> The partial pressure of O2 decreases with altitude. > At sea level, O2 accounts for about 21% of the total pressure > while at 18,000 feet O2 only provides about 12 or 13% of the total > atmospheric pressure. Isn't that because O2 is heavier than N2? So it runs to the bottom... -- Huw Powell http://www.humanspeakers.com/audi/ http://www.humanthoughts.org/