A normal oxygen level is 95 to 100 percent oxygen, as measured by pulse oximetry, says the Mayo Clinic. A pulse oximeter is a small device which clips on a finger and measures oxygen saturation in the blood, a measurement which is usually considered to be the equivalent of blood oxygen levels.
Any oxygen level under 90 percent is considered low, according to the Mayo Clinic. Hypoxia, or low blood oxygen levels, can be caused by disease, a blocked airway, high altitude or some medications. If blood oxygen levels drop as low as 80 percent, the person experiencing them may suffer from damage to the heart or brain, as reported on Wikipedia.
Symptoms of low blood oxygen levels include shortness of breath, especially when it comes on suddenly. When accompanied by headache, cough and fluid retention at high altitude, these symptoms may indicate the presence of high-altitude pulmonary edema, which can be fatal, says the Mayo Clinic. Extreme shortness of breath should also be treated by a doctor urgently.
In addition to the use of a pulse oximeter, which uses light to estimate oxygen levels in the blood, oxygen levels are also often tested by means of an arterial blood gas test, says Wikipedia. In this test, blood is drawn from an artery and tested not just for the levels of oxygen, but also for carbon dioxide and acidity.