Normal oxygen saturation levels (SpO2) are between 95 to 100 percent for both adults and children. 

Oxygen saturation levels below 95% are considered abnormal, and the brain may be affected when SpO2 levels drop below 80 to 85 percent. Bluish discoloration of the skin or mucous membranes (cyanosis) occurs when SpO2 levels fall below 67%.

However, older adults usually have slightly lower oxygen saturation levels than younger adults and children. People over 70 years of age may have oxygen levels closer to 95%, which can be normal for that age group, while normal SpO2 rates for infants and children are usually 97% and higher. 

Low levels of oxygen in the body’s tissues are called hypoxia. Low levels of oxygen in the blood are called hypoxemia. Both hypoxia and hypoxemia can occur separately, though they often occur together because when blood oxygen levels are low, the blood does not deliver enough oxygen to the body’s tissues. Hypoxia is often used to describe both low oxygen in the body’s tissues as well as low blood oxygen.