SMOKERS risk damaging their intelligence, suffering memory loss, slower reflexes and impaired problem solving.

Scientists have, for the first time, linked smoking with impaired mental agility in a study which shows that the habit can reduce a person’s IQ by up to 3%.

They claim this dip is enough to affect smokers’ quality of life, making them less efficient at performing everyday tasks such as counting change or doing crossword puzzles. The study also appears to contradict an earlier report which suggested that smoking might help protect people from dementia.

While smoking cigarettes is known to cause cancer, heart disease, birth defects and impotence, its impact on intelligence has been less clear.

Scientists at Aberdeen and Edinburgh universities contacted 413 people who had taken part in the Scottish national IQ survey in 1947, when they were aged 11. The subjects were tested again in 2000-02, when they were aged 64. The two sets of results, across a range of standard IQ measurements, including memory, learning and reasoning, were then compared.

Factors that might have skewed the results, such as alcohol consumption, occupational status, level of education, heart disease and hypertension, were screened out of the results.

Researchers discovered that the IQ scores of smokers were up to 3% lower than those of former smokers and non-smokers. Their psychomotor speed — co-ordination and dexterity — was also lower.

On average, the IQ of smokers showed a steeper decline over the 53 years than those of non-smokers. When lung function, which is known to have an effect on the working of the brain, was taken into account, the IQ scores of smokers were 4% lower.

The study, published in the current edition of the scientific journal Addictive Behaviors, found no significant difference between the IQs of ex-smokers and non-smokers.

“Our findings are significant because they show for the first time the long-term effect of smoking on cognitive ability,” said John Starr, of Edinburgh University’s department of geriatric medicine.

“The impact of smoking on the IQ of the people involved may appear small but it will impair their quality of life. They will experience niggly problems with any task that requires some sort of mental agility — whether that’s organising their daily life, remembering what to buy at the shops, doing crossword puzzles or playing bridge.

“The people involved in our research are still relatively young, they are not in their seventies or eighties when you would expect to see a noticeable effect.

“The impact of smoking on cognitive ability may be even higher than our study suggests because we excluded socioeconomic factors. It is known that people from a lower social class smoke more.”

Starr believes smoking may reduce IQ by damaging the blood vessels that supply the brain and releasing toxic chemicals into the bloodstream. It is known that a large number of cases of dementia involve damage to blood vessels supplying the brain. “Most people who have Alzheimer’s disease have some kind of vascular disease and that may relate to smoking,” said Starr.

Phil Hanlon, professor of public health at Glasgow University and an adviser to the Scottish executive, said: “What is elegant about this study is that IQ was measured using a standard mechanism across the whole population.

“The researchers have drawn on unique historical records and have had a long enough follow-up period to provide a picture of a genuine population as it has aged.”

Hanlon added that he was particularly concerned by the study’s implication that smoking may be a cause of dementia.

“This rings alarm bells. Do people with lower IQs have a greater chance of Alzheimer’s disease? I think they probably do. These people will also be at greater risk of multi-infarct dementia, where lots of little strokes eat away at the brain’s capacity. Has some of what we have seen in the way of dementia been due to smoking in the past and we have not been able to establish this? It could well be the case.”

Maureen Moore, chief executive of the anti-smoking group ASH Scotland, said: “This is an extremely interesting and valuable piece of research which confirms the damage that tobacco does to smokers.

“We know smoking is a killer through heart disease, cancer and respiratory disease. This study shows that the way smoking interferes with metabolic processes can affect IQ as well. We hope this will be seen as another piece of evidence as to why people shouldn’t smoke.”

Times Recommends

• Please help: maimed children of war want their lives back
• Unlocking the cruellest prison
• Prince Charles named in £81m court battle