The study, one of the largest ever done, looked at deaths among people aged 45 to 74 who completed census returns in 1981 and in 1996.

Those who had never smoked but lived with people who did were 15 per cent more likely to die in the three years after each census than were those who had never smoked and lived with non-smokers.

The census returns, which included data about smoking habits, enabled researchers from the Department of Public Health at Wellington School of Medicine and Health Sciences and from the Harvard School of Public Health to make a direct comparison, linking census records to deaths recorded in mortality statistics over the following three years.

For the 1981 census, that represented more than 10,000 deaths and for the 1996 census, about 9,000 deaths. By comparing death rates among those who lived with smokers with those who lived with non-smokers, it is possible to calculate the risks of passive smoking in the home.

The method used did not allow any account to be taken of exposure to second-hand smoke elsewhere — at work, for example — but earlier studies have shown that exposure at home is the greater risk.

The results were clear for both censuses, the report in BMJ.com, the online version of the British Medical Journal, concluded. For the 1981 census, the extra risk of death was 17 per cent for men, and 6 per cent for women. For the 1996 census, the extra risk for men was 16 per cent, and for women 28 per cent.

The link persisted even after correction was made for age, ethnicity, marital status and socioeconomic background.

“The results from this study add to the weight of evidence of harm caused by passive smoking and support steps to reduce exposure to other people’s smoke — in the home and in other settings,” the researchers said.

The major flaw in studies such as this is the risk of “misclassification bias”. People may claim never to have smoked when in fact they are former smokers in whom the damage has already been done. Such errors exaggerate the apparent effect of passive smoking.

However, the results are consistent with those from other studies that show increased risks of death from passive smoke at home. There is less persuasive evidence for the effect of passive smoke at work.

Deborah Arnott, director of the anti-smoking campaigners ASH, nevertheless described the research as “a vital piece of evidence on the need for tough action against second-hand smoke”. She said: “The case for a new law to end smoking in the workplace and in enclosed public places is now overwhelming. Second-hand smoke is a killer.”

Vivienne Nathanson, head of science and ethics at the British Medical Association, said that the research confirmed that passive smoking was “a very real risk”. She said: “It is high time the Government showed some courage and leadership by banning smoking in public places.”

Ban cuts heart attacks

BANNING smoking in public places had a remarkable effect on the city of Helena, Montana, cutting hospital admissions for heart attacks by 40 per cent.

The ban lasted just six months before opponents overturned it. This allowed Professor Stanton Glantz of the University of California to compare admissions for heart attacks before, during, and after the ban.

“The observations suggest that smoke-free laws not only protect people from the long-term dangers of second-hand smoke but also may be associated with a rapid decrease in heart attacks,” he said.

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