Environmental tobacco smoke is generated by the combustion of tobacco products. It is composed of side stream smoke (SS), emitted from the smoldering tobacco between puffs, and exhaled mainstream smoke (MS) from the smoker. We

When a cigarette is smoked, roughly half of the smoke generated is SS and the other half MS. Environmental tobacco smoke (ETS) is generated by the combustion of tobacco products. It is composed of side stream smoke, emitted from the smoldering tobacco between puffs, and exhaled mainstream smoke from the smoker. 

Environmental tobacco smoke, side stream smoke and mainstream smoke are complex mixtures of over 4000 compounds. These include more than 40 known or suspected human carcinogens, such as: 4-aminobiphenyl, 2-naphthylamine, benzene, nickel, and a variety of polycyclic aromatic hydrocarbons (PAHs) and N-nitrosamines.

Also present are a number of irritants, such as: ammonia, nitrogen oxides, sulfur dioxide and various aldehydes. Cardiovascular toxicants present include: carbon monoxide, nicotine and some PAHs, are also present.

Many carcinogens and other toxicants are generated in greater amounts in side stream smoke than in mainstream smoke. For example, Nnitrosodimethylamine, a potent animal carcinogen, is emitted in quantities 20–100 times greater in side stream smoke than in mainstream smoke, and SS:MS ratios of about 7–30 for the known human carcinogens 4-aminobiphenyl and 2-naphthylamine have been measured.