If humid air is lighter than dry air, why is hot and humid air harder to breathe?
—Robert Kozlik, Riverwoods
It is true that breathing is more difficult in hot and humid air than it is in cooler and drier air. Warmer air can hold more water vapor than colder air, considerably more if there is a large difference in temperatures between the hot and cold air. Adding water vapor to air reduces its density because the weight of a water molecule is less than the average weight of the mix of gases which comprise absolutely dry air. Therefore, moist air contains fewer oxygen molecules per unit volume than dry air, and we literally don’t get as much oxygen when we inhale moist air. This can cause breathing problems for people with certain respiratory diseases.