Even a few cigarettes a day dramatically raise your odds of heart attack, stroke, and early death, but the moment you quit, your body begins a powerful recovery that accelerates over the next two decades.
Study: Association between cigarette smoking status, intensity, and cessation duration with long-term incidence of nine cardiovascular and mortality outcomes: The Cross-Cohort Collaboration (CCC). Image credit: nookniicks/Shutterstock.com
Despite decades of research, important questions remain about how smoking and quitting truly affect cardiovascular disease and mortality. A new study published in PLOS Medicine takes a closer look, examining how smoking intensity and cessation relate to a wide range of heart and health outcomes.
The risks of smoking, and the unknown
Tobacco use causes more than eight million deaths each year. Smoking doubles the risk of cardiovascular disease (CVD) and can shorten life by five or more years due to its heart-related harms. In recent years, researchers have also uncovered additional links between smoking and cardiovascular conditions, such as atrial fibrillation and other cardiac arrhythmias.
Previous research has shown that the benefits of stopping smoking are almost immediate. Among younger adults, the excess risk of death falls by 90 % to 95 % within the first three years after quitting, preventing roughly five years of life lost. By ten years after quitting, excess mortality is almost eliminated, restoring nearly a decade of life expectancy.
While many smokers attempt to quit, others reduce the number of cigarettes they smoke. As a result, the number of people smoking fewer than ten cigarettes per day rose from 16 % in 2005 to 27 % in 2014, and the proportion of non-daily smokers increased from 19 % to 23 % over the same period.
However, the relationship between low-intensity smoking (two to five CPD) and CVD is not clear, nor is the duration of cessation associated with benefit quantified. The current study also aimed to shed light on this important area.
Smoking and heart outcomes study
The study used data from 22 prospective cohort studies that were included in the Cross-Cohort Collaboration Tobacco Working Group. The number of smoking years, the cigarettes per day, and years since quitting were analyzed against multiple health outcomes, including heart attack, stroke, heart disease, heart failure, atrial fibrillation, and death due to any of these causes.
Participants were followed up for up to 20 years, spanning a study that lasted almost five decades. Cox proportional hazard models were used to find the associations. Appropriate adjustments were made for socioeconomic, demographic, and cardiovascular risk factors.
The study included data from 323,826 adults, comprising 25 million person-years (PYs) at risk. Of these, 176,396 deaths were analyzed, covering 16 million PYs at risk.
Who was most at risk?
The median participant age was ~60 years, and 76 % were women. In total, 14 % were current smokers, 36.4 % were never-smokers, and 49 % were former smokers. Current smokers averaged about 20 cigarettes per day, while former smokers had quit for an average of 21 years.
Current smokers faced consistently higher risks for all outcomes than both former and never-smokers. Among men, smoking was linked to a 74 % higher risk of CVD, including heart attack and stroke; for women, the risk more than doubled. All-cause mortality was 117 % higher in male smokers and 143 % higher in female smokers compared with those who never smoked.
Smoking dose-response relationship
Each increase in smoking by ten pack-years (number of packs smoked per day times the number of smoking years) increased the risk for all outcomes by 2.4 % to 4.6 %. The high-risk threshold for all outcomes, except atrial fibrillation, appeared to be five pack-years or more, while the risk of atrial fibrillation increased above six pack-years.
All levels of smoking were linked to a higher risk. Those who smoked one or fewer CPD still had an increased risk for most outcomes (except for stroke and atrial fibrillation) compared to never-smokers.
Smoking just two to five CPD was linked to a higher risk for all outcomes. For example, the risk of atrial fibrillation was 26 % higher, and the risk of heart failure was 57 % higher. Similarly, death from CVD increased by 57 %, and all-cause mortality rose by 60 %.
Those smoking 11 to 15 CPD faced even greater dangers, with an 87 % higher risk of CVD and a 130 % higher risk of death from any cause compared to people who had never smoked.
The increase in risk was steepest in the first 20 pack-years of smoking, though it continued to climb afterward. A similar curve was seen for CPD, with the risk increasing sharply up to about 20 CPD before the curve began to level off.
Participants who entered the study after 2001 had higher risk estimates than those who entered earlier. The reasons could include better health among never-smokers and changes in smoking patterns.
Evidence from other studies agrees with this observation, suggesting that current smokers born in more recent years start earlier and smoke more heavily compared to those from earlier years, and are likely to be at even greater risk over the long term. This underlines the importance of smoking cessation promotion and support, as well as strategies to prevent smoking initiation.
The benefits of quitting
The greatest drop in risk occurred during the first decade after quitting, but benefits continued to accumulate more gradually over the long term. After 20 years of cessation, former smokers had roughly an 80 % lower risk than current smokers, a benefit that was especially pronounced among those who quit at younger ages.
The highest risk was among those with the most pack-years and the shortest duration since quitting.
The magnitude of health risk for former smokers within the highest pack-year group (>20 pack-years) was lower than the magnitude of health risk for current smokers within the lowest pack-year group (≤5 pack-years)
This signals the importance of both pack-years and smoking status in determining risk. While the duration since quitting was more indicative of the cardiovascular risk compared to pack-years, pack-years were more predictive of mortality risk.
These findings require further validation, as the included parameters, smoking status (vs. not smoking), duration of smoking, CPD, and duration since quitting, were self-reported and collected only at baseline, despite a very long follow-up period. This could have falsely lowered association levels due to changes in the smoking patterns over time. Moreover, it is unclear which of the participants used other forms of tobacco or electronic cigarettes.
Early quitting saves lives
The primary public health message for current smokers should be early cessation, rather than reducing the amount of smoking.
Smoking, even at very low levels, increases cardiovascular risk. Quitting provides significant reduction in the risk of CVD and death soon afterwards, with continued decreases over the two decades following cessation.
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