A question got posed in the Cilia
string that asked about the cleansing and healing that goes on after quitting. The lining tissue of the bronchus starts to heal very quickly, within days in fact. Over time this tissue as illustrated here and in the cilia string does return to normal. If a pathologist were to examine scrapings from the lining of the bronchus years after a person quit, he or she would not be able to determine if the patient were ever a smoker. This is quite significant considering that close to 90% of lung cancers that occur in the lung are from this specific region.
Underlying lung tissue doesn't have the same kind of repair capabilities. Once underlying lung tissue is destroyed, it does not regenerate. This is why a person who is diagnosed with emphysema will likely still have the disease after quitting. They may in fact breath better after quitting, not because of healing the destroyed lung tissue which is the cause of the disease, but rather because the healing of the bronchus lining and the subsequent cilia regeneration helps keeps the small airways cleared of mucous obstruction. So while a person doesn't grow new lung, what they have left is able to work more efficiently and with the clearing of the small airways because of the decrease in mucous production and the cleansing action of the restored cilia.
Also, the purpose of the lungs is to oxygenate the blood. When a person smokes they are taking in large amounts of carbon monoxide which is literally poisoning the oxygen carrying capacity of the blood. When a person quits smoking carbon monoxide levels return to normal in less than one week. So again, the lungs are able to function much more efficiently, not having to do the extra workload of getting more oxygen because of carbon monoxide suffocating effects.
Much of the discoloration of the underlying lung tissue may not be cleared out--but the chemicals do lose their potency over time. I wish I could say that when a person quits that their underlying lung tissue which is responsible for breathing returns to normal but it really is not the case. But when a person quits the tissue that is left is much better able to cope with the body's demands. And in the case of cancers of the bronchus, which is where a vast majority of lung cancers occur, this tissue does in fact return to normal if the person just quits before cancer is initiated in this area.
So while it can't be said that quitting smoking returns a person to a state of never smoking, we can say that the risk of initiating future diseases decrease, and the overall efficiency of the remaining healthy tissue increases. The sooner a person quits, the sooner the repairs start and the sooner the assault of the thousands of dangerous chemicals stops. Again, in the case of cancer you just don't know which cigarette would be the one that starts an irreversible and life threatening process. To minimize the risk now is no more complicated than knowing to never take another puff!