Sidestream smoke is smoke which goes into the air directly from a burning cigarette, cigar or smoking pipe.Sidestream smoke is the main component of Second-hand smoke (SHS), also known as Environmental Tobacco Smoke (ETS) or passive smoking  The chemical constituents of sidestream smoke are different from those of directly inhaled ("mainstream") smoke. Evidence has shown that sidestream smoke may be more harmful than mainstream smoke. Sidestream smoking has been classified as a Class A carcinogen by the U.S. Environmental Protection Agency.
Sidestream smoke is made up of many components. Some of the constituents of which are carbon monoxide,tar, nicotine, ammonia, benzene, cadmium and 4-aminobiphenyl. Some of the other compounds found in sidestream smoke are: vinylchloride, hydrogen cyanide, arsenic, acrolein, acetaldehyde, formaldehyde,catechol, cresol, hydroquinone, lead, methyl ethyl ketone, nitric oxide, phenol, styrene, toluene, and butane. Exposure to sidestream smoke yields higher concentrations of these compounds as well as increased concentrations of carboxyhemoglobin, nicotine, and cotinine in the blood. When comparing sidestream and mainstream condensate, side stream has 2-6 times more condensate per gram than mainstream smoke. Due to the incomplete combustion process responsible for the creation of sidestream smoke, there may be exposure to higher concentrations of carcinogens than are typically inhaled directly.