Back when I was a cadet, I had "issues" with heat exhaustion when humidity was extremely high. I didn't even have to be doing anything; just standing in the heat and humidity was enough to make me pass out sometimes.
Today in yoga class I noticed my heart was racing and my breathing fast. Why, when it's only been a week since my last class and I haven't been doing anything that would cause problems? When our instructor announced the temperature and humidity--106 degrees and 50%--it all made sense.