Lung Cancer deaths among men have risen at a steady rate since 1930.
 The lung cancer trend for women, though moving at a slower rate, has
 been consistently upward. The extraordinary increase in lung cancer has 
 not been observed for cancer of any other part of the body. Preceding
 and during the period of this rise, there has been a parallel increase 
 in percapita cigarette consumption. It is noteworthy that the disparity
 between the rate of increase of lung cancer in men and women corresponds
 in general to differences between smoking habits of men and women, by the
 amount smoked, and by the duration of smoking. Lung cancer is relatively 
 uncommon among those who do not smoke cigarettes and in those who smoke
 only cigars or pipes.
   Animal experiments show that tars extracted from cigarette smoke regulaly
 induce cancer at the test site when painted on the skin of mice. Lung cancer
 has been found in dogs expose to the inhalation of cigarette smoke for two
 or more years.Under special conditions, lung cancers similar to those in have 
 also been produced in the lungs of mice, rats, and hamsters by administering  
 polycyclic aromatic hydrocarbons-substances present in cigarette smoke.
   The amount of known cancer-producing substances in cigarette smoke is too
 small to meet the doage requirements established for the production of skin
 cancer in animals, and it is likely that other substances, not carcinogenic
 in themselves, promote cancer production. A number of these substances, called
 cocarcinogens, have been found in cigarette smoke.
   Several types of abnormal changes in the lining cells (epithelium) of the
 main airways-the trachea and brochi-are more common in cigarette smokers than
 in nonsmokers. The severity of the changes increases with the number of 
 cigarettes smoked and decreases after the discontinuation of cigarette smoking.
 The importance of the changes rests mainly on the belief that advance hyperplasia,
 with many atypical cells, immediately precedes the development of cancer.
   The 29 retrospecttive studies of groups of persons with lung cancer and of
 control subjects without lung cancer uniformly show an association between 
 cigarette smoking and lung cancer.More heavy smokers are found among lung cancer 
 patients. The relative risk of lung cancer ranges from 2.4 times greater for
 light smokers to 34.1 times greater for heavy smokers. The average male smoker
 incurs a tenfold greater risk of developing lung cancer than the nonsmokers. 
   All seven prospective studies show a remarkable consistancy in the higher 
 death rates of cigarette smokers from lung cancer. The risk of developing 
 lung cancer increases with the duration of smoking and the degree of inhalation
 and is decreased by discontinuing smoking.
   A causal relationship between cigarette smoking and lung cancer does not
 exclude the operation of other factors. Not all tpyes of lung cancer are 
 associated with cigarette smoking. Some lung cancer cases-less than 10%-occur
 among those who do not smoke cigarettes. The adenocarcinomas of the lung,
 which start in glandular linings, apparently have little relationship to
 smoking. Viruses may be involved in the causation of lung cancer and could
 be a determinant-a reason why some heavy smokers develop lung cancer while
 many do not.
   Occupational studies have demondstrated an increaesd death rate from lung
 cancer associated with several types of mining exposure, but unfortunately
 the contribution of cigarette smoking was not evaluated. In view of the
 relatively small size of the population exposed to industrial carcinogens,
 it is unlikely that such agents alone could account for the increasing lung 
 cancer risk in the general population. 
   Air pollution also may contribute causally to lung cancer.In Britain, where
 the death rate from lung cancer is the highest in the world, the higher levels
 of air pollution may be a factor in addition to cigarette smoking. Yet in 
 countries such as iceland, where air pollution is almost nonexistent, the
 correlation of cigarette smoking and lung cancer is well established.
   In summary, experimental, clinical-pathological, and epidemiological 
 evidence indicates that cigarette smoking is the main cause of lung cancer
 in men. Cigarette smoking is also a cause of lung cancer in women, but 
 accounts for fewer cases than in men. The risk of developing lung cancer
 increases with the number of cigarettes smoked per day and the duration
 of smoking, and it diminishes with cessation of smoking. Cigarette smoking
 appears to be much more important than other factors, such as air pollution,
 in the causation of lung cancer in the general population. The lung cancer
 risk for pipe and cigars smokers appears to be only slightly greater than
 for the nonsmoker, and it is clearly much less than for cigarette smokers.<br><br>

<b>Reference</b><br>
 Maistre, Charles Le. "Smoking and Health." <b>The Encyclopedia Americana</b>. 25 (1981): 72-73.