Why, in the 21st century, isn't the rule a prescribed blood oxygen level (or prescribed blood oxygen level for night ops and a prescribed blood oxygen level for day ops, if the science supports the difference)?
People who smoke may have an inaccurately high pulse ox reading. Smoking causes carbon monoxide to build up in your blood. A pulse ox can’t tell the difference between this other type of gas and oxygen may be one reason, that carbon monoxide could very well be coming from your exhaust system. Cold extremities and dark nail polish can also cause false low readings.