Oxygen concentration at 35% is normal, the formula is oxygen concentration = 21 + oxygen flow x 4. In chronic diseases such as bronchial asthma and chronic obstructive pulmonary disease, oxygen concentration at about 35%, that is, continuous low-flow oxygen can effectively protect lung function and improve the patient’s symptoms of hypoxia, and also improve the patient’s cough, chest tightness, shortness of breath, dyspnea and other symptoms. If the oxygen concentration is too low, hypoxemia can easily occur, and type I respiratory failure can occur in serious cases, leading to impaired consciousness, aggravation of the disease, and even life-threatening. If the oxygen concentration is too high, it will lead to the phenomenon of oxygen toxicity, and if it is a pregnant woman, it will easily lead to the blindness of the newborn, so the oxygen concentration is more suitable around 35%.