Secondhand smoke is a mixture of the smoke given off by the burning end of a cigarette, pipe, or cigar, and the smoke exhaled from the lungs of smokers. This mixture contains more than 4,000 substances, more than 40 of which are known to cause cancer and many of which are strong irritants. Exposure to secondhand smoke is sometimes called involuntary smoking, or passive smoking.

Secondhand smoke is a known cause of cancer in humans and also causes heart disease and stroke. Passive smoking is estimated to cause more than 700 deaths in Oklahoma among nonsmokers each year. It also causes irritation of the eyes, nose, throat, and lungs. Secondhand smoke-induced irritation of the lungs leads to excess phlegm, coughing, chest discomfort, and reduced lung function.