Lung Cancer and Smoking: What We Really Know

The American Cancer Society has already spent more than half a million dollars on its research on lung cancer, and the Board of Directors recently authorized a second half-million dollars with which to carry forward the investigations. In the article which follows, Dr. Charles S. Cameron, Medical and Scientific Director of the Society, summarizes the important evidence thus far and gives an answer to those who still deny that an association between smoking and lung cancer does exist.

During the last two years, a lot has been written—and said—about the dangers of cigarette smoking, especially as a possible cause of the alarming increase in cancer of the lung. But there is still much confusion about the tobacco-cancer issue. Most of the scientists who have given thought and study to the matter appear to agree that an association between cigarette smoking and cancer of the lung does exist. Whether that association is one of cause and effect is as yet unanswered in terms of major scientific opinion.

One of my colleagues expresses the situation in this way: If it has not been proved that tobacco is guilty of causing cancer of the lung, it has certainly been shown to have been on the scene of the crime. The American Cancer Society, along with a growing body of professional and scientific opinion, has taken this position: Although the complicity of the cigarette in the present prevalence of cancer of the lung has not been proved to the satisfaction of everyone, yet the weight of evidence against it is so serious as to demand of stewards of the public welfare that they make the evidence known to all.

Most authorities on the subject agree that before the early years of the twentieth century, cancer of the lung was encountered rarely. In a monograph on lung cancer which was notable in its day—1912—Adler could base his review on a mere 374 cases. Today cancer of the lung takes the lives of about as many white males each year as were reported to have died of all forms of cancer combined in 1900. During the period 1930-1948, the death rate from lung cancer among men rose from 5.3 per 100,000 to 27.1—an increase of 411 per cent. Some part of this remarkable increase can be laid to better and more widely available diagnosis, but the net impact of the factor of better diagnosis is considerably weakened by noting the trends in the post-mortem experience in large hospitals over the years. Cancer of the lung was perhaps less generally recognizable forty or fifty years ago than it is today—but that was hardly true in the autopsy room. Cancer of the lung now constitutes a substantially larger proportion of the total autopsy findings than it did thirty years ago.

There are certain curious features of this increase. In the first place, whereas the curve of the death rate from nearly every type of cancer affecting chiefly adults rises steadily with increasing age, that for cancer of the lung does not. As early as 1936 the rates by age for white males showed a flattened peak between the ages of 60 and 75, after which it fell off. The peak has since become high and sharp, and for the years 1945-1948 occurs at about the ages 65 to 70, after which the rates drop abruptly. The rate curves for women show later peak death rates, tending to resemble more closely the curves for other types of cancer. The only reasonable explanation for this phenomenon is as follows: From what is known about established environmental causes of human cancer, those causes appear to require years of operation, usually not less than twenty, but sometimes longer, in order to exert their effect. The lung cancer death rate curve suggests that whatever agent (or agents) is responsible for the present increase in cancer of the lung is of recent appearance in terms of its current prevalence, did not involve men who are now beyond the age of 70, but did involve men who are 65 to 70, and in the light of the usual exposure period necessary to produce cancer, about twenty to thirty years ago. This puts the critical exposure period in the 1920s and early 1930s, when the present susceptibles were relatively young men.

The second unique feature of lung cancer death rates over the years is the growing disparity between the sexes. In the period 1933-1936, the ratio was slightly over two male deaths to one female death from this disease. In the interval 1945-1948, five men died from lung cancer for every woman dying from the same cause. In 1949 the difference had increased to six to one, and today most opinions put the male-female ratio of deaths at eight or nine to one. It would appear that more men than women have exposed themselves to whatever factors are responsible for the recent rise of this disease.

Thirdly, cancer of the lung is commoner by a factor of more than 2 among white males living in cities than it is among country dwellers. The differences are much less marked for women, but are nonetheless discernible.

Now, why is this so? Suspicion falls first on substances which are inhaled, because almost all of the 400-odd cancer-producing substances discovered since Yamigawa provided the first demonstration of experimental cancer causation in 1915 exert their effect at the site of contact. What are we inhaling which is widespread, which is more prevalent in cities, which is recent, which is increasing, and to which more men than women are, or rather have been, exposed?

Prominent on the suspect list are industrial fumes; utility, industrial, and domestic soots derived from coal and fuel oil furnaces; exhausts from internal combustion engines (gasoline and diesel); asphalt or bituminous road surfaces; and cigarette smoke.

Industrial fumes have increased, of course; yet, because there is no satisfactory index for the amount of increase and because of their heterogeneous nature, it is most difficult to establish their relationship to the general increase of lung cancer. Coal consumption has not increased in the United States over a period of many years; therefore coal soot would not seem to be an important factor. But fuel oil sales have increased greatly, the volume of annual consumption now being about three and one-half times greater than it was thirty years ago.

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