|Graph from the British Medical Journal|
Why is this? No one knows. However, a logical theory is that people with a certain genetic makeup may be more susceptible to the effects of smoking. Another theory is that this poor genetic makeup may include, among others, the asthma gene.
However, if we refer to the Fletcher and Petco Curve
That said, Scott Cerreta, in his May, 2013, column in AARC Times, "It's Never Too Late To Stop Smoking," defined the two types of COPD:Average smokers:
1. Average smokers: Heavy daily smokers not susceptible to the effects of smoking, and do not seem to have a significant enough decline in lung function as determined by FEV1 (pulmonary function testing) to result in COPD flare ups.
2. Susceptible smokers: Heavy daily smokers who are susceptible to the effects of smoking, and do have a significant loss of lung function to cause COPD flare ups, and this generally begins to occur at the age of 35-45. Generally, the first symptoms are overlooked, and loss of lung function so gradual, that the person may not even notice it until struck with a severe flare up that requires a trip to the emergency room.