More simply put, it's second-hand smoke. Environmental tobacco smoke (ETS) is the combination of two forms of smoke from burning tobacco products: sidestream smoke, or smoke that is emitted between the puffs of a burning cigarette, pipe or cigar; and mainstream smoke, or the smoke that is exhaled by the smoker. When a cigarette is smoked, about one-half of the smoke generated is sidestream smoke. This form of smoke contains essentially all of the same carcinogenic agents that have been identified in the mainstream smoke, but at greater levels. Researchers estimate that ETS causes about 3,000 lung cancer deaths among nonsmokers each year.