Without oxygen blood oxygen drops down generally because the cause of hypoxia has not been corrected, such as chronic obstructive pulmonary disease, acute exacerbation of asthma, interstitial pulmonary fibrosis or cardiac insufficiency and other related diseases. 1. Chronic Obstructive Pulmonary Disease (COPD), advanced COPD or acute exacerbation of COPD may be accompanied by hypoxia or even respiratory failure, and oxygen saturation may increase when oxygen is inhaled, while oxygen may decrease when oxygen is stopped. 2. Acute exacerbation of bronchial asthma, acute exacerbation of bronchial asthma can also appear chest tightness, respiratory failure. A decrease in blood oxygen may also occur after stopping oxygen intake in cases where asthma is not yet under control. 3. Interstitial lung fibrosis, especially in the advanced stage, may have chest tightness and dyspnea as well as respiratory failure, and even need long-term oxygen maintenance treatment. 4. Cardiac insufficiency, cardiac insufficiency can lead to pulmonary edema, at this time there can also be hypoxia performance, stop inhaling oxygen oxygen may have a significant decline in blood oxygen. Oxygen without oxygen on the decline can also be seen in a variety of other reasons, such as pulmonary embolism, pneumothorax and other cardiopulmonary diseases, the need to consult a doctor in a timely manner, to clarify the cause of the disease to give the appropriate treatment.