Stopping smoking, even after many years, has substantial benefits for health. People who stop smoking before middle age avoid more than 90 per cent of the risk of lung cancer attributable to tobacco and those who stop even well into middle age avoid most of the risk, a study has found.
Professor Sir Richard Doll (emeritus professor of medicine, Radcliffe infirmary, Oxford), Professor Sir Richard Peto (professor of medical statistics and epidemiology, Radcliffe infirmary, Oxford) and colleagues compared lung cancer and smoking cessation rates in two large studies conducted in 1950 and 1990 in the UK. They found that, among both men and women in 1990, former smokers had only a fraction of the lung cancer rate of continuing smokers and that this fraction fell steeply with time since stopping.
The cumulative risk of lung cancer by age 75 is shown in the Table. The risk for women who stopped smoking early in life was lower than the risk for stopping at age 50 but the number of women who fell into this group was too small for statistical stability, the researchers say.
However, the cumulative risk of developing lung cancer has increased since 1950. They suggest that the reason for this is that older smokers - lung cancer mainly occurs above the age of 55 - in 1950 were likely to have smoked substantially fewer cigarettes during their lifetime than the same group in 1990. In addition, people now smoked more intensively when they were young.
There have also been changes in the pattern of smoking among women. Women in 1990 smoked more heavily than women in 1950 and a substantially higher proportion started smoking at an earlier age - 68 per cent in 1990 and 24 per cent in 1950 started before the age of 20. In addition, women now smoke cigarettes more like the way men do, the authors say.
The study also found that widespread smoking cessation since 1950 in the UK had roughly halved the number of cases of lung cancer that would now be occurring if former smokers had continued to smoke. Mortality in the first half of the 21st century will be affected most by the number of adults who stopped smoking whereas mortality in the second half of the century will be strongly affected by the number of young people who become smokers, the authors comment (British Medical Journal 2000;321:323).

Former smokers had only a fraction of the lung cancer rate as continuing smokers