I was recently in a hospital with a friend and as we walked along the ward I noticed a few patients had a probe attached to their fingers. Being curious I asked about these and was told that they were there to monitor oxygen saturation which refers to the fraction of haemoglobin that is saturated by oxygen. It is related to the partial pressure of oxygen in the inspired air and, therefore, to altitude. Normal oxygen saturation in an otherwise healthy individual at sea level is > 95%.

An...


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