I think this underlines the risk

Should the oxygen fail how long have we got??

Figure 8 - Times of Useful Consciousness(Effective performance time)

Altitude Conscious time
20,000 5 – 12 minutes
25,000 2 – 3 minutes
30,000 45 – 75 seconds
35,000 30 – 60 seconds
40,000 10 – 30 seconds
45,000 12 – 15 seconds
50,000+ 12 or less seconds

Concentrates the mind

G