Exposure to second-hand smoke boosts the risk of dementia and other cognitive problems, even among people who have never smoked, the largest study of its kind reported Friday.

Ill effects on non-smokers of constant exposure to tobacco smoke include an increased risk of lung cancer, diabetes, cardiovascular disease, stroke and death, earlier research has shown.

As for the impact on brain function, active smoking has been found to impair the mind but the evidence for passive smoking has until now been sketchy.

Using new methods in the largest clinical trial to date, a team led by Cambridge University professor David Llewellyn found that even people who had never smoked but kept constant company with smokers performed less well in cognitive tests.

The investigation focussed on nearly 5,000 adults over the age of 50 who were former smokers or who had never smoked.

The volunteers were divided into four groups according to their exposure to passive smoking.

This was determined by saliva samples, which were tested for a by-product of nicotine called cotinine.

Cotinine lingers in the saliva for about 25 days. The higher the levels of cotinine, the higher the exposure to recent second-hand smoke.

The volunteers then took neuro-psychological tests that assessed brain function and cognitive abilities, focusing on memory and the ability to work with numbers and words.

Using the lowest cotinine group as a benchmark, the researchers found a clear and progressively stronger link between impairment in brain function and exposure to second-hand smoke.