Passive smoking refers to the smoke that non-smokers are breathing in from active smokers. Passive smoking means that the passive smokers, or non-smokers, are breathing in the smoke either from the burning end of the active smoker's cigarette or the smoke expelled by the active smoker. Passive smoking is also called involuntary or secondhand smoking.

Passive smoking can lead to very serious health problems such as respiratory diseases, heart disease and lung cancer. Cigarette smoke has more than 400 chemicals in it, and cigarette smoking creates air pollution in the form of passive smoking. Passive smoking is considered a preventable cause of death which has killed thousands of people exposed to cigarette smoke in homes, workplaces and/or public places.

Children are often passive smokers and receive health injuries that they would not have had if their parents did not smoke. Even a small amount of secondhand smoke can affect babies, children and others. When active smokers smoke in the home, but try to smoke near an open window, some smoke still remains in the home. Children who end up being passive smokers have been shown to have an increased risk of getting cancer later in life as well as asthma and respiratory infections.

Passive smoking is unhealthy for everyone and while active smokers choose to smoke, passive smokers do not. Passive smoke is especially bad for those with asthma, but it is unhealthy for all people as well as animals. Symptoms of passive smoking include headache, sore throat and eye irritation. Dizziness, coughing and nausea may also occur.

Mainstream smoke refers to the smoke that enters the smoker's body and is not exhaled. The smoke that enters the smoker's body and is then exhaled differs chemically since the smoke interacts with the tissues in the smoker's body before being exhaled. Mainstream passive smoking occurs when the non-smoker breaths in smoke exhaled by the smoker. This type of smoke is harmful, but sidestream passive smoking has been shown to be even more harmful.

Sidestream passive smoking refers to the non-smoker breathing in smoke from the burning end of the smoker's cigarette. Most of the smoke in a room will be of the sidestream type which has a higher concentration of chemicals than the exhaled main stream type of smoke. Cigarette smoke contains chemicals such as carbon monoxide, benzene and ammonia and many of the chemicals in cigarettes are either irritants or carciogens.