First, let me begin by thanking Kareena and Ned for an interesting and important conversation about the causes of smoking and the decline in risk from quitting. In this thread, debate arose about the nature of the decline in lung cancer risk after quitting, in particular whether it ever reached the same levels as a never-smoker.

One particularly good source addressing the question is the Nurses’ Health Study. This study observed 104, 519 female nurses over time then was analyzed to determine the increase in risk for various illnesses in smokers. As you might guess, the risk was increased for just about every medical condition that the investigators queried—total mortality, vascular disease, coronary heart disease, cerebrovascular disease, total respiratory disease, COPD, lung cancer, all smoking-related cancers, other cancers, colorectal cancer and other causes. The only factor that was queried without a statistically significant association was ovarian cancer.

Everyone on this website understands the suffering caused by cigarettes. To what extent can the smoker reduce this risk by quitting? In the figures below, the top horizontal line, at 1, represents the risk of a current smoker. Each of the plotted values represents the risk of a disease relative to a current smoker (as measured by hazard ratio). The bottom horizontal line represents the risk of a never smoker.

So for lung cancer death, the nurse’s health study says that the risk approaches, but never reaches that of a never smoker. More importantly, it says that the risk drastically falls over time after quitting. Finally, it reminds us that smoking causes suffering and death in many ways other than lung cancer.

Another analysis pooled data from three large cohort studies of Japanese men, for a total of 110, 002 people—28, 715 never smokers, 25, 081 former smokers and 56, 206 current smokers. The smokers in this analysis all started smoking at age 18-22, and were analyzed for lung cancer mortality by number of years since smoking cessation. The mortality rate ratio was defined as the mortality rate relative to that of never smokers. The results were as follows:

The biology of smoking and lung cancer seems to be different for the Japanese than for Western populations. There is a lower risk of smoking from lung cancer in Japan and a higher risk for lung cancer among non-smokers. Nonetheless, the point of this chart is important and consistent with the nurses’ health study—risk declines continuously after quitting. In this study, the risk of lung cancer death reached that of never smokers for those who quit at ages 40-49 (circles) but not for those who quit between 50-59 (triangles), 60-69 (squares) or 70-79 (diamonds). In other words, it’s always worth quitting and the sooner, the better.

The American Cancer Society also evaluated this question. Unfortunately, I don’t have access to figures, but the basic result was that smoking cessation was beneficial at any age, with the risk of lung cancer coming closer to that of never-smokers the younger a smoker was at the time of quitting (abstract here). The American Cancer Society webpage provides a great list of reasons to quit, including a timeline of physiologic changes, starting from 20 minutes from quitting until 15 years after quitting, including reduction in risk in a variety of illnesses over time.

In the Nurses’ Health Study, 64% of deaths in current smokers and 28% of deaths in past smokers were attributable to smoking. As patients, loved-ones of patients, and oncologists, we are in a unique position to be ambassadors for smoking cessation.