I live at 1270 elevation so the lack of Oxygen affects me pretty much wherever I hunt. Its not uncommon to break, 10,000 but we are usually a little lower. Like suggested before, I start carrying a weighted pack on my PT hikes before season and get into camp a couple days ahead of time, which I also use for scouting. I have hunted higher and have never had altitude sickness, but it does not affect all.
