For the rapid decompression theorists here are some facts from RAF AVMED.
Hypoxia (anoxia) occurs when the body is short of oxygen.
Amount of oxyhaemoglobin in the blood depends on the amount of oxygen in the lungs (not atmosphere).
Partial Pressure of Oxygen
At sea level the standard atmospheric pressure is 760 mmHg. 21% of 760 will be from Oxygen because amount of oxygen in the air is 21%. Thus partial pressure of oxygen in the atmosphere at sea level is 160 mmHg. The amount of oxygen in the air can be described as its partial pressure in mmHg. As altitude increases the partial pressure of oxygen in the atmosphere falls. The rate of change of pressure is greatest as we climb from sea level and decreases with altitude.
At sea level, percentage of oxygen in the lungs is only 14.5% (in atmosphere it is about 21%). Thus the partial pressure of O2 in the lungs is 100 mmHg.
As altitude increases the partial pressure of oxygen in the atmosphere falls but the proportion of oxygen in the atmosphere remains the same. Also as altitude increases the partial pressure of water vapour and to an extent carbon dioxide in the lungs remains the same reducing the partial pressure of oxygen in the lungs still further. Reduction in cabin pressure to an equivalent altitude of 8000 ft (65 mmHg partial pressure of O2 in lungs) produces a detectable impairment of mental performance. Healthy people are able to compensate for altitudes up to 10,000 to 12,000 ft. Above this, risks become serious. Short term memory is affected early on.
At 18,000 ft the partial pressure is 50 mmHG (half that at sea level).
Unconsciousness occurs at about 35 mmHg of oxygen in the lungs (equivalent to prolonged exposure to altitudes between 20,000 and 25,0000).
Air can be used up to 10,000 ft. After that it needs to be mixed with oxygen up to 33,700 ft. Above this 100% oxygen upto 40,000 ft.
Above 40,000 ft 100% oxygen alone is insufficient and it must be supplied under pressure to the oxygen mask. This is pressure breathing.
Hypoxia does not lead to shortage of breath. If the oxygen supply system fails the normal reaction to lack of oxygen, "Panting" does not appear because, there is no excess of carbon dioxide.
Onset of hypoxia is insidious (like CO poisoning) and can be recognised only by being very aware of the symptoms.
Symptoms of Hypoxia:
Concentration difficulties.
Impaired judgement, mood changes, euphoria (euphoria can be experienced above 10,000 ft).
Drowsiness and lethargy.
Light headedness, dizziness, nausea.
Loss of muscular co-ordination.
Pallor and cyanosis.
Failure of the basic senses, especially colour vision, which becomes affected by 8000 ft. Night vision affected above 8000 ft.
Unconsciousness, coma and death.
Following factors increase the onset of hypoxia:
Exercise
Cold
Illness/age
Fatigue
The use of drugs/alcohol
Smoking. It can raise the physiological altitude by 4-5 thousand feet above the actual cabin altitude as the ability to transfer oxygen is reduced by 4% to 10%.
Time of Useful Consciousness
It is the length of time during which an individual can act with both mental and physical efficiency and alertness. It is measured from the moment at which an individual is exposed to hypoxia. It varies with altitude and these are the relevant levels believed to apply in this case
Altitude in ft .......... Time
30,000 .................. 30 seconds to 1 minute
35,000 .................. 15 to 30 seconds
Time of useful consciousness for people doing light to moderate work (effective performance time) falls by 40%.
Time of useful consciousness for people already short of oxygen (flying at a cabin altitude of 8000 ft) are half of the above values.
Can any 777 pilots confirm the cabin px at cruise altitude of 35,000ft? I know from my time as an FE that in many aircraft it's in the order of 5000 to 8000 ft so even before a failure all occupants are partially suffering from hypoxia.
Not a theory just some facts to help the discussion along.