Encyclopædia Britannica, Inc.

Medical evidence has established that cigar and pipe smoking cause cancer of the mouth and that cigarette smoking is linked directly with lung cancer. Today, in many countries, as many as one third of all cancer deaths are attributed to cigarette smoking. It is also known that smoking increases the risks of other diseases of the heart and lungs. Smoking by a pregnant woman increases the risks of miscarriage, premature birth, low birth weight, and death of the newborn. Smokeless tobaccos, such as chewing tobacco and snuff, have also been associated with increased risk of cancer of the mouth.

People who smoke may find the habit invigorating at some times and relaxing at others. These apparently opposite effects are produced by nicotine. Nicotine is a poisonous alkaloid found in tobacco smoke and in other tobacco products, and is the substance to which many smokers are addicted. (Other well-known alkaloids are cocaine, morphine, and strychnine.) It acts as a stimulant when inhaled in short puffs, but it acts as a tranquilizer when inhaled in deep drags. Because it also suppresses the appetite, many people use smoking as an aid in weight control. It is unclear whether nicotine alone has any long-term health effects. It is known to constrict the blood vessels and raise blood pressure, and these actions could put a strain on the heart. It may also be linked to some forms of cancer. Of the more than 4,000 substances found in tobacco smoke, about 60 are associated with cancer or tumor formation. Two of the most dangerous components of tobacco smoke are carbon monoxide (a gas) and particulate substances (including nicotine), which are collectively known as tar.

In the blood, carbon monoxide interferes with the supply of oxygen to all tissues and organs, including the brain and heart. Particulates accumulate on the mucous linings of the airways and lungs and impair their functioning. Continued exposure to particulates damages the lungs and increases the smoker’s chances of developing such conditions as chronic bronchitis and emphysema. Smokers who are still relatively young can reverse some lung damage by quitting, but in middle-aged and older smokers some damage seems to be irreversible.

One of the greatest dangers of smoking is the increased risk of cancer, particularly lung cancer, but also cancer of the bladder, mouth, and esophagus. In general, a person’s risk of developing a tobacco-related cancer depends on how long that person has been smoking, the number of cigarettes smoked per day, the tar content of the cigarette, and how deeply the person inhales the smoke. The risk of dying from lung cancer is 20 to 30 times greater for a heavy smoker than for a nonsmoker.

When people give up smoking, the risk of developing tobacco-related diseases declines. The speed and degree of this decline depends on how long and how heavily the person has smoked. Among those who have smoked more than 20 cigarettes a day for more than 20 years, a minimum of three years must elapse after quitting before a decreased risk for cancer is evident. More than ten years of abstinence is necessary before the degree of risk approaches that for those who have never smoked.

In the United States and in most other Western, industrialized countries in the 1970s and 1980s, concern increased among nonsmokers about the risks associated with passive smoking, or breathing the tobacco smoke exhaled by others. Studies conducted on passive smoking indicated that passive smokers had a higher risk of developing lung cancer than did nonsmokers. In response, many private companies, cities, and states began adopting work rules and laws requiring designated smoke-free areas in workplaces or in public buildings or banning smoking entirely in certain enclosed public areas.

(Encyclopaedia Britannica, 2015)