Dr Paolo Boffetta: The longer the exposure, the greater the risks

In the biggest study ever conducted on the subject, passive smokers were found to have a 16% to 17% increased risk of developing the disease.

Although studies in America, Japan and Asia have previously found a greater link between passive smoking and lung cancer, this study makes a link for the first time between the length of exposure and the chances of getting ill.

"When we look at duration of exposure, the risks are higher," said Dr Paolo Boffetta from the International Agency for Research on Cancer (IARC) in Lyon, a branch of the World Health Organisation (WHO). "People who are exposed for 20 or 30 years had a higher risk as compared to those who were exposed to just one or five years."

Health correspondent Richard Hannaford: Report will add weight to public health campaigners

More than 2,000 people in seven European countries took part in the research over seven years. Of those, 650 people had lung cancer but had never smoked and 1,542 were non smokers.

All were asked about their exposure to smoke at home, in the workplace, in vehicles and public places up to the age of 18. The research showed no association between childhood exposure to environmental tobacco smoke and lung cancer risk.

Speculation

Media reports in March had suggested that the WHO were shelving the study because it did not indicate a stronger link. Some newspapers had indicated that the research, if eventually published, would even suggest there was no link between passive smoking and lung cancer.

But Dr Boffetta said on Wednesday that all the speculation had been inaccurate.

"It is true that it took a long time to complete the study. To conduct these international studies is complicated and a long enterprise.

"But as soon as the final results were prepared and approved by all collaborators, we submitted it for publication in the scientific literature and this is the article which is just coming out now."

He said the other studies which had suggested a stronger link may indicate something different about Europe rather than the association between environmental tobacco smoke and cancer.

"There might be some explanation for the lower risks we found in Europe. Either the type of tobacco may be different, or, which is our favourite interpretation, less people are exposed to passive smoking as compared to when the other studies were conducted [in America and Asia] 10 or 15 years ago."

Reaction

Clive Bates, director of Action on Smoking and Health (ASH), said: "We consider this a significant study because it is the largest ever undertaken in Europe and it is by a reputable organisation.

"It confirms existing evidence that there is a risk, however small, between lung cancer and passive smoking."

The research is published in the Journal of the National Cancer Institute. It is accompanied by a commentary penned by William Blot and Joseph McLaughlin of the International Epidemiology Institute in Rockville, Maryland.

They said this and other studies now provided strong evidence of the dangers of passive smoking.