Secondhand smoke is the smoke that comes from burning tobacco products, such as cigarettes, cigars, hookahs, or pipes, and is breathed in by people who do not smoke. It is also called environmental tobacco smoke (ETS) and is a mixture of two forms of smoke that come from burning tobacco: mainstream smoke and sidestream smoke. Mainstream smoke is the smoke exhaled by a person who smokes, while sidestream smoke is smoke from the lighted end of a cigarette, pipe, or cigar, or tobacco burning in a hookah. Secondhand smoke contains more than 7,000 chemicals, many of which can cause cancer or heart disease. There is no safe level of exposure to secondhand smoke, and even brief exposure can cause serious health problems and be deadly. Secondhand smoke can cause coronary heart disease, stroke, and lung cancer in adults who do not smoke, and it is especially harmful to young children whose bodies are still growing. Children exposed to secondhand smoke are at an increased risk for sudden infant death syndrome (SIDS), acute respiratory infections such as pneumonia and bronchitis, middle ear disease, more frequent and severe asthma, respiratory symptoms, and slowed lung growth. Most people are exposed to secondhand smoke in their homes or their places of work, and people are more likely to be exposed to secondhand smoke if they live in multi-unit housing. Completely eliminating smoking is the only way to fully protect people who do not smoke from secondhand smoke exposure.