Second-hand smoke is tobacco smoke that affects a non-smoker. Second-hand smoke can come directly from a cigarette or other lit tobacco; it may also be exhaled by a person smoking.

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

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

Second-hand smoke is the most important risk factor for cancer among non-smokers, far greater than other known cancer-causing substances. Second-hand smoke is most harmful to:

• Anyone who lives or spends time with someone who smokes.
• A developing fetus, if the person who is pregnant smokes or is around people who smoke.
• People who spend most of their time in confined areas that are filled with tobacco smoke.

Second-hand aerosol from vapes does not contain as much nicotine and other harmful chemicals as second-hand tobacco smoke does. But there is a concern about possible health risks from second-hand aerosol exposure.