On Wednesday, November 9, 2016 at 1:42:05 PM UTC-7, Duster wrote:
It's all about getting oxygen to brain tissue (oxygen exchange efficiency), and avoiding embolism (aka "the bends"), not so much % O2 that's available to breath. At those altitudes, breathing 100% oxygen is about as efficient as breathing air at around 12,000ft (someone correct me if I'm off). Hence the need to increase "cabin" pressure in the Perlan 2 and as the USAF is doing now with the U2's. Remember Robert Harris and his 1986 49,009ft altitude record was done in an unpressurized Grob 102. He was forced to come down when his O2 system started to fail, apparently suffering from medical issues for some time thereafter. Now it kinda makes sense why Perlan 1 (Fossett) spent all that time and money to best Bickle's flight by only 1,600ft wearing NASA spacesuits with Enevoldson.

https://jonathanturley.org/2013/09/0...r-all-seasons/