Super-heated workouts may be trendy (not to mention burn more calories), but they're best for the highly fit or those who often do strenuous aerobic exercise, says Douglas Casa, PhD, an expert on athletic exertion in the heat. (Steer clear if you're overweight, pregnant, or have a medical issue like a heart condition.) Some safety tips:

Start slow
For your first several sessions, cut class short, do fewer reps, or lower your intensity to give yourself time to acclimate to the extreme heat. It will take about 7 to 10 workouts to adjust, Casa says.

Drink up
Since you'll be sweating a lot, hydration is key. Drink enough water (or a sports drink) before, during, and after so that your urine is the color of lemonade rather than apple juice.

Pay attention to your body
Watch for the warning signs of dehydration and heat illness, Casa says: "If you get weak, dizzy, have a headache, or notice hot and uncomfortable sensations around your neck, get to a cooler space, place cold, wet towels on your body, and drink some water ASAP."