A doctor writes some more...
What Jason said.
Except I don't see why anyone* would want to measure their sats outside a hospital. I don't really even find the GP measured sats useful - it rarely influences the decision for hospital assessment. It may be useful in the "theatre" of doctoring - here is a shiny machine! It says ping! It says you do not need to go to hospital! [Patient leaves happy they required not *just* their GP, but also a shiny machine saying ping]. I'd be interested in the views of GPs (if any are out there...)
People can tolerate very low sats if there is a gradual trend - on summiting Everest they're usually in the 70s... which given that a hospital will panic if less than 92% in a normal person is quite a drop (I'd actually start to worry at 94% in a truly healthy person). If you were subjected to that acutely you'd pass out, and not wake up until the O2 was cranked back up. Those with cyanotic heart disease can often be walking around with sats in the 70s, albeit they look a little blue around the edges.
Finally I would like to commend the 2015 (I think) Australasian oxygen guidelines to any clinicians out there, as they happily accept targeting O2 sats much lower than other guidelines due to the dangers of hyperoxia.
* except those with COPD (smoking related lung disease) - it may help decide when to visit your GP.