In a large, long-term study, British researchers found that while there seems to be no link between cognitive decline and smoking in women, in men, the habit is linked to swifter decline, with early dementia-like cognitive difficulties showing up as early as the age of 45.

The research adds to an already large body of evidence about the long-term dangers of smoking -- a habit the World Health Organisation refers to as "one of the biggest public health threats the world has ever faced."

Smoking causes lung cancer, which is often fatal, and other chronic respiratory diseases. It is also a major risk factor for cardiovascular diseases, the world's number one killers.

"While we were aware that smoking is a risk factor for respiratory disease, cancer, and cardiovascular disease, this study shows it also has a detrimental effect on cognitive ageing and this is evident as early as 45 years," said Severine Sabia of University College London, who led the study and published it in the Archives of General Psychiatry journal.

In an interview she said one explanation for the gender difference found in this study might be the larger amount of tobacco smoked by men, or the fact that there was a significantly lower proportion of women than men among those involved in the research.

Sabia's team looked for possible links between smoking history and cognitive decline in the transition from midlife to old age using data from 5,099 men and 2,137 women who are involved in a large research project called the Whitehall II study, which is based on employees of the British Civil Service.

The average age of those taking part was 56 when they had their first cognitive assessment.

The study used six assessments of smoking status over 25 years and three cognitive assessments over 10 years, and found that smokers showed a cognitive decline as fast as non-smokers 10 years older than them.

"A 50 year old male smoker shows a similar cognitive decline as a 60 year old male never smoker," Sabia explained.

She also found that men who quit smoking in the 10 years before the first cognitive testing point were still at risk of greater cognitive decline, especially in executive function -- which covers various complex cognitive processes involved in achieving a particular goal.

Long-term ex-smokers, however, did not show a faster decline in their brain functions or cognitive abilities.

Sabia said more research is now needed to find out why there was a difference between men and women in this study, and to look into possible mechanisms that might link declining brain function to smoking.

(Reporting by Kate Kelland, editing by Paul Casciato)

• A Permanent Economic Emergency