Absolutely useless Gingernut
Many moons ago when we started using pulse oximeters on patients on ventilators we were staggered to see the readings when we put them on each other in flight. You would expect over 96% on any walking patient, and on intensive care we sweat if we get down to the low 90s. Anything in the 80s......little hope
Then we saw our readings were in the 80s. Just into the 90s in a few,,,, and we were flying week in week out with little sign of brain rot....The reality is an oxygen saturation in the 80s caused by depressurisation in a healthy person causes no issues except perhaps night blindness. And that is reversible. So they cant really be used as a safety device.
The problem is that the amount of oxygen available in the blood is not a straight line relationship with the reading on the pulse oximeter. When it gets to the 80s just a tiny reduction in actual oxygen can lead to a big drop in saturation, so a pulse oximeter is worthless IMHO unless you are on oxygen when you can use the reading to titrate the flow rate against the saturation. And then you should aim for 94%...