Jarrah said:On page 38 of The History Of Manned Space Flight, Dr. David Baker tells us: On Earth, atmosphere consists of a 760mmHg mixture of 21% oxygen and 79% nitrogen, plus several trace gases that can be ignored for temporary and artificial environments. Because the quantity of oxygen in the lungs regulates the amount entering the blood, any appreciable increase in partial oxygen pressure of 160mmHg will cause irritation of mucous membranes and could disturb the function of critical enzymes.
However, even a moderate fall below the normal partial pressure value quickly causes brain damages and severe side-effects.
The option available to Mercury engineers were that the capsule could contain either a two-gas mixture of oxygen and nitrogen at sea-level pressure (760mmHg) or a single gas atmosphere of pure oxygen at about 160mmHg. Tests revealed that the pure oxygen pressure limits were between 150mmHg and 345mmHg.
Oxygen pressure for a single gas atmosphere outside these limits would cause severe, if not permanent, damage."
150mmHg = 2.9psi
345mmHg = 6.67psi
Depending on whether you ask Michael Collins, Gordon Cooper or Frank Borman; Apollo 1s cabin pressure was either nearly 16psi or 16.7psi or 20.2psi. Either way, these pressures are outside the safe pressure limits for breathing pure oxygen.
Why did NASA not classify Apollo 1 as hazardous, when they knew since Mercury that breathing pure oxygen at those pressures alone was a hazard?
The reports I have read list slightly different tolerance (Po2=425mmHg) but not enought to make a difference to what you are saying.
So: is breathing 100% oxygen at 16 psi or greater (or at a Po2 of 425 mmHG or greater) hazardous? Yes, if it is done for anything other than short periods... but the astronauts were all in their A1C pressure suits, breathing oxygen at around 2.5-3.5 psi. If there were an emergency whereby they had to remove the helmets, they pressure would have been dumped and the short exposure to 100% oxygen at elevated pressures would have had little - if any - effect on the affected crewmember.
Even so, we once again come to the question as to why this test was not classified as hazardous... and once again the answer is they got complacent and just didn't think it was that hazardous. None of the astronauts did.