High-altitude cerebral edema and high-altitude pulmonary edema most commonly occur at very high-altitude; however, they can occur in some people at high-altitude.
This review article hit all the items I could come up with off the top of my head and added quite a few more: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3678789/
This web interface is asking me to identify myself as an expert, but I'm really a generalist physician with an interest in altitude related illnesses.
There are risks related to the demographic population. Those impacted most significantly are those arriving acutely to altitude and less so those who reside at altitude of course.
I can answer a lot more thoroughly but am uncertain if this is an active question. For now, I will just note the changes which occur in the environment before explaining the how and why on the body. Someone else might find this useful
• Atmospheric pressure drops by about tenth for every 1000m of altitude.