I think this curve is neat because it shows that we will all develop COPD if we live long enough, yet usually never enough damage will occur so it will present with symptoms.
Yet you can see that the longer you smoke the faster your lung function declines. However, quitting smoking, at any point in your life, can improve your lung function, thus helping you to live a longer life with COPD.
Essentially, this curve shows that smoking essentially speeds up the aging process, and that fewer cigarettes you inhale the longer you will live.
The American Thoracic Society published a good article on the curve called "Natural Histories of Chronic Obstructive Pulmonary Disease" in 2008. The article was written by Stephen I. Rennard and Jorgen Vestbo, who explained that the curve was the result of a 1976 study by Fletcher and Peto who studied the lung functions by measuring the FEV1 of a variety of participants, some who smoked and others who did not.
The result was this chart that shows the natural progression of COPD. The writers propose that this curve comes with limitations as it only measures FEV1, yet even so, it still provides us with a vivid picture of just how effective cigarette smoke is at reducing lung function.
It would be neat to see a similar curve showing the progression of lung function for those who inhale second hand smoke.