Notes: The data are for unacclimatized, sedentary humans at one level of activity (VO₂ = 500 mL/minute), who are breathing 8 ppm CO.
This means that there will be an increase in COHb at altitude even in the absence of inhaled CO.
Sources: U.S. Environmental Protection Agency, and J.J. McGrath, Carbon Monoxide Toxicity, pg. 149, CRC Press, 2000.