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:

• 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.

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