Higher altitude means lower air pressure, simple as that. Remember, evaporative equilibrium is directly related to pressure. Additional factors involved in evaporation don&#x27;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&#x27;re outdoors. Combined though and at 1800-2000m above sea level for example you can easily see water loss from perspiration and exhalation <i>double</i>, 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.