A 30-year study of "light" smokers has shown that people who only light up with a drink in the evening still run a sharply increased risk of dying early.

Previous research has shown that, although tobacco smoke has a long- term effect on the lungs, which can result in lung cancer, it has an immediate effect on the blood, which can cause heart disease and heart attacks.

Researchers from Norway, who monitored the smoking habits of 43,000 people from the mid-1970s to 2002, found that those who smoked one to four cigarettes a day - defined as "light" smokers - had three times the risk of heart disease compared with those who had never smoked.

Men who were light smokers were three times as likely to die of lung cancer but, among women, the risk was five times higher than among women who had never smoked.

Overall, light smokers had a 50 per cent higher risk of dying from any cause than non-smokers and the risk rose with the number of cigarettes smoked.

The steepest increase in risk was between one and four cigarettes a day. Above that level the risk rose more slowly. Writing in the medical journal Tobacco Control, the researchers blame the tobacco industry for fostering the notion since the 1970s that smoking a few cigarettes a day is not harmful to health.

Surveys in Norway in the 1990s showed that one-third of the population believed that light smoking was safe.

"The results from this and other studies imply that smoking control policymakers and health educators should emphasise more strongly that light smokers are also endangering their health."

Controversy has surrounded the effects of light smoking for more than a decade. Scientists have been puzzled by the disproportionate risks associated with passive smoking, which may mimic light smoking. A non-smoker who lives with a person who smokes 20 cigarettes a day has one-third of the risk of their partner even though they are actually exposed to only 1 per cent of the smoke, equivalent to one cigarette every five days.

Laboratory evidence suggests that is because the effect on the blood of toxins in tobacco smoke peak at low levels of exposure, increasing its stickiness (the tendency of the platelets to aggregate) and inflaming the arteries, increasing the risk of thrombosis - a blood clot that triggers a heart attack. The result is that the risk of heart problems increase rapidly for people who smoke one to five cigarettes a day but then rises more slowly as smoking increases to 20 cigarettes a day.

The finding will add to concern about the Government's proposal to exempt pubs that do not serve food from the complete smoking ban that is due to come into effect in 2008.

Campaigners including the British Medical Association and the Royal College of Physicians have called for a complete ban on smoking in all enclosed public places, including pubs.

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