By now, you probably know that you should stop smoking. But that doesn't stop journals from publishing more studies about the ill effects of secondhand smoke. This study is particularly interesting because, unlike many other studies that ask people to think back over the years and remember how much they (and others around them) smoked, it asked people about their exposure and then followed them over time to look for cancer and other lung diseases.

What the researchers wanted to know: What's the link between secondhand smoke and respiratory diseases, including cancer?

What they did: The researchers looked for cancers in about 123,000 nonsmokers and former smokers (people who'd stopped smoking at least 10 years before the study started). The people filled out questionnaires between 1993 and 1998, when they first joined a large European study on cancer. The researchers used cancer registries and health insurance records to follow up on them, or they kept up with the people and their families directly. By the time this analysis was done, the people had been followed up for a median of seven years. The researchers looked at each person diagnosed with respiratory diseases and compared their exposure to secondhand smoke and pollutants with that of people who matched them by sex, age, and other characteristics. For this study, "respiratory disease" includes diagnoses of lung, mouth, and other cancers and deaths from chronic obstructive pulmonary disease (such as emphysema).

What they found: Former smokers were at a higher risk of respiratory diseases if they were exposed to secondhand smoke. The researchers didn't find a significant increase in the risk of respiratory diseases for people who'd never smoked and were exposed to secondhand smoke at the time the study started, although the data did point in that direction. The study wasn't entirely prospective; researchers also asked some people to recall their childhood exposure to tobacco smoke. Being exposed to secondhand smoke for several hours a day in childhood increased the risk of lung cancer about 3.5 times.

What the study means to you: This research suggests that secondhand smoke has negative effects. The researchers might get more conclusive results with longer follow-up.

Caveats: The information on smoke exposure was pretty limited—the people in the study filled out the questionnaires only once, when the study started, and they weren't asked how much smoke they were exposed to.

Find out more: A little information on secondhand smoke from the Environmental Protection Agency

This study used the people enrolled in the EPIC study, for European Prospective Investigation into Cancer and Nutrition.

Read the article: Vineis, P. et al. "Environmental Tobacco Smoke and Risk of Respiratory Cancer and Chronic Obstructive Pulmonary Disease in Former Smokers and Never Smokers in the EPIC Prospective Study." British Medical Journal. Published online Jan. 28, 2005.

Article online: http://bmj.bmjjournals.com