A normal PO2 (partial pressure of oxygen) in arterial blood is 75 to 100 mmHg for a healthy adult at sea level. This number reflects how much oxygen is dissolved in your blood and is one of the key measurements from an arterial blood gas (ABG) test. Your specific normal depends on your age, your altitude, and whether the sample came from an artery or a vein.
What PO2 Actually Measures
PO2 measures the pressure exerted by oxygen molecules dissolved in your blood plasma. It’s different from the oxygen saturation reading (SpO2) you get from a pulse oximeter clipped to your finger. SpO2 tells you what percentage of your hemoglobin is carrying oxygen, while PO2 tells you how much oxygen is physically dissolved in the liquid portion of your blood. Both matter, but PO2 gives a more detailed picture of how well your lungs are transferring oxygen into your bloodstream.
The two numbers are related but not interchangeable. A PO2 of about 95 mmHg corresponds to an SpO2 of roughly 97%. At a PO2 of 60 mmHg, SpO2 drops to around 92%. Below 60 mmHg, oxygen saturation falls steeply: a PO2 of 50 mmHg corresponds to about 89% saturation, and 40 mmHg to roughly 75%. This steep drop is why clinicians pay close attention when PO2 dips below 60.
Normal Ranges by Severity
PO2 values fall into well-defined categories:
- Normal oxygenation: 80 to 100 mmHg
- Mild hypoxemia (low oxygen): 60 to 79 mmHg
- Moderate hypoxemia: 40 to 59 mmHg
- Severe hypoxemia: below 40 mmHg
- Hyperoxemia (too much oxygen): above 100 mmHg
Values in the 75 to 80 range are still considered within the normal reference range, especially for older adults, though they sit at the lower boundary.
How Age Shifts Your Normal
PO2 naturally declines as you get older. A useful rule of thumb: take 100 and subtract the number of years you are over 40. A 60-year-old’s expected PO2 would be around 80 mmHg, and a 70-year-old’s might sit between 70 and 80 mmHg at sea level. Both of those are perfectly normal for those ages, even though they’d be on the low side for a 25-year-old.
This age-related decline happens because the lungs gradually lose some of their efficiency at gas exchange over decades. The airways and air sacs become slightly less elastic, and the matching between airflow and blood flow in the lungs becomes less precise. None of this means something is wrong. It’s expected physiology.
How Altitude Changes the Numbers
The reference range of 75 to 100 mmHg assumes you’re near sea level. At higher elevations, there’s less atmospheric pressure pushing oxygen into your lungs, so normal PO2 drops accordingly. A young, healthy person at sea level typically has a PO2 of 90 to 95 mmHg. At about 2,800 meters (9,200 feet), roughly the elevation of cities like Bogotá or Aspen, that same person’s PO2 drops to around 60 mmHg. At 6,100 meters (20,140 feet), it falls to about 35 mmHg.
If you live at a significant elevation, your body compensates over time by producing more red blood cells and adjusting how efficiently hemoglobin grabs and releases oxygen. A PO2 that would indicate moderate hypoxemia at sea level can be entirely normal for a long-term high-altitude resident.
Arterial vs. Venous PO2
The 75 to 100 mmHg range applies only to arterial blood, which is blood that has just been freshly loaded with oxygen in your lungs. Venous blood, which is returning from your tissues after delivering oxygen, has a much lower PO2: the normal range for venous blood is 30 to 55 mmHg on room air. This lower number isn’t a sign of a problem. It simply reflects the fact that your tissues have already extracted the oxygen they need.
Most of the time, when someone refers to “PO2” without specifying, they mean the arterial value (sometimes written as PaO2). Venous PO2 (PvO2) is measured less frequently and for different clinical reasons, usually to assess how well the body’s tissues are using oxygen rather than how well the lungs are supplying it.
When PO2 Is Too High
Most people worry about PO2 being too low, but excessively high levels carry their own risks. A PO2 above 100 mmHg on room air is unusual, but it commonly occurs in patients receiving supplemental oxygen. Research on cardiac intensive care patients found that a PO2 above 150 mmHg was associated with increased hospital mortality, and patients with levels above 300 mmHg had more than double the mortality risk compared to those in the 60 to 100 mmHg range.
For most people breathing normal air, hyperoxemia isn’t a practical concern. It becomes relevant during hospitalization when supplemental oxygen is being delivered, which is why oxygen therapy is carefully titrated rather than simply given at the highest possible flow.
How PO2 Is Tested
PO2 is measured through an arterial blood gas test, which involves drawing blood from an artery, usually at the wrist. This is different from a standard blood draw, which pulls from a vein. The arterial draw can be more uncomfortable because arteries sit deeper and have more nerve endings nearby. Results are typically available within minutes.
A pulse oximeter on your finger gives you SpO2, not PO2. As long as your SpO2 reads 95% or above, your PO2 is very likely in the normal range. If SpO2 consistently reads below 92%, PO2 is probably around 60 mmHg or lower, which falls into the mild to moderate hypoxemia range and warrants further evaluation.