title: The 21st century hazards of smoking and benefits of stopping: a prospective study of one million women in the UK.
abstract: BACKGROUND
  Women born around 1940 in countries such as the UK and USA were the first generation in which many smoked substantial numbers of cigarettes throughout adult life. Hence, only in the 21st century can we observe directly the full effects of prolonged smoking, and of prolonged cessation, on mortality among women in the UK.
  
  
  METHODS
  For this prospective study, 1·3 million UK women were recruited in 1996-2001 and resurveyed postally about 3 and 8 years later. All were followed to Jan 1, 2011, through national mortality records (mean 12 woman-years, SD 2). Participants were asked at entry whether they were current or ex-smokers, and how many cigarettes they currently smoked. Those who were ex-smokers at both entry and the 3-year resurvey and had stopped before the age of 55 years were categorised by the age they had stopped smoking. We used Cox regression models to obtain adjusted relative risks that compared categories of smokers or ex-smokers with otherwise similar never-smokers.
  
  
  RESULTS
  After excluding 0·1 million women with previous disease, 1·2 million women remained, with median birth year 1943 (IQR 1938-46) and age 55 years (IQR 52-60). Overall, 6% (66,489/1,180,652) died, at mean age 65 years (SD 6). At baseline, 20% (232,461) were current smokers, 28% (328,417) were ex-smokers, and 52% (619,774) were never-smokers. For 12-year mortality, those smoking at baseline had a mortality rate ratio of 2·76 (95% CI 2·71-2·81) compared with never-smokers, even though 44% (37,240/85,256) of the baseline smokers who responded to the 8-year resurvey had by then stopped smoking. Mortality was tripled, largely irrespective of age, in those still smoking at the 3-year resurvey (rate ratio 2·97, 2·88-3·07). Even for women smoking fewer than ten cigarettes per day at baseline, 12-year mortality was doubled (rate ratio 1·98, 1·91-2·04). Of the 30 most common causes of death, 23 were increased significantly in smokers; for lung cancer, the rate ratio was 21·4 (19·7-23·2). The excess mortality among smokers (in comparison with never-smokers) was mainly from diseases that, like lung cancer, can be caused by smoking. Among ex-smokers who had stopped permanently at ages 25-34 years or at ages 35-44 years, the respective relative risks were 1·05 (95% CI 1·00-1·11) and 1·20 (1·14-1·26) for all-cause mortality and 1·84 (1·45-2·34) and 3·34 (2·76-4·03) for lung cancer mortality. Thus, although some excess mortality remains among these long-term ex-smokers, it is only 3% and 10% of the excess mortality among continuing smokers. If combined with 2010 UK national death rates, tripled mortality rates among smokers indicate 53% of smokers and 22% of never-smokers dying before age 80 years, and an 11-year lifespan difference.
  
  
  CONCLUSIONS
  Among UK women, two-thirds of all deaths of smokers in their 50s, 60s, and 70s are caused by smoking; smokers lose at least 10 years of lifespan. Although the hazards of smoking until age 40 years and then stopping are substantial, the hazards of continuing are ten times greater. Stopping before age 40 years (and preferably well before age 40 years) avoids more than 90% of the excess mortality caused by continuing smoking; stopping before age 30 years avoids more than 97% of it.
  
  
  BACKGROUND
  Cancer Research UK, Medical Research Council.
authors.affiliation: Cancer Epidemiology Unit, University of Oxford, Oxford, UK. kirstin.pirie@ceu.ox.ac.uk
authors.forename: Kirstin
authors.initials: K
authors.lastname: Pirie
authors.forename: Richard
authors.initials: R
authors.lastname: Peto
authors.forename: Gillian K
authors.initials: GK
authors.lastname: Reeves
authors.forename: Jane
authors.initials: J
authors.lastname: Green
authors.forename: Valerie
authors.initials: V
authors.lastname: Beral
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D003625:Data Collection; D005260:Female; D006801:Humans; D008875:Middle Aged; D016016:Proportional Hazards Models; D011446:Prospective Studies; D012307:Risk Factors; D012907:Smoking / Q000401:mortality*; D016540:Smoking Cessation / Q000706:statistics & numerical data; D012959:Socioeconomic Factors; D013997:Time Factors; D006113:United Kingdom / Q000453:epidemiology; D055815:Young Adult