Carcinogens In Our Atmospheric Environment
( Originally Published Early 1960's )
Our lungs are one of our main contacts with the outside world. We breathe about 20 times every minute, inhaling and exhaling a pint of air with each breath.
When the air we breathe contains impurities, the lungs ordinarily have ways of getting rid of them. This is done by coughing, or by more complicated processes within the lining of the bronchial tubes or of the lung tissue. Excessive exposure to impurities that are inhaled brings about changes in the bronchial linings and the lungs which eventually result in disability and illness. If the impurities contain cancer producing chemicals, prolonged exposure can lead to cancer.
As early as 1879, German pathologists showed that an important proportion of the high death rate from lung disease in the miners of Schneeberg and Joachimsthal was due to lung cancer, attributable to the inhaling of various metallic ores of that region, including uranium. Recent studies among uranium miners of the United States support the supposition that radioactive ores have a high carcinogenic potential for the human lung. Other occupational inhalants that have been associated with increased risk to lung cancer include chromates, nickel, asbestos and coal gas.
Lung cancer in the United States and in Europe has shown the greatest increase of any cancer site in males during the past 30 years. It was formerly considered to be a rare disease. There are now over 40,000 deaths per year in the United States from this cause, and it has become one of the leading killers of men over 40 years of age. If the present trends continue, it is estimated that about 1 million persons who are now school children will die eventually of lung cancer.
Analyses of this cancer epidemic indicate that a great pro-portion of the increase is related to two factors: one is air pollution caused by industrial wastes, automobile exhausts and household sources. In general, city dwellers have up to 3 times more lung cancer than rural inhabitants. The other major source is an individual form of air pollution, the smoke from burning tobacco, especially cigarettes. The identification of cigarette smoke as a major cause of lung cancer and other lung diseases will rank as one of the major scientific public health studies of this century, equivalent to the identification of water pollution as a source of many gastrointestinal diseases such as typhoid.
The first studies were of the so-called retrospective type, in which patients with lung cancer were asked about their previous habits, and their answers compared with those of patients with other diseases. Almost all lung cancer patients replied that they had been long term, heavy cigarette smokers. The next studies were of the prospective type, in which large groups of men were identified by their smoking habits, and then followed for several years. Richard Doll and Bradford Hill, using 80,000 British physicians; Cuyler Hammond and Daniel Horn, of the American Cancer Society, using over 180,000 volunteers; and Harold Dorn of the National Institutes of Health, who followed over 200,000 U.S. Government life insurance policy holders, reached the same conclusions. Deaths from all causes were increased among smokers, according to the amount smoked, particularly if cigarettes were used; the most striking proportional rise was in lung cancer.
With these epidemiological clues in hand, scientists began to study the relationship in the laboratory. Chemists isolated and identified at least a dozen carcinogenic chemicals of the hydrocarbon type in the "tars" from tobacco smoke. It is now suspected that the action of these chemicals is enhanced by phenols and other cocarcinogens, and that tobacco smoke may contain yet further carcinogens.
Tobacco smoke tars produce cancers of the skin of mice and other laboratory animals when painted on the skin for long periods. Cancers of the bronchi have been produced in dogs, following prolonged painting of the bronchi with such tars. No laboratory animal has been trained to smoke cigarettes like man, and no actual production of lung cancers by inhalation has been recorded.
The membranes lining the lungs absorb cancer producing chemicals from tobacco smoke. The protective mechanisms by which the lungs rid themselves of impurities are first paralyzed and then destroyed by tobacco smoke. Prolonged exposure of the lungs of animals to tobacco smoke produces changes in cells that resemble early stages of cancer development. These changes are also seen in the lungs of heavy smokers who die of causes other than lung cancer.
Deaths from lung cancer are about 6 times more frequent among men who smoke regularly in any form (cigarettes, pipe or cigars) than among nonsmokers. Deaths from lung cancer are 6o times more frequent among men who smoke over 2 packs of cigarettes per day than among nonsmokers.
Women smoke less than men, but among them, too, cigarette smoking increases the risk of lung cancer. Among nonsmoking men and women who live in the country, the risk of developing lung cancer is approximately the same. The younger one starts smoking, the greater the risk of developing lung cancer. The risk is reduced by stopping smoking, although it does not return to the low frequency among those who never smoked.
The strong association between lung cancer and cigarette smoking is probably due to customary deep inhalation of the bland cigarette smoke. The more acrid, alkaline cigar and pipe smoke is seldom inhaled deeply into the lungs.
General air pollution, or pollution from industrial sources, is a matter for community action. The problem of smoking is a matter of individual decision. The available facts indicate that the following is sound medical advice to be considered in arriving at the decision :
1. If you are not a smoker, do not start; if you are a smoker, stop.
2. If you must smoke, smoke a pipe or cigars rather than cigarettes.
3. If you must smoke cigarettes, smoke as few as possible (certainly under a pack per day), inhale as little as possible, and discard each cigarette as soon as possible.
4. If you are a cigarette smoker and 40 years or older, get an X-ray film of your chest every 6 months.