At an elevation of 4,500 feet, the air in Salt Lake City has about 17 percent less oxygen than the air at sea level.

The eight intermountain states of the American West, sometimes called the Suicide Belt, have high elevations and the associated thin air. Now, researchers say the low oxygen in these areas is linked with signs of depression, and could potentially even contribute to suicides in some regions.

In 2012, Colorado, Wyoming, Utah, Montana, Idaho, Nevada, Arizona and New Mexico all had suicide rates exceeding 18 per 100,000 people, while the national rate was 12.5 per 100,000 people, according to the American Foundation for Suicide Prevention.

These states tend toward higher elevations, and several studies have identified living at higher elevations as an independent risk factor for suicide. Other studies have also found that rates of depression increase with elevation and may contribute to increased suicide risk.

In the new study, researchers at the University of Utah and one colleague at Tufts University found that female rats exposed to high-elevation conditions — both simulated and real — exhibited increased depressionlike behavior. The behavior could have been due to the animals experiencing hypoxia, a condition in which an individual gets insufficient oxygen, the researchers said. [5 Myths About Suicide, Debunked]

Male rats showed no increased signs of depression when exposed to the same levels of hypoxia, the researchers found. Female mammals, including humans and rats, naturally produce less of the brain chemical serotonin than males. The neurotransmitter is thought to contribute to feelings of well-being and happiness, and as such, the higher levels in males may make them less susceptible to depression, researchers say.

"The significance of this animal study is that it can isolate hypoxia as a distinct risk factor for depression in those living at altitude," said Shami Kanekar, a research assistant and professor of psychiatry at the University of Utah, and a lead author on the study. It also suggests an increased risk of depression for people who have conditions such as chronic obstructive pulmonary disease (COPD) or asthma, which may reduce their ability to take in oxygen, she said.

In the experiments, the rats were kept for a week in Salt Lake City, which has an elevation of 4,500 feet (1,370 meters), and then in a lab under conditions that simulated the oxygen levels at sea level, then the oxygen levels at 10,000 feet (3,050 m) and 20,000 feet (6,100 m). The researchers used a widely accepted behavioral test in which depression in rats is gauged by the persistence exhibited by the rodents in a swim test.

"In female rats, increasing [the] altitude of [the animals'] housing from sea level to 20,000 feet caused a parallel increase in depressionlike behavior," Kanekar said.

The findings bolster the argument that physiological changes triggered by the low oxygen at higher altitude can contribute to depression.

"There are many potential risk factors that contribute to depression and suicide at altitude, and we are not discounting any of these other factors at all," said Dr. Perry F. Renshaw, a professor of psychiatry at the University of Utah and a lead author of the study. "Several such factors that are prevalent in the intermountain West include poverty, rural residence, low population density, gun ownership and psychiatric disorders such as bipolar disease."

But the new study shows that one factor inherent to living at higher elevations — low oxygen levels — can cause depression, Renshaw told Live Science.

Renshaw noted that the study had limitations. For example, the brains of humans and rats are very different, particularly in the frontal lobe, which is thought to be involved in decision making and impulse control, among many other functions.

Renshaw said he suspects depression in thin-air locations might be partly caused by low levels of serotonin. Hypoxia impairs an enzyme involved in producing serotonin, which could lead to depression, Renshaw said.

The possible link could be particularly important for women living in higher elevations, Renshaw said.

The big question, Renshaw said, is "should we be treating women who are depressed, and particularly those in the Rocky Mountain states, differently?"

Renshaw's team is examining the effectiveness of antidepressants, especially drugs called SSRIs (selective serotonin reuptake inhibitors), which are the most commonly prescribed antidepressants in the United States. Studies using animals have suggested that SSRIs such as Prozac may not work when brain serotonin levels are low.

Utah has the highest use of antidepressants in the country and the highest rate of depression, according to a 2007 study conducted on behalf of the nonprofit organization Mental Health America, Renshaw said.

"The fact that both depression and suicide rates increase with altitude implies that current antidepressant treatments are not adequate for those suffering from depression at altitude, leading to high levels of unresolved depression that can contribute to higher levels of suicidal ideation and suicide attempts," Kanekar said.

Recently, Renshaw said his team began a new study that seeks to increase serotonin levels in women diagnosed with depression to the levels found in women at sea level, to see if this change could help antidepressants to work more effectively.

The number for the National Suicide Prevention Lifeline is 800-273-8255.

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