Oxygen Use in Aviation (Aircraft Owners and Pilots Association)
"...you have to use supplemental oxygen if you fly more than 30 minutes at cabin pressure altitudes of 12,500 feet or higher."
Outdoor Action Guide to High Altitude: Acclimatization and Illnesses (Princeton)
"The major cause of altitude illnesses is going too high too fast. Given time, your body can adapt to the decrease in oxygen molecules at a specific altitude. This process is known as acclimatization and generally takes 1-3 days at that altitude."
Development of severe hypoxaemia in chronic obstructive pulmonary disease patients at 2,438 m (8,000 ft) altitude (European Respiratory Journal)
"Patients with lung disease are more vulnerable to low atmospheric pressure than healthy individuals."