Not everyone who develops lung cancer does so because of smoking. Smoking or being exposed to secondhand smoke causes the vast majority of lung cancers. The more people smoke and the longer they smoke, the greater their chances of developing the disease, reports the U.S. National Cancer Institute.
Lung cancer starts when abnormal cells grow out of control in the lung. They can invade nearby tissues and form tumors. Lung cancer can start anywhere in the lungs and affect any part of the respiratory system.
Early lung cancer doesn't usually cause any symptoms. This is why it's not usually found early. In its advanced stage, cancer may affect how your lungs work. The first signs of lung cancer may include:
Feeling short of breath.
Having blood in any mucus that you cough up.
Sources: Healthwise, HealthDay, American Cancer Society and National Cancer Institute