Respiratory infections, increases in inhaled pollution, and the occurrence of other medical problems will further reduce their ability to absorb oxygen and to expel carbon dioxide. These problems can send COPD patients into hypoxemia. Such stresses are unavoidable, so COPD patients suffer repeated episodes of significantly worsened symptoms called “acute exacerbations.” Acute exacerbations resolve slowly over weeks or months even with medical treatment, and sometimes acute exacerbations must be managed in a hospital.