How did Alexander von Humboldt explain the cause of mountain sickness?

During his expedition to the Andes in the early 19th century, Alexander von Humboldt experienced severe physical distress while attempting to climb Chimborazo.1

Rather than viewing this discomfort as a mere byproduct of exertion, Humboldt turned his experience into a scientific observation. He became the first person to correctly ascribe mountain sickness to the lack of oxygen present in the rarefied air found at great heights. This insight was a significant scientific contribution, as it identified the physiological cause of the condition long before it was fully understood by the broader scientific community.12