NEW and stronger evidence that passive smoking can cause lung cancer, especially among children, has emerged from a major European study.

While not the last word on the subject, the results strengthen the belief that breathing in other people’s smoke is hazardous. In a finding bound to be seized on by campaigners, it shows that being exposed to smoke at work is worse than being exposed in the home. Some earlier studies have shown the opposite.

The results show that being exposed to passive smoke increases the risks of respiratory disease by 30 per cent, and lung cancer by 34 per cent. But these results are not statistically significant, because the study has not been running long enough to accumulate sufficient cases. It also shows that the risk to children increases rapidly with greater exposure. Those exposed to tobacco smoke at home every day for many hours are nearly four times as likely to contract lung cancer later in life as those whose parents were non-smokers.

The strength of the study, part of Epic (the European Prospective Investigation into Cancer and nutrition) is that it is prospective: it recorded people’s exposure before they developed cancer. Most earlier studies have been retrospective, asking cancer patients to look back and remember how exposed they were to passive smoking years before, an unreliable procedure.

At the start of the study, 123,479 people in ten European countries who had never smoked provided information about their exposure to passive smoke at home or at work. They were then followed for seven years, by which time 97 had developed lung cancer, 20 cancers in the larynx and pharynx, and 14 had died of chronic obstructive pulmonary disease.

Comparing the incidence of the disease against the exposure to passive smoke declared at the start of the study enabled the extra risks to be calculated.

For the entire group, the risk of lung cancer was increased by 34 per cent. But much of this increase was accounted for by “former smokers” - those who had once smoked but had given up for at least ten years. Their risks were more than doubled, suggesting that their earlier smoking had done damage that was then exacerbated by passive smoke. People who had never smoked saw their risk increase by only 5 per cent.

Men were more vulnerable than women, with a 96 per cent increased risk, against 20 per cent, possibly because there were more former smokers among the men.

Exposure to passive smoke at home had a negligible effect of only 3 per cent, while exposure at work increased the risk by 65 per cent.

The most striking results in the study, published in BMJ Online First, were for people exposed as children. These showed a clear “dose-response” relationship, with extra exposure producing extra risk. Those exposed to smoke daily had double the risk of lung cancer, while those exposed daily for many hours saw their risks increase 3.6-fold.

The study continues, and as particpants grow older and die in greater numbers its statistical power will increase. Within a few years it should be able to answer definitively what the extra risks really are.

“There are already more than 50 studies that show passive smoking increases the risks of respiratory diseases,” said Porfessor Paolo Vineis, of Imperial College London, the lead author of the study.

“Although not all our results are statistically significant, they are consistent with these earlier studies. Passive smoking clearly increases the risk of lung cancer. People should not smoke in the presence of their children.”

He believes that the greatly elevated risk in children is an index of intense exposure, rather than evidence that children are more vulnerable. But the overall results, showing that exposure at work is more dangerous, argue against this. He admits that this result is slightly surprising, but suggests it is because work exposure can be long-term.

The team carried out a second study, concentrating on those who had developed diseases and comparing them with matched controls who had not. This produced more alarming results: risks of lung cancer were increased by 76 per cent.

The discrepancy between the two ways of using the same data could be due to chance, Professor Vineis said, or it could be that in the smaller study the team had corrected better for confounding variables. If the latter is true, then the risks of passive smoking are even higher than the headline figure.

Professor Robert West, Cancer Research UK’s director of tobacco studies, said: “This study raises a terrifying spectre for smoking parents. Exposing their children to cigarette smoke not only damages the child’s airways at the time, but may lead to development of lung cancer in later life.

“As a society we recognise that non-smokers need to be protected from carcinogens when at work, but we are not doing enough to protect the most vulnerable non-smokers of all, children.”

Chris Ogden, Director of Trade and Industry Affairs for the Tobacco Manufacturers’ Association, said that he found the study weak and confusing. He added: “This is just another study that is being spun by the opponents of tobacco.”