Irisheyes, I'm in Texas and we work our horse EVERYDAY in the round pen. I am for certain our temps. And humidityare much higher than yours. It's all about the horses getting acculumated (sp?) to their environment. Some do better than others. Are your round pens in the sun? (Not being sarcastic, just a question) Putting them in the shade helps tons!!! It also helps when you need to seclude one away from everyone else and they need shade.
We hose them down sometimes BEFORE working them. It helps to cool them off. Then we hose them down every time after they work. Have NEVER had issues....even in 100 degree 100% humidity weather.
I'm just kind-of shocked by your comment.... How again do wild horses survive the heat?