Experts have long agreed secondhand smoke causes cancer, but how much so has been a question that has remained hazy for equally as long.

A new study, however, begins to bring into detailed focus some of the cancer risks that nonsmokers face when exposed to passive smoking. Working with data from two large studies in Europe and the United States, researchers have found nonsmokers exposed to secondhand smoke had a lung cancer risk 18 percent to 32 percent higher than those not exposed, with risks increasing proportionate to the length of exposure. "We pooled them together to try to look at two things: to try to look at long-term exposure, and to try to get out some of the other factors out, such as dietary and occupational factors," says lead researcher Paul Brennan, of the International Agency for Research on Cancer. "What this study has been able to show clearly is the more exposed one is to passive smoke, the greater the lung cancer risk." In the study, Brennan and his researcher team measured lung cancer risk from 1,263 nonsmoking lung cancer patients, as well as 2,740 control subjects, about their exposure to secondhand smoke -- from a spouse, at work and in social settings. Nonsmokers whose spouses had ever smoked had an increased lung cancer risk of 18 percent. For those who lived with smoking spouses for more than 30 years, the increased risk was 23 percent. Those exposed to smoke in the workplace had an increased lung cancer risk of 13 percent. For those exposed for more than 21 years, the risk jumped to 25 percent, the study found. In social settings, those exposed to secondhand smoke saw their lung cancer risk rise by 17 percent. For those exposed for 20 or more years in social settings, the risk was 26 percent above that of never-exposed nonsmokers. Nonsmokers exposed to the most secondhand smoke from all sources combined had the highest levels of increased risk for lung cancer, or 32 percent when exposure was long term. The study found no evidence that other measured risks, including diet and occupation, had an effect on lung cancer risk. "What the pooled analysis does is confirm the earlier ones, and it gives a more precise estimation of the risks. This is useful and important in reducing the level of uncertainty, i.e. the range of estimates, in the calculated risk," says David Phillips, a professor at England's Institute of Cancer Research. "Given that many millions of nonsmokers are exposed to passive smoking at work, the impact on public health is very significant." The United States has led the way on the issue of exposure to cigarette smoke, with laws, and lawsuits, led by California in the 1990s. That trend has begun to reach Europe's smoky cafe culture, where secondhand smoke is being increasingly seen as a costly public health problem. But governments efforts to reduce nonsmokers' exposure in Europe face fierce resistance from the tobacco industry. In the Netherlands, for example, the restaurant and hospitality industry, a sector with perhaps the highest exposure to secondhand smoke, was recently granted an exemption from new workplace antismoking laws set to go into effect in 2004, according to a spokesman for the Ministry of Health. "Reducing the opportunities for smokers to do so in public is likely to lead to a reduction in smoking overall," says Phillips. "The tobacco industry is bound to resist such moves, which will further undermine the already diminishing social acceptability of smoking." "The point is that now there is good health evidence to support such moves, not just the 'smoking is unpleasant for nonsmokers' argument," he adds.

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