A pulse oximeter reading of below 90 percent is considered low, reports Mayo Clinic. A pulse oximeter measures oxygen saturation level, which is then used to estimate the amount of oxygen in the blood. Oxygen levels can also be tested using a sample of blood from an artery.
Blood carries oxygen to the organs and tissues of the body, explains Mayo Clinic. If the amount of oxygen in the blood drops below normal, the risk for hypoxia increases. Hypoxia is when the blood doesn't contain enough oxygen to meet the requirements of the body, as defined by WebMD. A lack of oxygen in the blood can cause shortness of breath, cough, increased respiratory rate, wheezing and sweating.