As soon as you stop smoking, your risk of lung cancer starts to go down. 10 years after you’ve stopped smoking, your lung cancer risk is half that of someone who has continued to smoke.
Source: Lung cancer myths and facts @ NHS Choices
Image by Challiyil Eswaramangalath Vipin from Chalakudy, India (cigarette.. like a cigarette in the rain) [CC-BY-SA-2.0], via Wikimedia Commons