Do NOT mess around with this.
About 20 years back, while riding my motorcycle cross country, I had done about 600 miles that day, mostly at lower elevations. Started to feel a bit of fatigue and a headache coming on so elected to stop in Breckenridge for the night. Elevation 9600 feet or so.
Pulled up to the Holiday Inn right off the highway and requested a room. Desk clerk looked up at me and said "Of course, sir. Why don't you take a seat over there, I'll have someone bring you a glass of juice and get working on setting up your room." What nice folks here, thought I. A few minutes later, in walks 2 EMTs with an O2 bottle and a mask. Stop at the front desk and the clerk head points to me. Before I could more than say boo, they had me masked up and insisted I drink the juice and then some water.
Turns out the clerk noticed finger nails and lips blue and some mild disorientation (slurred speech, wandering eyes).
They had had 3 guys fly in from NY the week previous who promptly headed for the bar right after check in. One guy tells his buddies, after 3 or more cocktails, he is feeling a little jet lagged and headed for his room. Next morning buddies couldn't rouse him and it turns out nobody could, he died in his bed that night.