The preset alarm and "official" oxygen deficient atmosphere is 19.5% which is way high. Put it this way, if you chose that alarm point, you would exclude yourself from a lot of cornish mines.
Yes, I agree. Our incident was without a gas detector and we were all heavy smokers at the time plus we had to exert ourselves considerably to exit the situation.
As said earlier a lot will depend on the particular case.
Edit: Looks like most will be OK down to 16%.
"One of the major problems is the fact that, in the initial stages, those affected do not notice that they are losing control and, instead, have total confidence in their abilities.
Studies have shown that the maximum safe level of oxygen is 23.5% volume, the minimum is 19.5%, and the average oxygen concentration in air is 21%.
At 17% the first signs of deprivation show themselves in the form of judgement errors and, at 16%, anoxia is apparent.
Between 16 and 12% the breathing and pulse rate increases and muscular co-ordination is slightly impaired.
Below 12% and above 10%, emotional upsets, abnormal fatigue upon exertion and disturbed respiration become evident while, between 10 and 6% the sufferer will experience nausea and vomiting, an inability to move freely, and loss of consciousness.
Less that 6% volume will bring convulsive movements and gasping respiration, breathing will stop, and a few minutes later heart action will cease".