I have climbed Mt. Kinabalu (13,455ft), along with many other people, with no ill effects. Reinhold Messner (probably spelt wrong) climbed Mt. Everest with no oxygen many years ago and others have done it since. Climbing takes a lot of effort. Just sitting in an aeroplane requires very little effort. So your figure of 8,000ft is way out. That figure is for comfort and to avoid having to pressurise the cabin to sea level which would be a much greater pressure differential.