Has your doctor discussed the use of supplemental oxygen, maybe just a simple compressor (I think it is) when sleeping? I'm sure it takes some getting used to wearing the breathing tub in the nose. My only experience with that is following surgery... my breathing problem seems to be sinus related. But, I am a past smoker, going on 30 years now, so I may have some lung damage from that.
I don't recall what the "correct" oxygen level is, but 90 doesn't sound too bad, guess it can approach 100% where the number represents saturation, or ability of blood to carry oxygen.
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