Secondhand smoke is the combination of smoke from the burning end of a cigarette and the smoke exhaled by smokers. You can be exposed to secondhand smoke in homes, cars, the workplace, and public places, such as bars, restaurants, and recreational settings. In the United States, the source of most secondhand smoke is cigarettes, followed by pipes, cigars, and other tobacco products. Secondhand smoke contains more than 7,000 chemicals. Hundreds of the chemicals are toxic and about 70 are known to cause cancer.