British and U.S. researchers said on Friday that passive smoking can significantly raise a person’s risk of dementia and other cognitive problems to the extent that people who live with a smoker stand a higher risk of developing Alzheimer’s.

The study published in the British Medical Journal is the first to find a link between passive smoking and mental decline. David Llewellyn of Britain's University of Cambridge, who led the study said, "Our results suggest that inhaling other people's smoke may damage the brain, impair cognitive functions such as memory, and make dementia more likely."

Researchers said the people who had the highest exposure to second hand smoke were found to have a 44 % increased risk as compared to those who were not exposed at all.

The study was conducted by researchers at Cambridge University and Peninsula Medical School, Exeter.  The researchers analysed data from three separate British health surveys of 4,800 people non smoking adults aged 50 or more.  Saliva samples were tested for levels of cotinine, which are a marker of nicotine exposure and found in the saliva for about 25 hours after exposure to second hand smoke. The adults were then divided into groups based on the cotinine levels.

Tests to assess brain function which involved verbal memory like recalling words immediately and after a while, as well as numerical calculations, time orientation and verbal fluency were given to the people. The researchers reported that the people who were exposed to high levels of second hand smoke stood a 44 % increased risk of cognitive impairment.   

The increased risks of lung cancer and heart disease for passive smokers is a fact earlier research has substantiated, and previous studies have also identified smoking to increase the risks of dementia and other forms of cognitive impairment. The link of second hand smoke causing dementia and other cognitive impairments has been shown for the first time.

The researchers said, "Given the ongoing international policy debate on exposure to second-hand smoke, this is a topic of major public health significance."

The researchers said one possible explanation could be the fact that exposure to second hand smoke increases the risk of heart disease and stroke, both of which are known to increase the odds of dementia and other cognitive problems.

Dr Llewellyn, said it was well established that smoking can increase the risk of Alzheimer's and other forms of dementia, 'But we found a big increase in risk to non-smokers exposed in the workplace to other people's smoke, or at home,' he said.

'It was quite shocking because potentially millions are at risk.'

According to the World Health Organization more than half of children worldwide are exposed to second-hand smoke in their homes. Dr Llewellyn said, 'Given that passive smoking is also linked to other serious health problems such as heart disease and stroke, smokers should avoid lighting up near anyone who doesn't smoke, not just children and babies.'