Researchers there are trying to explain a long-observed phenomenon: that suicide rates are much higher in certain parts of the western United States. Of the 10 states with the highest suicide rates, eight are in a region darkly deemed “the suicide belt.” In the heat map of suicide deaths from the CDC, the suicide belt cuts across the country in dark red—including Colorado, Montana, Utah, Idaho, and Wyoming.

One thing that these states have in common is high elevations. In 2011, a handful of scientists from Utah put forth a provocative theory: that the high altitudes are contributing to the high suicide rates. They said that a lack of oxygen can lead to hypoxia, or oxygen deprivation, and affect fundamental processes of the brain, like the production of the neurotransmitter serotonin, which is thought to play a key role in mood. They think it could also potentially affect the functioning of depression medications like SSRIs.

But critics of the theory say that suicide from low oxygen is too simple of an explanation, and ignores the overwhelming sociocultural attributes of these areas—access to guns, poverty, social isolation, a dearth of mental health services.