Participants ride exercise bikes during a group outdoor fitness promotion in central Sydney on July 5, 2011.

Participants ride exercise bikes during a group outdoor fitness promotion in central Sydney on July 5, 2011. (Greg Wood, AFP/Getty Images / June 18, 2012)

When Kay Kay Clivio teaches her hot yoga class at Pure Yoga in New York City, she likes to keep the room between 95 and 100 degrees Fahrenheit (35 to 37.7 degrees Celsius).

"Heat has a healing, therapeutic aspect," she said. "The hot room opens your muscles more and makes you more flexible."

Students in her hot class are encouraged to drink plenty of water and take breaks as needed.

"It's not usual to get nauseous in the first classes. Heat turns up a lot of toxins. Like steam cleaning a carpet, it really dredges stuff up. So the body reacts," Clivio said. "A lot of the benefit is that you're detoxifying."

For her non-hot yoga classes, Clivio likes to set the thermometer at 75 degrees Fahrenheit (23.8 degrees Celsius).

"If you have people in the room, the heat will come up," she explained. "I don't ever think the air conditioner should be on."

Besides flushing toxins and increasing flexibility, Clivio said heat can reduce stress and even sharpen the mind.

"It pushes you out of your comfort zone," she said.

Sadly, one thing exercising in heat cannot do is burn more calories.

"You do not burn more calories by sweating, per se," said Olson.

She said the energy cost of walking a 15-minute mile is the same, whether the temperature is balmy or sweltering.

"The idea that a hot room or area to work out and increase sweat results in increased calorie burning is a myth," she said. "Sweating is a passive process."

(Editing by Patricia Reaney)