Secondhand smoke is tobacco smoke that affects a nonsmoker. Secondhand smoke can come directly from a cigarette or other lit tobacco; it may also be exhaled by a person smoking.

Secondhand smoke is sometimes referred to as environmental tobacco smoke (ETS), involuntary smoking, or passive smoking.

Secondhand smoke contains the same cancer-causing and lung-damaging chemicals that affect smokers. Repeated exposure to secondhand smoke over many years can cause damage to a nonsmoker's lungs that is similar to that of a smoker's.

Secondhand smoke is the most important risk factor for cancer among nonsmokers, far greater than other known cancer-causing substances. Secondhand smoke is most harmful to:

• Children and spouses of smokers, or anyone who lives with a smoker.
• People who spend most of their time in confined areas that are filled with tobacco smoke.