and previous smokers were assessed separately.

The researchers used established neuropsychological tests to assess brain function and cognitive impairment. These focused on memory function, numeracy and verbal fluency – for example naming as many animals in a minute. The results of the tests were added together 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.

The authors argue that the link between second-hand smoke and cognitive impairment could be explained given that heart disease increases the risk of developing dementia and second-hand smoke exposure is known to cause heart disease.

In an accompanying editorial, Dr Mark Eisner from the University of California, says that while the serious negative health effects of second-hand smoke like 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.

He writes: “Emerging evidence suggests that parental smoking may impair childhood cognitive development. 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 concludes by hoping that greater public awareness about the dangers of second-hand smoke, especially 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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BMJ-British Medical Journal