We all are fairly aware of the perils of smoking: emphysema, heart and vascular diseases, stroke, mouth and lung cancers, and possible adverse effects on the unborn children of smoking parents. Currently smoking-related ailments kill 5.4 million people a year and the number is growing rapidly, especially in poor nations where tobacco is under taxed and there are no mandated health warnings on tobacco products.

What you don't know is that a new study, led by Severine Sabia at France's National Institute of Health and Medical Research, points a finger at middle-aged smoking as a contributor to memory loss-which may be a predicator to dementia-by damaging blood vessels in the brain.

"Dementia is rare among middle-aged people, but cognitive function at this age is closely related to dementia," said Sabia, "Our results show that smoking is associated with poorer cognition and a decline over five years".

Sabia and colleagues studied nearly 5,400 people who were part of a survey of more than 10,000 British civil servants aged 35 to 55 started in 1985. Test subjects completed memory, reasoning, vocabulary and verbal fluency tests at varying intervals during a 17-year period and were questioned about their smoking habits. The research found that smokers scored in the lowest 20 percent of all subjects on memory test, verbal ability and mathematical reasoning-compared to people who had never smoked. While ex-smokers were 30 percent less likely to have poor vocabulary and low verbal frequency scores than current smokers. Ex-smokers also reported an overall healthier life style-more exercise, less alcohol consumption, and better eating habits.

Although the findings, published in the Archives of Internal Medicine, add some weight to the theory, the evidence is far from conclusive-perhaps people who never smoked or quit smoking have better overall health habits, they may be better educated, and may have a higher level of cognitive functioning to begin with. Also many smokers had died or did not participate in follow-up tests over the course of the study and data collected was from so-called white-collar civil servants which may not represent the general population.

Dr. Norman H. Edelman, scientific consultant to the American Lung Association, said: "This study is generally a confirmation of previous work. But there is a fundamental question: Are they stupid because they smoke or do they smoke because they are stupid?"

"With the ageing population and the projected increases in older adults with dementia, it is important to identify modifiable risk factors," Sabia said. "Our results suggest that smoking had an adverse effect on cognitive function. Thus, public health messages should target smokers at all ages."