It can be deadly in a short time frame.
Those who aren't acclimated to altitudes of 8000 ft and over can fall victim.
At 10K feet the effects low air pressure become more noticeable and the risk of altitude sickness increase greatly.https://en.m.wikip.../Altitude_sickness
It's best to acclimate yourself over a period of days or weeks before going above 10K feet.
Avoid alcohol, and limit physical exertion.
Once acute altitude sickness develops it is a medical emergency. Descending to a lower altitude and/or 02 are needed stat.
The victim will most likely be unable to walk out.
This was a preventable death from acute altitude sickness @11K feet.
A couple bottles of O2 may have saved her.
She needed a few days of incremental exposure to adjust rather than bum rush up. They should have halted or better yet come down to a lower altitude at the first signs of sickness. An experienced climber in excellent condition would have more leeway, but it has killed professional climbers as well. http://www.philly....822.html?mobi=true