Plane & Pilot have an article saying:
Altitude-chamber tests have shown that as oxygen deprivation increases, some victims experience a sense of increasing well-being, even euphoria, while they’re losing the ability to function in a thoughtful, coordinated manner.
But it's not too clear from that article how many people experience euphoria from oxygen deprivation. I'm pretty sure the loss of cognitive function is 100%, eventually, even though the dose/concentration might vary with the subject. But does everyone experience euphoria before that happens or does it happen just to some individuals with a certain vulnerability? This video suggests not everyone experiences euphoria, and if that's so what's the prevalence?
Optionally (i.e. if it's easy to answer based on the same source(s) for my main question), are there any proposed/studied pathways by which this euphoric consequence of oxygen deprivation might come about? And is it relevant to anything else, say to the "choking game" that some children (unfortunately) practice, or to asphyxiophilia/hypoxyphilia?