If I smoked in my 20s then moved away from major cities and pollution at 40, is my risk for lung cancer high?
Higher than normal. Any time you smoke, you increase the risk of lung cancer. Even if you quit, the risk is still higher than someone who has never smoked. But, this doesn't mean that you should, then, keep smoking. The sooner you stop smoking the better. And the issue of whether someone develops lung cancer or not depends on many factors in addition to smoking, such as genetic disposition, etxc. But stop smoking.
100 cigarettes. If you smoked more than 100 cigarettes or 5 packs in a lifetime, then risk of lung cancer is higher than general population, but quitting smoking lowers that risk.
Not as high if quit. The risk for lung cancer depends on how long you keep smoking. If you smoke all your life, you will be at a much higher risk than if you stop at some point. If you still smoke, you will still be at high risk even if you move away from pollution. If you stopped in your 20's, then your risk has gone down significantly.