NEW YORK (Reuters) – When it comes to exercise, some like it hot, some like it hotter, and some like it under the air conditioner.

Experts say heat can be help your workout, boosting circulation and increasing flexibility, but it’s a relationship that can also turn nasty.

“Your body is engineered to exercise in the heat,” said Michele Olson, Ph.D., professor of exercise science at Auburn University at Montgomery, Alabama. “This is why we sweat.”

Olson, who is also an expert with the American College of Sports Medicine, said it’s actually the evaporation of sweat from the skin that cools our bodies to maintain our normal body temperature.

Heat also speeds up the heart rate, increasing the circulation of blood around the body, including the muscles, decreasing friction and making it more comfortable to stretch, she said.

“This is why we warm up before exercise,” she explained.

But heat-related illnesses that strike during a sport or recreational activity send nearly 6,000 people in the United States to emergency rooms every year, according to a report released in 2011 by the Centers for Disease Control and Prevention.

Sometimes the problem is really not the heat, but the humidity.

“If it is too humid, sweat cannot evaporate and this can prove dangerous, causing a heat injury, such as dehydration, heat cramps or heat stroke,” Olson said.

Dr. Cedric X. Bryant, chief science officer of the American Council on Exercise, said it’s usually a combination of the two.

“With humidity the environment is not conducive to evaporation, so the body stores more heat, the core temperature goes up, and your physical performance in negatively impacted,” he said.

When ambient temperature is 90 degrees Fahrenheit (32 degrees Celsius) or more and the humidity is 60 percent, he said exercise should be indoors.

At 90 degrees Fahrenheit (32 degrees Celsius) and 30 percent humidity you’d still be able to exercise safely, he said, depending of the intensity of the workout, which determines how much heat is produced.

“Generally speaking if temperature is 80 degrees Fahrenheit (26.6 degrees Celsius) and humidity is below 50 per cent, that’s going to be pretty comfortable,” Bryant said.

Acclimatization, the gradual adaptation to environmental changes such as heat, can also reduce risk of injury, according to Bryant. The body usually takes 10 to 14 days of heat-exposed exercise to adapt.

To beat the heat, adequate hydration before, during and after exercise is a must, Bryant said.

He recommends drinking copious amounts of fluid 30 minutes before exercise, drinking at least six ounces every 20 minutes during exercise, and drinking beyond thirst after exercise.

“Water is generally best,” he said. “But consider a sports drink if you’re going to exercising for more than an hour.”

During the dog days of summer, Bryant suggests working out in the morning, before the day heats up, and protecting skin and eyes with sunscreen and sunglasses.

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.”

(Reporting by Dorene Internicola; editing by Patricia Reaney)