People living in high-altitude areas have higher rates of suicide and depression, according to a review published in the Harvard Review of Psychiatry. Researchers believe the connection may be explained by reduced blood oxygen levels due to lower atmospheric pressure.

Researchers analyzed 12 studies, including population-based data on the relationship between altitude and suicide or depression. Most studies concluded that areas in higher altitudes had greater risks of suicide and depression. In general, the link was stronger for suicide than for depression.

The highest suicide rates were clustered in the intermountain states: Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming. Alaska and Virginia also had high suicide rates. In a 2014 study, the percentage of adults with "serious thoughts of suicide" ranged from 3.3 percent in Connecticut (average altitude 490 feet) to 4.9 percent in Utah (average altitude 6,100 feet).

The study found that although people living at higher altitudes had decreased death rates from all causes, suicide rates were higher, and they increased dramatically between 2,000 and 3,000 feet.

Suicide rates had a stronger link with higher altitude than with owning firearms.

Suicide rates per 100,000 people adjusted for population density were 17.7 at high altitude, 11.9 at middle altitude, and 4.8 at low altitude.

Dr. Brent Michael Kious of the University of Utah, Salt Lake City, and coauthors suggest the reason for increased suicide rates might be "chronic hypobaric hypoxia" —  low blood oxygen related to low atmospheric pressure. That theory is supported by studies in animals and short-term studies in humans.

Kious and his colleagues suggest two pathways by which hypobaric hypoxia might increase the risks of suicide and depression: by altering the metabolism of the neurotransmitter serotonin and/or through its effects on brain bioenergetics.

The study suggests some possible treatments to alleviate the effects of altitude on depression and suicide risk: supplemental 5-hydroxytryptophan (a serotonin precursor) to increase serotonin levels, or creatinine to influence brain bioenergetics.