People regularly exposed to secondhand smoke may have increased risks of dying from various causes, a long-term study from China suggests.
Researchers found that compared with adults who lived and worked in smoke-free environs, those exposed to secondhand smoke were more likely to die of heart disease or lung cancer over 17 years.
And they were also more likely to die of stroke or the lung disease emphysema -- two diseases that have had relatively weaker links to secondhand smoke.
The findings, which appear in the medical journal Chest, cannot definitively prove that secondhand smoke is the culprit. But the researchers were able to account for some other key factors, like a person's age, education, job, and blood pressure and cholesterol levels.
And the links between secondhand smoke and mortality remained, say the researchers, led by Dr. Yao He of Chinese PLA General Hospital in Beijing.
"This is exactly the type of study design you want to see," said Joanna Cohen, director of the Institute for Global Tobacco Control at Johns Hopkins Bloomberg School of Public Health in Baltimore.
Cohen, who was not involved in the research, pointed out that the study followed people over many years, and it found evidence of a "dose-response" relationship -- meaning people's risks climbed as their secondhand smoke exposure increased.
Those things are considered key in building the case for a cause-and-effect relationship.
A number of studies have found that non-smokers who regularly breathe in other people's tobacco smoke have an increased risk of developing heart disease or certain cancers, including lung tumors.
In the U.S., the most recent Surgeon General's report said there was "suggestive" evidence that secondhand smoke might boost people's risk of stroke and emphysema, also known as chronic obstructive pulmonary disease or COPD.
But the evidence was considered insufficient to say there was a "causal relationship," Cohen noted.
"This type of study," she said, "is important for adding to evidence of a causal relationship."
Cohen also said it was "huge" that the information was coming from China. "It's the country with the most number of smokers," she pointed out. And, she said, it is trailing other nations in anti-smoking education and tobacco control.
The current findings are based on 910 adults who were followed over almost two decades.
At the start, 44 percent said they lived with a smoker, while 53 percent said they inhaled secondhand smoke at work.
Over the following years, 249 study participants died. And the risks of death from heart disease, stroke, lung cancer and emphysema were all two to three times higher among people exposed to secondhand smoke.
Among men, for example, 11 percent of the 271 men exposed to secondhand smoke died of stroke. That compared with 6.5 percent of the 168 men who lived and worked in smoke-free surroundings.
The numbers of people who died of each specific cause were fairly small, which is a limitation.
"When numbers get small," Cohen said, "it makes it more difficult to get a precise estimate" of risks.
But she said the results do support evidence that secondhand smoke may boost the risks of not only heart disease and certain cancers, but stroke and emphysema as well.