Their report published in the British Medical Journal found a 44 per cent increased risk for people exposed to high levels of second-hand smoke, and is the first large-scale study to show the association between the two.

"Our results suggest that inhaling other people's smoke may damage the brain, impair cognitive functions such as memory, and make dementia more likely," David Llewellyn of Britain's University of Cambridge, who led the study, said in a statement.

Research has tied passive smoking to a range of conditions including lung cancer and heart disease. More than half of children worldwide are exposed to second-hand smoke in their homes, according to the World Health Organization.

Previous studies have also identified smoking as something that increases the chances of dementia and other forms of cognitive impairment but it was not as clear whether the same held true for second-hand smoke.

Llewellyn and colleagues examined saliva samples from nearly 5,000 non-smoking adults over the age of 50 using data from three separate British health surveys.

Then they tested the saliva for cotinine – a product of nicotine found in saliva for about 25 hours after exposure to second-hand smoke – to measure exposure to cigarettes.

Tests aimed at gauging brain function such as verbal memory and keeping track of time showed an association between exposure to second-hand smoke and cognitive impairment.

One possible explanation is that exposure to second-hand smoke increases the risk of heart disease and stroke, conditions known to boost the odds of dementia and other cognitive problems, the researchers said.