They do say this:

"Despite a negative correlation (r = −0.31, p < 0.001) between county altitude and the all-cause mortality rate, there was a strong positive correlation (r = 0.50, p < 0.001) between altitude and suicide rate at the county level (Fig. 1)."

This is why it is important to take genetics into account when looking at these studies.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5460084/

Not all of us were meant to live at higher altitudes.

Yeah, and that made me wonder whether survivorship bias could be at play. That is, populations at lower elevations see less opportunities for suicide due to individuals dying of things like heart disease, etc., before they have a chance to off themselves at a later age.

But they do mention that they "[controlled] for percent of age >50 yr." Does that imply that, per county, they excluded high-elevation suicides that wouldn't have happened at lower elevations, using known mortality rates for common causes-of-death for age > 50 individuals?

I'm sure the answer is in the paper somewhere, but I need to get back to work. :-)

There's also a lower incidence of cancer in the population living in high altitudes despite being exposed to more ultraviolet light.

I believe the leading hypothesis is that the higher oxygen at lower altitudes causes more oxidation.

Lower BMI as well.

https://slatestarcodex.com/2016/12/05/thin-air/