Hemoglobin becomes saturated with oxygen through a process known as aerobic metabolism. Aerobic metabolism is one of the by products of breathing, explains the Mayo Clinic. If oxygen saturation levels drop too low, a person runs the risk of passing out and eventually dying. Oxygen saturation levels fluctuate depending on elevation, but they should generally stay within the range of 95 percent and 100 percent. Extremely high elevations may be slightly below this range.
Sources: hopkinsmedicine.org, mayoclinic.org