In many people a momentary drop to the 80s or even below is not an emergency or anything close to it. Not saying that it is good. Someone that is awake, sitting up, and struggling to breathe should be considered an emergency regardless of oxygen levels (and in this situation 80% would be very concerning).

EDIT: your comment is otherwise entirely correct, particularly at sea level.

90% you should really be traveling to see someone. At least an urgent care, if not an ER.