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I've noticed that my blood oxygen is nearly always in the 95-98 range whereas my wife is 98-99. I'm healthy with no known conditions whereas she has asthma.

What is the normal range for healthy humans and what does a slightly lower average level mean, if anything? Does it correlate positively or negatively with fitness for example, as resting heart rate?

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According to Minnesota Dept. of Health:

A normal level of oxygen is usually at least 95% or higher. Some people with chronic lung disease or sleep apnea can have normal levels of around 90%. The SpO2 reading on a pulse oximeter shows the percentage of oxygen in someone’s blood.

The instructions that came with my home-use pulse oximeter (in German) roughly concur: 94-99% "normal". 90-93 "consult an MD", <90% "call an ambulance". It does also come with a table for compensating for (earth's) elevation too, as that affects O2 concentration. Basically below 1500m altitude 90 is the safety limit, but at 5800m for example that decreases to 80% and to 70% at 7500m. (It even comes with a citation for this part: Hackett PH, Roach RC, "High-Altitude Medicine" in Auerbach PS (ed.) Wilderness Medicine, 3rd ed., Mosby, 1995)

From a more recent study on that (high-altitude angle), there's actually a fairly substantial spread in how individual SpO2 responds to altitude

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