Circumstance one: Morning class is in a classroom in the center of a building, so no windows, and all doors give on to one of two high-traffic hallways.
Circumstance two: Today was one of many days that that building's erratic climate control system was heating the room to about 76 degrees.
Circumstance three: Today I gave a midterm in morning class, meaning that opening the doors to get (limited) cross-ventilation was impractical, due to distracting hallway noise.
Circumstance four: Pre-spring break stress/lack of sleep/apathy has apparently led some of my students to pay less attention than usual to personal hygiene.
Circumstance five: My morning class is 85% male, mostly in their very early twenties.
Result: Well, you can guess.