When it is really hot I will split my workout, say 5 outside and finish with 2 inside. Get the benfefit of heat training without all of the negative consequences. I much prefer to run outside so I usually limit the treadmill portion of the run to 3 miles or less.
I do the same thing for really hot (95+). Usually divide it evenly inside/outside. I usually do the treadmill portion first and then head outside.