British and U.S. researchers said on Friday that passive smoking significantly raises a person's risk of dementia as well as other cognitive problems by causing damage to the way the brain works.

The report published in the British Medical Journal said that people who were exposed to high levels of other people's smoke stood 44 % increased risk of suffering cognitive impairment which could affect memory as well as the ability to perform calculations.

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."

Although it has already been established that smoking can increase a smoker's risk of dementia this is the first large scale study to examine the effect of second hand smoke. According to the World Health Organization more than half the children worldwide are exposed to second hand smoke in their homes and passive smoking has already been linked to various conditions such as lung cancer and heart disease.

In the study Llewellyn and colleagues examined saliva samples from 4,800 non-smoking adults over the age of 50 in England. The sample of people included some who previously were smokers but quit. They were divided into four groups based on the levels of cotinine levels in their saliva which is a marker of nicotine exposure found in the saliva for about 25 hours after exposure to second hand smoke.

All the groups went through neuropsychological tests to assess aspects of brain function such as verbal memory involving recalling words immediately and after a delay, numerical calculations, time orientation and verbal fluency of naming as many animals as possible in one minute.

The results were collected together to yield a global score for cognitive function and those who came in the lowest 10 % were said to suffer from cognitive impairment. The results showed that the people who were in the highest concentration group had a 44% increased risk of suffering cognitive impairment as compared to those in the lowest group.

The researchers felt one possible explanation for the findings 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 risk of dementia and other cognitive problems.

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