I found the mention of the Zenith balloon disaster interesting. It led me to reading this article in The Medical Times and Gazette from 1875. It shows what experts at the time thought about the cause of death:
'The deaths of the two aeronauts in question, he added, were not caused by want of air or by an insufficiency of oxygen. At an altitude of 7000 metres, the density of the atmosphere is reduced by half, and the quantity of oxygen contained in a given volume of air is reduced in the same proportion; but the diminution of oxygen in the case under notice could not have caused death. It is known by experience and proved by experiment that a human being is capable, without any great inconvenience, of living for some time in an atmosphere containing half the normal quantity of oxygen. No amount of inhalation of oxygen would have prevented the bleeding from the mucous membrane of the mouth and lungs, which was accompanied by extreme lassitude and temporary paralysis of the respiratory muscles, which proved fatal-symptoms caused by low atmospheric pressure. Another point to be taken into consideration is whether the gases generated during digestion in the stomach and intestines would not under such conditions act in such a way as to push the diaphragm upwards, and thus interfere with the movements of respiration.
The moral lesson to be drawn…is that man should keep within his own sphere as from his organization he is not fitted to soar to the lofty regions of the air.’