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A normal blood oxygen level depends on what exactly you’re measuring. The partial pressure of oxygen in a sample of arterial blood (PaO2) is normally 75–100 mm Hg. The partial pressure of oxygen in a sample of venous blood (PvO2) varies but is lower than this. The arterial oxygen saturation (SaO2), which can be measured by wearing a pulse oximeter, is normally >95%.
One of the primary functions of the blood is to carry oxygen around the body, delivering it to the tissues. The arteries carry blood away from the heart and contain more oxygen; the veins carry blood back to the heart and contain less oxygen. Having a low blood oxygen level means less oxygen is available for use by the body’s cells, which hinders cellular energy production. This can cause shortness of breath, headache, confusion, and—if oxygen levels are very low—organ damage and death. Low blood oxygen levels typically result from lung disease.
Low blood oxygen levels are addressed by treating the underlying problem. If this isn’t possible, supplemental oxygen (breathing oxygen through a tube) may be necessary. In some circumstances, such as in individuals with chronic obstructive pulmonary disease (COPD), it may be more appropriate to keep the SaO2 between 88–92%.
What we mean by normal
In medicine, using the term “normal” can sometimes be off-putting. Saying something is “normal” implies that everything else is “abnormal.” Additionally, saying something is “normal” may not be accurate, since something that is “normal” for you may not be “normal” for somebody else. Therefore, instead of saying certain values are “normal,” alternative terminology may be to say that these values are “healthy” or “within the reference range.”
Additionally, some values have well-defined cutoffs, while others do not. For example, when looking at hemoglobin A1c levels, a value of 6.5 or greater is always diagnostic of diabetes. On the other hand, when looking at testosterone levels, some use cutoffs of 270–1,070 ng/dL while others use cutoffs of 300–1,000 ng/dL.
The information above represents values that are commonly used as cutoffs. However, depending on the specific source you’re looking at or the laboratory you go to, their values may be a little different.
Dr. Tzvi Doron is Board Certified in Family Medicine by the American Board of Family Medicine and is Ro's Chief Clinical Officer.