Thanks for your answers. Was hoping it would be like athletes training at attitude. Would get easier over time and much better when you went down.

Elevation does affect breathing. If you don't use O2 at sea level, you may very well need it at altitude. There is a test you can take called the High Altitude Simulation Test which will give you an idea of at what altitude you'll need O2. It does NOT get better over time.

Hello Don. Greetings from that Small town by the big Lake.
The main issue with elevation is probably barometric pressure. That pressure decreases as you increase in height above sea level, but increases as you dive below the surface. Think scuba diving here. Other things also come into play like air quality and wind circulation patterns, but who cares when you just can't get a good breath? A breathe OUT, that is! Higher pressures actually help collapse the lungs to force out the bad air which smothers us.
Let me stop here to allow others to comment about their places like Colorado (at 5300 feet above sea level) or the muggy swamps of Florida (at less than 100 feet. 🐊)
See - Except for the Sears Tower (1450 feet + antennas), Chicago's not that bad after all !!

Yes unless you live up hear you dont want to try it im at a bit over 6 thousand im used to it but go above 8 and well not fun and noo nothing about copd really gets better