This is a tragic early experience with high altitude sickness. How were they to know?

Zénith

It was now April the 15th, and the lofty flight was embarked upon by M. Gaston Tissandier, accompanied by MM. Croce-Spinelli and Sivel. Under competent advice, provision for respiration on emergency was provided in three small balloons, filled with a mixture of air and oxygen, and fitted with indiarubber hose pipes, which would allow the mixture, when inhaled, to pass first through a wash bottle containing aromatic fluid […]

Ascending at 11:30 under a warm sun, the balloon had by 13:00 reached an altitude of 16,000 feet, when the external air was at freezing point, the gas high in the balloon being 72 degrees, and at the centre 66 degrees. Ere this height had been fully reached, however, the voyagers had begun to breathe oxygen. At 11:57, an hour previously, Spinelli had written in his notebook, ‘Slight pain in the ears--somewhat oppressed--it is the gas.’ At 23,000 feet Sivel wrote in his notebook, ‘I am inhaling oxygen--the effect is excellent,’ after which he proceeded to urge the balloon higher by a discharge of ballast. The rest of the terrible narrative has now to be taken from the notes of M. Tissandier, […] one of the most thrilling narratives in aeronautical records…
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