Higher altitude means lower air pressure, simple as that. Remember, evaporative equilibrium is directly related to pressure. Additional factors involved in evap

Higher altitude means lower air pressure, simple as that. Remember, evaporative equilibrium is directly related to pressure. Additional factors involved in evaporation don't help either: though not perfectly universal, usually higher altitudes are also more arid as well, which further increases the rate of evaporation, as do higher wind speeds if you're outdoors. Combined though and at 1800-2000m above sea level for example you can easily see water loss from perspiration and exhalation double, which over the course of a day adds up to a significant amount of water even without any other activities. In general at low to medium-high altitudes dehydration is a much much more common issue then reduced oxygen, and some of the symptoms can be confused.