Adapted from Fletcher CM, Peto R (1977). The natural history of chronic airflow obstruction. BMJ, 1(6077): 1645–1648.

The graph illustrates the change in lung function over time for smokers, nonsmokers, and smokers who quit. Each point on the line represents the average level of lung function for each group at a particular age. Peak lung function is achieved at age 25 and it then decreases.

For example, at 60 years of age, a nonsmoker has nearly 100% lung function, while a smoker has 50% and a smoker who quit at age 45 has about 75%.