Excellent explainer in this months "Pulmonary Paper" Ryan Report... on why "88% is the magic number" he describes it as a hill you 
are
on. As you go down towards 88%, the slope is gradual, but at 88%, it
gets very steep. Once below 88%, it gets dangerous, and can be difficult
to go back up. This is why keeping your pulse-ox handy, the more you
know about your body the better.
This also supports the advice of when you feel an SOB attach, STOP what your doing, and immediately start PLB to get your system back into the green zone {i.e, SpO2 > 88%}.
ed