Cigarette smoking is the cause of most cases of emphysema. Tobacco smoke damages the lungs' alveoli, the tiny air sacs through which inhaled oxygen is transferred to the bloodstream and carbon dioxide is passed back to the lungs to be exhaled. The lungs become less elastic and breathing becomes increasingly difficult. The genetic form of emphysema occurs earlier in life (worsened by, but not dependent upon cigarette smoking). It is caused by a rare genetic deficiency of the protein alpha1-antitrypsin. In the absence of antitrypsin, which normally functions to protect the lungs from damage, the walls of the alveoli are attacked by chemicals released in alveoli in response to tobacco smoke and air pollutants.