Hey Lisa:
I have no a/c (ghastly reality) and in 90 + degrees F it has to become a question of mind over matter to keep going on dog days. I try to schedule different workouts according to the weather forecast, i.e. if we have several cooler days coming up, I'll do long cardios and Interval training then, if it's going to be stinking hot and humid, I may just weight train and nothing else because I am no good in heat and hate to be drenched in sweat by the time I've finished setting up my workout space.
Also I use 2 fans placed either side of me on a diagonal, I drink water all the way through, 3 or 4 8 oz glasses, mop down with a towel every time I drink, close the shades to keep the blasted sun off me, and keep in touch with how I feel. If I start to get heavy legs and/or light headed, I know to stop or back off, but this has happened to me only a few times and mainly when power walking outside in intense sunshine. Keep hydrated, stop for quick breathers when you need to, remember that your body is working much harder than when it does these routines in December and be kind to it. I go easier on myself in Summer, I usually do shorter CTX type cardio and switch my focus to strength training to see if I can conquer a new goal there, whatever works for you that keeps you active and happy but not totally exhausted after every workout.
Rememebr that the heat exhaustion feelings sometimes only come on after the workout is over: this has happened to me too, sometimes I may get a crashing headache or feel nauseous after having my shower when sitting eating dinner. Once I got so hot I couldn't stop sweating for the rest of the day and my skin felt really rough, bumpy and icky, my body was over-heated and could not cool down and all I'd done is Powermax.
Bottom line, go easy until a cooler day comes along. Avoid the emergecy room.
Take care,
Clare