Health experts have issued a first-ever international warning about the dangers of secondhand smoke, saying that so-called “passive smoking” is even more dangerous than previously thought.

Twenty-nine scientists, brought together by the World Health Organization, reviewed some 3,000 research papers on smoking and spent eight days compiling and debating the data.

Their statement about passive smoking, issued June 19, is the most definitive yet from an international organization, say experts. “This is the first time there has been a formal evaluation by scientists that has concluded that involuntary smoking causes lung cancer,” said Professor Richard Doll, one of the participants. The scientists concluded that non-smokers living with smokers have a 20 to 30 percent increased risk of developing cancer.

There are some 4,000 chemicals contained in secondhand smoke, according to a pamphlet produced by the health ministries department of Seventh-day Adventist world church. Along with various forms of cancer, the deadly smoke has been linked to heart disease, sudden infant death syndrome, and asthma.