Marjorie,
I spent many years working at high altitudes seasonally. I'm wondering if maybe the greater exposure one gets at high altitudes gives greater protection due to whatever beneficial effects one gets from the sunshine/vitamin D in the thinner atmosphere, such that it would be protective in the thinner atmosphere whether or not more rbc's are produced.
Being at high altitudes consistently forces the body to produce more rbc's because the oxygen content of the air is lower at high altitudes. I don't know whether it actually ends up creating a situation of extensive oxygen saturation simply because of rbc production stimulation. The hemoglobin content is also important.
A.A.
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