The normal level for blood oxygen is 95 to 100 percent, according to Mayo Clinic. A blood oxygen level under 90 percent is considered low.
Blood oxygen is measured by taking a blood sample from the arteries or approximated using a pulse oximeter, explains Mayo Clinic. A pulse oximeter is a small device that attaches to the finger and measures the level of oxygen saturation in blood. Blood oxygen levels below 90 percent indicate hypoxemia. Individuals with hypoxemia experience shortness of breath. Causes of hypoxemia are varied and include anemia, sleep apnea, interstitial lung disease, high altitudes and a blocked airway.