Normally, air humidity between 40% and 70% is normal. Air humidity is the ratio of the actual water pressure in the air to the saturated water pressure at the same temperature. The higher the humidity, the more humid the air is. The human body feels most comfortable when the air humidity is between 40% and 70%. If the air humidity is too high, it can cause nausea, constipation, loss of appetite and dizziness. If the air humidity is too small, less than 40% of the time, dust, bacteria and other easy to adhere to the oral cavity, nasal mucosa, irritation of the larynx triggered by coughing, resulting in bronchitis, asthma and other respiratory diseases. The human body’s optimal air humidity is 40% to 50%, high or low humidity will make the human body feel uncomfortable.