Tobacco smoking, especially cigarettes, is linked to lung cancer, which is the most common cause of cancer death in most countries. Eighty percent of lung cancer cases are due to cigarette smoking. The toxins from cigarettes damage the alveoli so they leak and pop, causing emphysema. Lung cancer usually develops in the epithelium of the airways, spreads through the walls of the airways, and enters the bloodstream and lymphatic system. These changes are usually not detected until secondary growths or other symptoms occur. Only 13 percent of lung cancer patients live as long as five years after the initial diagnosis.