Smoking is considered a health hazard because tobacco smoke contains nicotine, a poisonous alkaloid, and other harmful substances such as carbon monoxide, acrolein, ammonia, prussic acid, and a number of aldehydes and tars; in all tobacco contains some 4,000 chemicals. In 1964 definitive proof that cigarette smoking is a serious health hazard was contained in a report by the Surgeon General's Advisory Committee on Health, appointed by the U.S. Public Health Service. The committee drew evidence from numerous studies conducted over decades. They concluded that a smoker has a significantly greater chance of contracting lung cancer than a nonsmoker, the rate varying according to factors such as the number of cigarettes smoked per day, the number of years the subject smoked, and the time in the person's life when he or she began smoking. Cigarette smoking was also found to be an important cause of cancers of the esophagus, nasopharynx, mouth, larynx, kidney, and bladder as well as a cause of chronic obstructive pulmonary disease, emphysema, and heart disease, stroke, and other cardiovascular diseases. Since then it has been found to be an independent risk factor in male impotence. Smoking also increases risks associated with oral contraceptive use and exposure to occupational hazards, such as asbestos. Pipe and cigar smokers, if they do not inhale, are not as prone to lung cancer as cigarette smokers, but they are as likely to develop cancers of the mouth, larynx, and esophagus. Those who use snuff or chewing tobacco (sometimes called "smokeless tobacco") run a greater risk of developing cancer of the mouth.
Inhalation of tobacco smoke by nonsmokers has been found to increase the risk of heart disease and respiratory problems; this has created a movement for smokeless environments in public spaces, including government buildings, office buildings, and restaurants. Fetal damage can be caused if a mother smokes or is exposed to smoke during pregnancy. Children of smokers have a higher risk of asthma and lung disease.