The largest-ever study of the hazards of smoking and benefits of stopping for women in the UK has shown that female smokers lose at least ten years of lifespan, but that stopping before the age of 40, and preferably well before the age of 40, avoids more than 90% of the increased risk of dying caused by continuing to smoke, while stopping before the age of 30 avoids over 97% of it.
The research, based on results from the Million Women Study, is published Online First in The Lancet to mark the 100th anniversary of the birth of Sir Richard Doll, one of the first people to identify the link between lung cancer and smoking.
1·3 million women were recruited to the study between 1996 and 2001, at ages 50 to 65 years. Participants completed a questionnaire about lifestyle, medical and social factors and were resurveyed by post three years later. The NHS central register notified the researchers when any participant died, giving the cause of that death. Women were traced for an average of twelve years from the time they first joined; thus far, 66 000 study participants died.
Initially, 20% of the study participants were smokers, 28% were ex-smokers, and 52% had never smoked. Those who were still smokers at the 3-year resurvey were nearly three (2·97) times as likely as non-smokers to die over the next 9 years, even though some reduced their risk by stopping smoking during this period.
This threefold death rate ratio means that two-thirds of all deaths of smokers in their 50s, 60s, and 70s are caused by smoking, as most of the difference between smokers and non-smokers came from smoking-related diseases such as lung cancer, chronic lung disease, heart disease, or stroke. The risks among smokers increased steeply with the amount smoked, although even for those who were light smokers (1–9 cigarettes per day) at the start of the study, mortality rates were double those for non-smokers.
The key finding is that both the hazards of smoking and, correspondingly, the benefits of stopping are bigger than previous studies have suggested; smokers who stopped around age 30 avoided 97% of their excess risk of premature death, and although serious excess hazards remained for decades among those who smoked until age 40 before stopping, the excess hazards among those who continued smoking after age 40 were ten times bigger.