Oxygen intake contributes to depression, scientists have surmised after a study shows that thin air and high altitudes causes depression in female rats. "The significance of this animal study is that it can isolate hypoxia as a distinct risk factor for depression in those living at altitude (hypobaric hypoxia) or with other chronic hypoxic conditions such as COPD, asthma or smoking, independent of other risk factors," says the lead author on the study.