Mayo Clinic defines the normal human oxygen level as 95 percent to 100 percent. Levels below 90 percent are considered low. Oxygen levels are usually measured with a pulse oximeter device attached to the finger; this device measures the amount of oxygen saturating the blood.
Low oxygen levels can be caused by acute conditions, such as pneumonia, or chronic conditions, such as emphysema. Various heart problems may also cause low oxygen levels. Symptoms of low oxygen levels include shortness of breath during mild activity, shortness of breath during exercise and shortness of breath when sleeping, according to Mayo Clinic. People are encouraged to contact their doctor when severe shortness of breath comes on quickly and is not relieved.