Oxygen partial pressure is used to determine the presence and degree of hypoxia in clinical practice. A partial pressure of less than 80 mmHg indicates hypoxemia, and a partial pressure of less than 60 mmHg indicates respiratory failure. Low partial pressure of oxygen is often caused by alveolar hypoventilation, reduced pulmonary diffusion, and imbalance of ventilation to blood flow ratio, such as in patients with chronic obstructive pulmonary disease, severe pneumonia, interstitial pneumonia, adult respiratory distress syndrome, pulmonary edema, and pulmonary embolism. A low partial pressure of oxygen is most often indicative of tissue hypoxia, and appropriate oxygen therapy should be administered at this time. For example, if hypoxemia is not accompanied by elevated partial pressure of carbon dioxide, type I respiratory failure should be treated with high concentration of oxygen, and if it is accompanied by elevated partial pressure of carbon dioxide, type II respiratory failure should be treated with continuous low-flow oxygenation.