Environmental tobacco smoke is generated from the sidestream (the burning end) of a cigarette, pipe or cigar or from the exhaled mainstream (the smoke puffed out by smokers) of cigarettes, pipes, and cigars.
Environmental tobacco smoke was classified as a "known human carcinogen" by the US government in 2000, based on the causal relationship observed between passive exposure to tobacco smoke and human lung cancer and based also on studies that have conclusively shown an increased risk of lung cancer in nonsmoking women living with smoking husbands or working with smoking co-workers.
To be more concrete, if someone in your house or restaurant or office or anywhere around you smokes, you are smoking, too. You, too, can get lung cancer and the other diseases now known to be associated with smoking. And you, too, are running the same increased risks to become sick and also, incidentally, to die.