The normal range of serum carbon dioxide is 23~31mmol/l. Low serum carbon dioxide may be related to respiratory or metabolic factors, such as hyperventilation, vomiting, diarrhea and so on. 1. Respiratory factors: if the respiratory rate is too fast and the expiratory or inspiratory time is longer, then hyperventilation can occur, and too much carbon dioxide is exhaled out of the body, then low serum carbon dioxide can occur, which is commonly seen in hyperthermia, strenuous exercise and bronchial asthma and other diseases. 2. Metabolic factors: carbon dioxide in the serum mainly in the form of bicarbonate exists, when because of diarrhea and other causes of excessive loss of bicarbonate in the body, metabolic acidosis can occur, the serum carbon dioxide decreased. In short, low serum carbon dioxide, should consult the doctor in a timely manner, to clarify the cause of the disease, and actively cooperate with the doctor’s treatment.