Your lungs have many functions. The most important is to remove carbon dioxide from the bloodstream and to replace it with oxygen–allowing you to breathe. Your lungs also help to filter blood clots in your veins and act as a layer of protection for your heart. Lung cancer is mainly caused by smoking–in fact, 87% of lung cancer cases are smoking-related. Other risk factors include radon and asbestos exposure, and there has been some research into genetic risk factors as well.
The number one cause of lung cancer is cigarette smoke, including secondhand smoke. Even if you are not a smoker, you could still be at risk for lung cancer because of secondhand smoke, also known as passive smoking. Secondhand smoke is dangerous because it comes from two sources: the end of a lit cigarette and the smoke blown out of a smoker’s mouth.