pressure breathing
Air is a mixture of gasses, one being oxygen. Each component may be said to have a partial pressure in accordance with the percentage of the whole
As air pressure reduces the partial pressure exerted by the oxygen component reduces and at about 10,000 ft the oxygen content becomes marginal to sustain normal activity and so extra oxygen is required, As altitude further increases an increased proportion of oxygen in the overall gas mixture is required to the point that even 100 % oxygen does not reach the level of oxygen partial pressure at 10,00 ft. This is at about 25,000 ft. At this point the need is for an increased pressure of oxygen to sustain activity, so called pressure breathing. The military practice this in special chambers and also practice sudden cabin pressure failures with the associated physiological effects.
The normal flight crew masks will supply 100 % oxygen and will prevent extraneous gas ingress by using a small excess pressure but they are by means "pressure breathing" and cannot sustain activity for long above 25,000 ft
The physiological effects of sudden loss of cabin pressure above 25,000 ft include but are not limited to;Gut gas expansion causing pain and evacuation downwards with part of other gut contents. If there is excessive gut gas the gut may become twisted with associated extreme pain.
Partial blockages of sinuses and middle ears with associated extreme pain and possible eardrum puncture. Pain in teethe fillings