There are many individual variations of hypoxia, even within the same person.


Generally, old age tends to reduce the efficiency of the pulmonary system,

^] Smoking drastically reduces oxygen intake efficiency, and  reduce tolerance by 3,000-6,000 feet
A rapid decompression can reduce the these numbers by up to 50 percent
AND,
less time is available for rational thinking,



The table below reflects various altitudes with the corresponding average ^{time til unconscious  are for an individual at rest}

Altitude in Flight level	Time of Useful Consciousness	Altitude in meters	Altitude in feet
FL 150	30 min or more	4,572 m	15,000
FL 180	20 to 30 min	5,486 m	18,000
FL 220	5-10 min	6,705 m	22,000
FL 250	3 to 6 min	7,620 m	25,000
FL 280	2.5 to 3 mins	8,534 m	28,000