This report adds to the growing evidence on the relationship between smoking and mortality.13, 14
The original report from the Nurses’ Health Study on smoking and cause-specific mortality included 2,847 deaths and evaluated 6 mortality-specific outcomes: total mortality, total cardiovascular diseases, total cancer, total cancer excluding lung cancer, and external causes of injury. This updated report on smoking and mortality in the Nurses’ Health Study cohort includes an additional 16 years of follow-up, 12,483 deaths, and additional estimates for coronary heart disease, cerebrovascular disease, respiratory disease, COPD, lung cancer, smoking-caused cancers, colorectal cancer, ovarian cancer, and other causes. Since smoking behavior changes over time, updating participants’ smoking status every 2 years enables more accurate evaluation of the detrimental effects from long-term smoking and the risk reduction over time from sustained cessation. The 9,636 additional deaths that have accrued over time also allow for better precision in estimating the extent of risks associated with smoking and smoking cessation on causes of death previously studied.
As expected, smoking increased the risk of dying from all major cause-specific mortality, with hazard ratios 8 to 14 times higher for lung cancer mortality and COPD mortality compared to total mortality. The increase in the HR with increasing cigarettes smoked per day varied by outcome. An increasing trend was less pronounced for deaths due to vascular disease, suggesting that the first few cigarettes account for most of the increased risk; in contrast, an increase in the number of cigarettes smoked per day substantially increased risk for respiratory death.
Cohort studies consistently support an increased risk associated with current smoking on colorectal cancer risk, but only after accounting for an induction period of 30–40 years.15
Our mortality estimates are higher for current smoking and similar for former smoking compared to those from the American Cancer Society Cancer Prevention Study II, which reported HRs of 1.41 (95% CI: 1.26–1.58) and 1.22 (95% CI: 1.09–1.37) for current and past smoking status among women.16
The 2004 Surgeon General’s report concludes that the evidence is suggestive but not sufficient to infer a causal relationship between smoking and colorectal cancer,6
mainly because of the possibility that the higher death rates from colorectal cancer may be due to less screening in smokers and a later stage of disease at diagnosis. However, we observed only modest differences in colorectal cancer screening in our cohort. In 1992, 5% of smokers reported screening by sigmoidoscopy and 31% by the stool occult blood test in the past two years, versus 10% and 42%, respectively, for past smokers, and 8% and 38% for never smokers. There was a small difference in the percentage of never smokers compared to past and current smokers who had an advanced stage of colorectal cancer at diagnosis. It is unlikely that these small differences in screening and stage at diagnosis explain the smoking and colorectal cancer mortality relation.
The 2004 Surgeon General’s report also concluded that the evidence was inadequate to infer a causal relationship between smoking and ovarian cancer.6
Although we observed a positive but non-significant relation between current smoking and ovarian cancer morality, we found no significant trend with increasing cigarettes smoked per day, pack-years of smoking, and age starting smoking, nor an association between smoking cessation and ovarian cancer mortality, even after 20 years of quitting. Previous studies suggest an increased risk of ovarian cancer incidence associated with current smoking for mucinous epithelial tumors.17, 18
Smoking cessation was beneficial for each cause-specific mortality outcome examined. Unlike the Cancer Prevention Study I, which did not update smoking status during follow-up and found that the risks associated with lung cancer and COPD mortality remained even after 20 years, we observed a monotonic decrease in risk compared to current smokers with increasing years of smoking cessation, with risks equivalent to that of never smokers after 25 years.19
By stopping the updating of covariates after diagnosis, we minimize the bias due to symptom-induced smoking cessation or reducing smoking levels (the “ill quitter” effect). Inability of other studies to update smoking exposure over time or utilize smoking information just before diagnoses may obscure the harms of continuing smoking and the benefits of cessation, because current smokers may quit smoking over time and some past smokers may resume smoking.
In the British Doctors Study, men born in the 1920s likely had more intense early cigarette exposure than earlier birth cohorts, and coupled with improvements in treatment, an estimated two-thirds of those persistent smokers were likely to die from smoking.20
We did not see differences in early cigarette exposure between those born between 1920–1929 and 1930–1939, translating into similar hazards for total mortality in both groups; however, youth are starting to smoke at younger ages. One nationwide survey reported that 13% of eighth grade students first smoked by age 11,21
and 22% of all high school students report themselves as current smokers.22
It is likely that deaths attributable to smoking will increase over time unless there is a substantial increase in cessation.
In summary, our findings indicate that 64% of deaths in current smokers and 28% of deaths in past smokers are attributable to smoking. Quitting reduces the excess mortality rates for all major causes of death examined: most of the excess risk of vascular mortality due to smoking can be eliminated rapidly upon cessation and within 20 years for lung diseases, where the damaging effects of smoking are greatest. Early age at initiation increases mortality risk, so implementing and maintaining school tobacco prevention programs, in addition to enforcing youth access laws, are key preventive strategies.23, 24
Effectively communicating risks to smokers and helping them quit successfully should be an integral part of public health programs.