The type of yoga I teach is pretty hot, and I remember one woman showed up to my class early while I was setting up. She pointed to the unit up above us and said, "Could you turn up the AC?" I looked at her and said, "Actually, that's heat." Needless to say, she didn't stay for the class.
Hot yoga has been a huge craze for years. Bikram yoga is held in rooms of 105°F or higher, and many other styles, including Ashtanga and Vinyasa, heat practice rooms too, but only to temperatures in the 80s or 90s. The heat serves a purpose — to warm up the muscles so a practitioner can get deeper into poses, and also to provide a detoxifying effect (read, sweat!). Many people also believe sweating profusely during a yoga class is a sign that they're dropping pounds, although the weight they're losing is mostly water weight.
Not only can exercising in extreme heat be dangerous, leading to heat stroke and dehydration, but many people just can't take it. According to fitness expert Fabio Comana, the body is designed to withstand temps between 97 and 100 degrees, so if your core temp raises above 105°F, he says "you'll start to damage protein." Some people are addicted to insanely hot yoga though, while others prefer practicing in more moderate temps, so tell me . . .