By now, most Americans know that smoking cigarettes is a dangerous proposition, one that can leave them vulnerable to cancer, heart disease, lung disease, and other ailments. Most Americans also know that secondhand smoke is bad for them, too. Several recent studies highlight just how bad.

Researchers from New Zealand provide new evidence that secondhand smoke can be deadly. In the April 5 online edition of BMJ, they report that people who have never smoked but live with a smoker are more likely to die prematurely than nonsmokers who live with other nonsmokers. After taking into account factors like age, ethnicity, marital status and socioeconomic status, the researchers found that nonsmokers exposed to secondhand smoke at home had a death rate about 15% higher than that of nonsmokers whose home air was clean.

"The results of 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," wrote coauthor Tony Blakely, MBChB, MPH, PhD, senior research fellow at Wellington School of Medicine and Health Sciences.

The finding comes on the heels of a European study that quantified the risk of lung cancer faced by nonsmokers exposed to secondhand smoke at home, work, and in social settings. Researchers from the International Agency for Research on Cancer analyzed the results of two large studies on 1,263 lung cancer patients who had never smoked.

They determined that exposure to secondhand smoke at home raised lung cancer risk by 18%; long term-exposure at home -- 30 years or more -- raised the risk by 23%. Inhaling secondhand smoke at work or in social settings raised lung cancer risk by about 16% (and by as much as 27% when exposure lasted 20 years or more). They published their results in the International Journal of Cancer (Vol. 109, No. 1: 125-131).

Findings like these come as no surprise to public health and tobacco control experts.

Secondhand smoke contains some 4,000 chemicals, more than 60 of which are known or suspected to cause cancer. The US Environmental Protection Agency classifies secondhand smoke as a Group A carcinogen (cancer-causing substance), right up there with arsenic, asbestos, and benzene.

The dangers of secondhand smoke were suspected as early as the 1960s, said Tom Glynn, PhD, senior director of International Tobacco Control for the American Cancer Society. But it wasn't until the 1970s and '80s that concrete data of its harmful effects began to accumulate.

The EPA now estimates that, in the US alone, secondhand smoke causes some 3,000 lung cancer deaths in nonsmokers every year.

As awareness of the threat has increased, so have efforts to curb public exposure to it. But merely separating smokers from nonsmokers may not be sufficient, as a recent study in the journal Tobacco Control (Vol. 13, No. 1: 17-22) showed.

Researchers from Australia's University of New South Wales compared levels of tobacco pollutants in smoking and nonsmoking areas of Sydney gaming clubs. Levels of nicotine and smoke particles were about 50% lower in the nonsmoking areas overall. Nonsmoking areas that were separate rooms had slightly cleaner air than non-smoking areas that were just sections of a larger room where smoking was allowed.

The finding shows that simply keeping smokers separate doesn't do enough to protect nonsmokers from secondhand smoke, the authors concluded. Only an outright smoking ban will do, they wrote.

ACS and other health organizations support efforts to enact smoke-free policies on a local and state level. Currently, some 1,703 communities in the US have laws restricting smoking in workplaces and public facilities.

"It's a public health issue, pure and simple," said Glynn. "We know [secondhand smoke] makes people ill, and even kills them, and we have an obligation to do all we can to reduce that."