Cancer is a class of diseases characterized by out of control cell growth. Lung cancer occurs when this uncontrolled cell growth begins in one or both lungs. The lungs are two large spongy organs located inside the chest cavity. Air is breathed into the trachea and moves down two tubes called bronchi, each going to a lung. Lung cancer occurs most often in adults between the ages of 40 and 70 who have smoked cigarettes for at least 20 years.