What actually happens when fighter pilots take off their masks

Capt. Zoe Kotnik clips on her mask in her F-16 prior to a sortie at Eglin Air Force, Florida, November 2017.
US Air Force/Samuel King Jr.
A fighter pilot's mask is primarily used to prevent the pilot from getting hypoxic, which is the lack of oxygen to the brain.
The effect of flying without the mask depend on how high the jet is flying and how long the pilot goes without the mask.
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