A normal lung is pink. In respect of lung cancer at least, it is like playing Russian Roulette or a deadly dice game with every cigarette that one smokes. The damage is measured in packet years or cigarette years . It is reported that lung cancers have usually existed in the lung up to about 15 years before they are large enough to be noticed or diagnosed. So even if one rolls an unlucky throw of the dice, one will not know for 15 years.
Lung cancers expand exponentially, the initial rate of increase in terms of volume is very slow.
It is also know that few smokers started smoking when they are adults - or few adults are stupid enough to start smoking. After another 10 years of having stopped smoking ones changes of catching lung cancer is reduced to about twice that of a non smoker. One is not exactly cured but one rolls the dice less frequently. As long as there may be a cancer already growing in the lungs or residues of tar in the lungs the risk will not go away. The long delay time for lung cancer is probably one of the major reasons that smoking is so difficult to give up.