A low blood oxygen level, or hypoxemia, is considered anything under 90 percent, notes Mayo Clinic. A normal range is 95 to 100 percent. Blood oxygen level may be measured with a blood test or a pulse oximeter, which clips onto the finger and measures oxygen saturation through estimation.
The body needs an appropriate amount of oxygen to function properly, states Mayo Clinic. Cells, organs and tissues need oxygen to live. Hypoxemia may be caused by a variety of conditions, such as acute respiratory distress syndrome, narcotics that depress the breathing, or pneumonia. Anemia, sleep apnea and COPD can also cause hypoxemia, as well as a host of other conditions.