Updated September 13, 2013.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Definition:
Also known as secondhand smoke, environmental tobacco smoke (ETS) is a combination of diluted sidestream smoke and mainstream smoke.

Dilution of sidestream smoke is dependent on several factors:

• room volume
• air exchange
• temperature
• humidity
• number of people in the room

Research has shown that there is no safe level of exposure to secondhand smoke. Breathing in ETS contributes to a wide variety of diseases, including heart disease and many cancers.
See also:
Secondhand Smoke Facts
Secondhand Smoke and Children