An oxygen saturation of 90% is a normal low value, and greater than or equal to 90% is clinically normal. If a patient has decreased oxygen saturation, it is usually due to respiratory disease or cardiovascular disease. Examples include severe pneumonia, tuberculosis, lung tumors, chronic obstructive pulmonary disease, acute respiratory distress syndrome, pulmonary embolism, etc. Cardiovascular diseases include cardiac insufficiency, and severe anemia can also cause a drop in oxygen saturation. Therefore, when a patient has a drop in oxygen saturation, he or she needs to actively seek medical attention, improve relevant tests, clarify the diagnosis, and take appropriate treatment according to the diagnosis. For example, if a patient has decreased oxygen saturation due to severe pneumonia, he or she should receive oxygen and anti-infection treatment. If the patient’s oxygen saturation drops due to cardiac insufficiency, anti-cardiac failure treatment is required.