The 565-page monograph, Changes in Cigarette-Related Disease Risks and Their Implication for Prevention and Control contains newly analyzed data from five of the world's largest epidemiologic studies on smoking and health.
Two of the studies began in the 1950s when an understanding of the health effects of smoking was just beginning to emerge, while the other three studies started more recently. Study populations ranged from 60,000 to 1.2 million and the length of follow-up varied from six to 26 years. A total of nearly 490,000 deaths was available for analysis across all studies. Taken as a group, these five studies represent over 20 million person-years of observation.
According to the new monograph, the risks for all smoking-related causes of death, including lung cancer, other cancers, heart disease, stroke, and chronic obstructive lung diseases have increased among both men and women. And while men who smoke still experience higher risks for these diseases than women, the greatest increases in risks were found in female smokers.
"The findings are extremely troubling for anyone concerned with public health," said NCI Director Richard D. Klausner, M.D. "Especially troubling is the large increase in relative risks observed among women for cancer of the lung and other smoking-related cancer sites. We must do all we can to continue to keep women fully informed about the dangers of cigarette smoking."
The increase in mortality risk occurred during a time when significant declines in machine-measured tar and nicotine yields of cigarettes were being reported. The average tar level per cigarette has declined nearly 70 percent since 1955, from approximately 38 mg to 12.5 mg today. Similarly, nicotine levels fell from an average of 2.6 mg per cigarette to under 0.9 mg over the same time period. Yet the relative risks for all major smoking-related causes of deaths increased.
David M. Burns, M.D., the monograph's senior editor, of the University of California, San Diego, said the increase in relative risk was due to a greater lifetime dose of cigarette smoke received by smokers in the more recent studies compared with smokers included in the studies from the 1950s and early 1960s. For example, women in the contemporary studies started smoking in their teens, while many of those in the older studies began smoking later in life.
According to Burns, "Smokers in the newer studies consumed more cigarettes per day than smokers followed in earlier studies, and much of the difference between the two sets of risks disappears when duration of smoking history and number of cigarettes smoked per day are held constant." A substantial literature base also exists which clearly shows that smokers today are smoking each cigarette more intensively than smokers did 40 years ago, with larger puffs and deeper patterns of inhalation, added Burns, but it is unclear to what degree these differences have contributed to the observed increase in mortality risks.
In the new analysis, American Cancer Society (ACS) investigators compared six-year follow-up data from two ACS studies known as Cancer Prevention Studies (CPS) I and II (see table). CPS I was initiated in 1959 while CPS II began in 1982. The two studies used nearly identical study designs and methodologies, and each included more than 1 million persons. These studies essentially represent two groups of smokers born approximately a generation apart.
The difference in lung cancer risk between men who smoked and those who did not smoke doubled between studies. In statistical terms, the relative risk increased from 11.9 to 23.2. Relative risk is a ratio used to compare the probabilities of an outcome such as cancer in two different groups. Smokers studied in CPS I were about 12 times more likely than nonsmokers to die of lung cancer, while smokers studied in CPS II were about 23 times more likely than nonsmokers. Among women, the relative risk increased more than fourfold, from 2.7 in CPS I women to 12.8 in CPS II.
The morality risks for all other smoking-related cancers combined, which included cancers of the larynx, oral cavity, esophagus, bladder, kidney, and pancreas, increased from 2.7 to 3.5 in male smokers and from 1.8 to 2.6 among females smokers. Relative risk for coronary heart disease (CHD) for men rose from 1.7 in CPS I to 2.3 in CPS II, while the CHD risks in female smokers rose from 1.4 to 1.8. Similar increases were noted for other causes of death.
Results from two other studies that began in the 1970s confirm the results observed among female smokers in CPS II. Data based on 36,035 women in the Kaiser Permanente Study and 121,700 women in the Nurses' Health Study show that women smokers had nearly twice the risk of death from all causes compared with women who did not smoke. This relative risk of 1.9 was identical to the relative risk found for women in CPS II. Among women in the Kaiser Permanente Study, relative risks for all the major smoking-related diseases were similar to those found among women in CPS II.
For example, the mortality risk for lung cancer among women smokers in the Kaiser Permanente Study was 15.1, compared with 12.8 among CPS II women. For CHD, the relative risks were 1.7 and 1.8 in Kaiser and CPS II, respectively; and for chronic obstructive lung disease the relative risks were 9.0 in Kaiser and 12.8 in CPS II.
"The major prospective studies summarized in the monograph are important cornerstones for documenting smoking-induced diseases," said Donald Shopland, coordinator of NCI's Smoking and Tobacco Control Program, "not only because of the size of the populations involved but because the participants were essentially healthy at the start of the studies."
Shopland added that the findings make clear the enormous risks posed by cigarette smoking. Preventing adolescent smoking will have the greatest benefit for society in the long run, however, the benefits of cessation to adults who currently smoke are substantial, and declines in smoking among this group could reduce death rates in as little as five years.
Health professionals may order single copies of Changes in Cigarette-Related Disease Risks and Their Implication for Prevention and Control from the NCI's Cancer Information Service at 1-800-4-CANCER. A summary of the publication will also be available.
Attachment: Characteristics of Study Populations for Five Major Prospective Studies reported in Changes in Cigarette-Related Disease Risks and Their Implication for Prevention and Control.
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* CPS = Cancer Prevention Study ** Mean number of years follow up