I see here some misinformation about the subject and would like to correct it. As a diver I know what is going on in human body with pressure and various gases.
Nitrogen narcosis.
It starts normally at the depths of 30-40m of water. With ambient pressures of 4-5 bar. Narcosis can not happen in flight. Never! narcosis is not related to gaseous nitrogen in brain. If gas appear in your brain you are having serious decompression sickness. It is very serious and life threatening1
Decompression sickness.
Its mild forms are called also bends. If ambient pressure drops too fast, either due to too fast cabin decompression or too fast ascent to surface, nitrogen, dissolved in body tissues starts exit tissues and to create tiny nitrogen bubbles in joints, muscles and if problem is serious in your bloodstream. The result is pain in joints, and numbness and skin iching in less serious forms. Bubbles in bloodstream block bloodsupply and if these bubbles block bloodsupply in heart or brain, result is serious. This will certainly incapacitate just anybody.
There is an important factor - older, less fit, more fat and tired persons are much more prone to decompression sickness than younger, fit, and healthy persons. The difference is remarkable. Really.
When flying at 10000m (was it 36000 foot?) then ambient pressure is approx. 0,27 bar, and using pure oxygene is pretty normal as oxygen partial pressure must not drop below 0,18 bar. Below then hypoxia developes. The problem here is that removing nitrogen part and decreasing ambient pressure to 0,27bar would make the normal, dissolved nitrogen exit tissues quite fast. There is no problem if you breath oxygen at sea level. Problem could arise when ambient pressure is decreased as well. As decreased pressure will force nitrogen out of tissues more rapidly.
Thats why some pilots who fly high, need to breath pure oxygen before flight - to reduce nitrogen, dissolved in tissues, to avoid incapacitating deco sickness.
So, with uncompressed cabin, breathing pure oxygen, flying at normal flight levels is possible, only you must reduce your dissolved nitrogen before flight.
Hope that helps with understanding of deco sickness, partial pressures, nitrogen narcosis. Though I've never seen any deco tables for flightcrew
