I live in Ohio, and I've stopped riding if the humidity is over 80%. If I have trouble breathing and the air is like molasses, I know he can't work in it. I read something interesting on another forum recently--add temp and humidity together, and if it's 120 or under, you can do what you want. If it's between 120 and 180, be careful, find shade, hydrate, etc. If it's over 180, don't ride.
So 70 degrees and 70% humidity--140, just be careful.
100 degrees and 10% humidity--110, if your horse is used to it, go for it!
90 degrees and 80% humidity--170, not recommended!