|
|
|
Cigarette Smoking as a Cause of CancerDonald R. Shopland*
|
Tobacco use, particularly in the form of cigarette smoking, is the single most preventable cause of excess mortality in the United States. Each year, more people die prematurely from smoking than die from automobile accidents, drug abuse, AIDS, and alcohol combined (USDHS, 1989). An estimated 434,000 Americans died as a result of their smoking last year alone. Former Surgeon General C. Everett Koop has called cigarette smoking "...the chief, single, avoidable cause of death in our society and the most important public health issue of our time" (USDHS, 1982). A series of authoritative reports by the U.S. Public Health Service and other international scientific organizations has conclusively documented a causal relationship between cigarette smoking and cancer of at least eight major sites (Shopland et al., 1991). These reports have uniformly identified smoking as a major cause of cancers of the lung, larynx, oral cavity, and esophagus--that is, cigarette smoking is responsible for a majority of the cases and deaths from cancer of these sites. These reports have also demonstrated that smoking substantially elevates the death rates for cancers of the bladder, kidney, and pancreas in both men and women, and, possibly, cervical cancer in women. A number of published reports have suggested an association between smoking and other cancers, including cancer of the stomach, liver, prostate, colon, and rectum. Recent evidence published by investigators at the National Cancer Institute and the American Cancer Society (Shopland et al., 1991) conclusively demonstrates that the cancer risks among current cigarette smokers are greater today than at the time of the first Surgeon General's report in the early 1960s. Table 1 reports the relative risks of early cancer mortality for the eight major smoking-associated cancer sites among smoking men and women compared to nonsmokers. These data are taken from the large American Cancer Society Cancer Prevention Study II of more than 1.2 million individuals (685,748 women and 521,555 men) followed prospectively since 1982 (Shopland et al., 1991). This study clearly shows that, for each site, mortality risks among current smokers are higher than those among nonsmokers. Mortality risks in former smokers are lower than in those who continue to smoke, but higher than in those subjects who had never smoked. The risk of developing any of the smoking-related cancers is dose-related; that is, the more cigarettes consumed daily, the younger the age at which one initiates smoking, and the more years one smokes, the greater the risk. Among male cigarette smokers, the risk of lung cancer is more than 2,000 percent higher than among male nonsmokers; for women, the risks were approximately 1,200 percent greater. Lung cancer is the single largest cause of cancer mortality among both men and women and accounts for more than one in every four cancer deaths nationally in the U.S. In addition to cigarette smoking as a cause of cancer in smokers, environmental tobacco smoke (ETS) (also called involuntary or passive smoking) is now recognized as a significant cause of lung cancer in nonsmokers (National Research Council, 1986; DHS, 1987; EPA, 1993). Nonsmokers who live or work with smokers experience a 30 to 50 percent elevated risk for lung cancer. An estimated 3,000 to 6,000 nonsmoker lung cancer deaths annually are attributed to ETS (EPA, 1993). While the number of ETS related lung cancer deaths may seem small when compared to the number attributed to active smoking, the number is actually quite large when compared to other indoor and outdoor environmental pollutants (Table 2), many of which are regulated by the U.S. Environmental Protection Agency. By way of comparison, two British scientists have estimated that exposure to asbestos fibers among people who live or work in asbestos-containing buildings carries an annual risk of lung cancer of less than 1 in 1 million (Doll and Peto, 1986). Notwithstanding this small risk, great efforts are made to remove asbestos from work sites, schools, and other public buildings because the risks are deemed to be unacceptable. Yet, according to these same investigators, the relative risk for lung cancer due to ETS "is more than 100 times higher than the estimated effects of 20 years' exposure to the amount of chrysotile asbestos normally found in asbestos-containing buildings" (Peto and Doll, 1986). Smokeless tobacco users are at increased risk for cancers of the oral cavity, particularly cancers of the cheek and gum (DHS, 1986), and evidence also suggests an association between use of smokeless tobacco and cancers of the larynx and esophagus. Pipe and cigar smokers experience substantially elevated risks for cancers of the oral cavity, larynx, pharynx, and esophagus, which equal and often exceed the risks observed in regular cigarette smokers (USDHEW, 1979). Pipe and cigar smokers experience a slightly increased risk for lung cancer; however, among pipe and cigar smokers who inhale, the risk of lung cancer is on the same order of magnitude found in cigarette smokers. The total magnitude of the cancer burden caused by smoking is staggering. Of the 514,000 cancer deaths expected to occur this year in the United States, slightly over 164,000, or nearly one-third, are directly linked to cigarette smoking (Table 1). An additional 14,000 deaths can reasonably be attributed to pipe and cigar smoking among men. In all, it is estimated that cigarette smoking causes approximately 23 percent of all cancer deaths in women, but the combination of pipe, cigar, and cigarette smoking is responsible for 42 percent of all male cancer deaths (Shopland et al., 1991). If cancer deaths associated with tobacco use were excluded from national cancer mortality figures, we would be witnessing a substantial downturn in both overall cancer deaths and rates. |
