How much older do we become if we smoke?
Cigarette smoking is probably the largest contributing factor to the large burden of cancer in our society. Smoking does not only increase the chances of a person developing lung cancer, but also bowel, stomach, pancreas, bladder or head and neck cancers.
Obviously, this is not new. We have known this for decades and despite all the efforts and achievements in fighting the tobacco epidemic from different sectors of the society, still nowadays nearly 1 billion people are smokers.
Efforts to decrease the number of smokers have been focused on the youth. The number of smokers among adults aged 65 years and older has meanwhile grown, possibly because older adults may be unaware of the benefits of cessation.
Perhaps we scientists may have not clearly communicated and persuaded the general public about the harms of smoking and the benefits of cessation, especially at a later age.
What have we done differently?
Well, in order to explain what we have done, first one needs to understand what has been done until now. Traditionally, risk ratios are estimated. They compare an exposed with an unexposed group and give information on how much likely to develop a disease are those exposed to a risk factor compared with those unexposed.
Smokers have a 44% higher risk of developing cancer than never smokers, and are twice as likely to die from cancer as never smokers.
For example, as we observed in our study, smokers have a 44% higher risk of developing cancer than never smokers, and are twice as likely to die from cancer as never smokers.
The goal of our study was to present more striking and tangible estimates of the increase in cancer risk associated with smoking, and of the decrease in cancer risk associated with cessation.
In order to do that we quantified rate advancement periods (or RAPs) in a very large setting of European and American adults participating in ongoing cohort studies of the EU-funded CHANCES Consortium, for which data have been harmonized.
What did we find?
According to our findings, smokers reach the same risk of cancer than never smokers on average eight years earlier. In other words, a smoker aged 62 years reaches the same risk of cancer than a never smoker reaches at 70 years.
Thus, smokers are advancing their risk of cancer by on average eight years. Likewise, smokers are advancing their risk of dying from cancer by on average ten years.
If a current smoker reaches a given level of cancer risk at 60 years, an ex-smoker that had quitted on average five years before will reach the same level of risk at 62 years.
On the other hand, if a current smoker reaches a given level of cancer risk at 60 years, an ex-smoker that had quitted on average five years before will reach the same level of risk at 62 years.
In other words, recent quitters delay their risk of cancer by two years. The longer the time since cessation is, the larger the delay in cancer risk, in comparison to current smokers.
We repeated our analysis among adults aged 65 years or more and obtained the same findings: being smoker at old age still advances your risk of cancer. Also at advanced age, quitting smoking was beneficial in delaying the risk of cancer.
Of course, our findings varied greatly between cancer sites, the advancement being even larger for lung cancer but smaller for breast cancer. For lung cancer, per every year since smoking cessation, lung cancer risk was delayed in nearly one year, compared to smokers.
For lung cancer, per every year since smoking cessation, lung cancer risk was delayed in nearly one year, compared to smokers.
Our findings should be interpreted with care: rate advancement periods, as their name implies, do not advance the event, that is, the occurrence of or death from cancer, but rather the risk of developing the event. Also, we are assuming that the risk of cancer is increasing with age independently of whether a person smokes or not.
The implications of our research
In previous projects we have shown the same findings for cardiovascular disease risk and mortality. Cancer and cardiovascular disease are the two most common causes of death among adults, which explains how we have also observed that smoking advances the overall risk of mortality.
In summary, our research on smoking and health has shown that smokers, compared with never smokers, become older by advancing their risk of disease and of dying. Nevertheless, quitting can help to delay this risk, even at old age.
Bradford Hill criteria of causality dictates that smoking is a dose dependent activity.
Quantifying risk proportional to dose and manner of smoking, rather than a simplistic smoke/never smoke comparison might not only make better sense, but it might be instinctively more persuasive. The smoke/never smoke comparison is an entirely artificial construct, comparing an allegedly homogeneous group on the one hand, with a widely heterogeneous group on the other. I suspect that the reality may be that a low level of smoking is in fact quite undetectable from usual background pollution, one of those inconvenient truths risk mongers and anti-smoking activists would prefer to quietly ignore?