Researchers at the University of Utah studied physiological differences of rats when kept at various elevation levels and found increases of hypoxia, or decreases of oxygen in tissues. This caused depression in the rats.
"Because rats are not subject to the same psychological and societal pressures as people, the current study bolsters the argument that physiological changes triggered by hypobaric hypoxia (the low oxygen at high altitude) can contribute to depression. What these changes are, and whether they also occur in people, will be the subject of future studies."
An interesting idea that may offer more answers in our quest to reduce suicide.