A new systematic review, now published in the Harvard Review of Psychiatry, found that people living in high-altitude areas of the United States, such as intermountain states, have higher-than-average rates of suicide and depression.

The researchers, from the University of Utah in Salt Lake City, suggest that low atmospheric pressure at high altitudes may lower blood oxygen levels.

This may affect mood and make people living at these altitudes more susceptible to suicidal thoughts, they explain.

Individuals are much more likely to think about, attempt, or complete suicide if they have major depressive disorder, and around 16% of people experience this mental health condition at some point in their lives.

Major depressive disorder occurs when someone has at least 2 weeks of low mood, self-esteem, and energy across most situations.

Rates of major depressive disorder vary substantially from region to region, which could suggest that environmental factors play a role in some cases of major depressive disorder and suicide.

In the US, around 123 people take their own lives every day, making suicide the 10th leading cause of death nationwide.

Highest suicide rates in intermountain states

The researchers analyzed 12 studies that looked at the relationship between altitude and suicide or major depressive disorder.

They found that most of these studies reported an association between living in higher-altitude areas and increased rates of depression and—especially—suicide.

The review demonstrates that the highest rates of suicide "were clustered in the intermountain states" listed below:

• Arizona
• Colorado
• Idaho
• Montana
• Nevada
• New Mexico
• Utah
• Wyoming

The researchers identified a dramatic increase in suicide rates among communities living at altitudes between 2,000 and 3,000 feet.

Studies have found a strong relationship between increased rates of suicide and gun ownership, but in this review, the association between suicide and altitude was even stronger than the link between suicide and gun ownership.

Interestingly, although communities based in higher altitudes were found to have increased rates of suicide, the review reports that these communities actually have "decreased rates of death from all causes."

However, the studies evaluated in the review could not account for all factors that might contribute to regional variations in suicide rates, such as cultural differences or levels of substance abuse.

The authors propose that the mechanism driving the association between increased suicide rates and living at higher altitudes could be a condition called "chronic hypobaric hypoxia," which occurs when atmospheric pressure results in low blood oxygen. Previous studies revealed that hypobaric hypoxia can cause severe brain damage.

The team suggests that chronic hypobaric hypoxia could have an adverse effect on the brain or perhaps alter how it processes serotonin, which is a neurotransmitter known to affect mood.