Thank you for explaining. So less pressure and not less gas. So, for a mountain climber the o2 is present, but the lungs can’t take enough in?Not sure what you mean by oxygen content Mark, but the percent of oxygen remains about 21% regardless of altitude. The problem lies in the air pressure. There is less of every gas at a given volume at higher altitudes, which I guess is why people typically describe the air as "thinner". Don't mean to be nitpicky, but I think a common misperception is that there is less oxygen as a percentage at higher altitudes. Bottom line is that living at 5200 feet I've never known if it's easier to smoke a pipe at sea level or if the nic hit is harder. I don't have anything to compare it to.