I live at 4, 500 ft. When I venture to Wy for elk (floor is 7,000 and timberline is about 9,000), I get there three days before the season and do the light chores of getting camp in, a little scouting, and some fishing. For the first couple days one has the sensation that he can quite pull in a full breath. By the end of the 3rd day, that sensation has gone away, and life is good.
