Harvard Reports on Cancer Prevention
Volume I: Human Causes of Cancer
Cancer Causes & Control:
An International Journal of Studies of Cancer in Human Populations
Official Journal of the International Association of Cancer Registries
Volume 7 Supplement November 1996 ISSN 0957-5243

Smoking

Introduction
By far, the largest known preventable cause of cancer in the United States is cigarette smoking, which accounts for 30 percent of all cancer deaths - some 400,000 deaths in 1995. The percentage of cancer deaths in the US due to tobacco is still increasing, reflecting the changing patterns of use over time. Smoking cigarettes at an early age has become more common among women since the 1950s. Tobacco consumption is related causally to cancer of the lung, mouth, larynx, esophagus, bladder, kidney, and pancreas. Studies documenting these relationships go back more than 40 years. The most convincing evidence comes from prospective follow-up studies, such as the original British Doctors Study conducted by Doll and Hill¹ and an American Cancer Society study of one million men and women.² Numerous other studies have documented these important relationships between smoking and cancer.

In addition to the cancers listed above, recent evidence addressing the relation between smoking and colon cancer has been published in the past few years. Those cohorts with long-term follow-up, including the British Doctors Study, the American Veterans Cohort Study, the Nurses' Health Study, and the Health Professionals Follow-up Study, show that having smoked 30 or more years in the past increases the risk of colon cancer.³ Further, approximately 25 percent of colon cancer can be attributed to cigarette smoking in these prospective studies. Smoking also is related to colon polyps, precursors of colon cancer. No consistent relation has been observed between smoking and breast cancer or prostate cancer. An association between cancer of the uterine cervix and cigarette smoking has been noted; however, because of the possibility of confounding by unidentified factors (possibly a sexually transmitted agent), this association has not been identified as causal. It is biologically plausible, though, that cigarette smoking may influence the risk of cervical cancer through several mechanisms.

Benefits of quitting
Studies addressing the relation between time since quitting and risk of cancer indicate that former smokers experience a lower risk of all cancers than current smokers. An important aspect of the relation between smoking and cancer from the perspective of prevention is the course of declining risk after an adult st ops smoking. The benefits of quitting smoking in terms of lung cancer risk are seen clearly across numerous prospective studies. Within five years of quitting, the risk among former smokers is typically half that of current smokers, and within 10 to 15 years of quitting, their risk is comparable to that of never smokers in the majority of studies. Based on these consistent results, the 1990 Report of the US Surgeon General concluded that smoking cessation reduces the risk of lung cancer compared with continuing smokers and that the reduced risk of lung cancer among former smokers is observed in both males and females, in smokers of filter and nonfilter cigarettes, and for all types of lung cancer. Further, smoking cessation lowers the risk of laryngeal cancer.⁴

Having smoked cigarettes more than 15 years in the past shows little relation with risk of pancreatic cancer, and, for this cancer, the benefits of quitting are substantial. The pattern of reduced risk after cessation from smoking for pancreatic cancer follows much the same decline as that observed for lung cancer.

Age at initiation of smoking
The second major factor determining risk of lung cancer is the age at onset of smoking. Early onset of smoking increases risk of lung cancer, and continuing to smoke in adult years promotes the final stages of cancer formation and invasion of the surrounding lung. Several fact ors that clearly influence adolescent smoking include ease of access to cigarettes and price (taxes are important - as higher taxes tend to lower rates of adolescent smoking). Adolescent smoking is a socially learned behavior, and programs aimed at reducing smoking among youths must focus on the broad array of influences on adolescent smoking. School-based education programs are a necessary component but are not sufficient to counter the impact of marketing on adolescent smoking patterns.

Though numerous studies suggest the adverse effects of smoking on risk of lung cancer are reduced among men and women with a high consumption of fruit and vegetables, the greatest reduction in risk can be achieved through cessation of smoking and avoiding or preventing adolescent smoking.

Summary
Cigarette smoking increases risk of a wide range of cancers, and early onset of smoking is related particularly to risk of lung cancer and colon cancer. The adverse effects of smoking may occur at different points in the pathway to cancer. Smoking may act early, as seen with colon cancer; it may act early and late, as with lung cancer; or it may act only late in the pathway to cancer, as seen with cancer of the pancreas. The benefits of quitting are substantial and quite rapid for cancer, particularly for cancers of the lung and pancreas.

Summary Points

Suggestions

Suggested Further Reading

References