Smoking is by far the biggest cause of lung cancer. It causes more than 8 out of 10 cases (86%) including a small proportion caused by exposure to second hand smoke in non smokers (passive smoking).
Here are some facts about smoking and lung cancer
The more you smoke, the more likely you are to get lung cancer but the length of time you have been a smoker is even more important than how many cigarettes you smoke a day
Starting smoking at a young age is even more harmful than starting as an adult
Stopping smoking reduces your risk of lung cancer compared to continuing to smoke. The sooner you quit, the better your health - but it's never too late
Passive smoking (breathing in other people's cigarette smoke) increases the risk of lung cancer, but it is still much less than if you smoke yourself