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Students at Bikram Yoga Toronto on Bloor Street work out during a class on June 16, 2011.Kevin Van Paassen

A group of 25 men and women fold their sweat-drenched bodies forward, clasping their heels with their hands. They're in a room set to about 41 C.

A young woman, her shoulders shaking, breaks from the pose and lies down on the yoga mat under her feet. Every inch of exposed skin is beaded with perspiration that won't evaporate. She looks overwhelmed.

The booming voice of a svelte yoga instructor cuts through the oppressive heat. "Maybe you feel a little bit nauseous sometimes. Maybe you feel a bit dizzy. Good. It's working. You're getting all the toxins out," she says.

At Bikram Yoga Bloor in downtown Toronto, students engage in what seems like athletic masochism (the practice's founder, Bikram Choudhury, refers to his studio as a "torture chamber") to "release their toxins" and treat myriad conditions including asthma, carpal tunnel syndrome and hypertension.

Students Ann Jervis, 57, and Hayley Dineen, 23, remember unpleasant first experiences with Bikram yoga, but stuck it out because of the perceived advantages of the practice.

But some health professionals question the efficacy of the trendy style of yoga, practised for 90 minutes in stifling heat.

"As a scientist, I wouldn't say there's a huge stock in sweating out your toxins," says Stephen Cheung, the Canada Research Chair in Environmental Ergonomics, whose area of expertise is heat stress. The body only releases them through sweat to a very limited extent, he says.

The extreme temperature and humidity in Bikram yoga and its less regimented spinoff Moksha yoga can be risky for those with heart conditions, as well as for those with low or high blood pressure in the normal range, says Nieca Goldberg, medical director of New York University's Women's Heart Program.

Review sites such as Yelp, yoga forums and Twitter are rife with tales of students feeling dizzy, passing out and being tended to by paramedics.

Bikram is no stranger to controversy: About six years ago, when it first gained popularity in North America, participants complained of injuries and pulled muscles. Doctors blamed the hot conditions, which sometimes allow students to stretch too deep.

But the dizziness and blackouts are of concern to Dr. Goldberg, a cardiologist, because hot yoga's proponents give students the impression that light-headedness is to be expected.

When Sheila Madsen attended her first Moksha yoga class in Toronto two years ago, she was overwhelmed by the heat and felt dizzy. She continued, because an instructor told her things would get easier. She says that since she could get through tennis practice and Pilates class without difficulty, she wondered why she was struggling so much at Moksha yoga.

"Some people do well in high levels of humidity. I do not," Ms. Madsen, 64, explains. "I couldn't get my heart rate down sometimes for half an hour, which is really dangerous."

She would go into "child's pose," a resting position, but the room's conditions made relaxing difficult. On four occasions, she left the 39 C room to recover. Then she quit.

The science behind fainting is simple. "Your blood vessels normally expand to go to your exercising muscles," Dr. Goldberg says. "There's an even magnified response when you're doing it in a very hot environment. That's taking blood away from the blood vessels that are going to your brain, so you faint."

She has treated otherwise healthy patients who fainted in hot yoga classes.

Dana Moore, co-owner of Bikram Yoga Bloor, says there have only been a few cases of fainting in the studio's two-year history. Instructors, trained in first aid, usually spot warning signs and guide students down onto their mats. And, as at all reputable studios, students must fill out a form disclosing any injuries, ailments or health conditions. Staff call new students to see how they're feeling within two days of their first class.

When a student faints, "we always recommend staying in the room and that is really more for the safety of the student," she says. "[Outside the room] no one's there to keep an eye on them."

But Dr. Cheung says, "That sounds completely counterintuitive to the whole point of fainting."

In a small study conducted at Dallas's University of Texas Southwestern Medical Center in 2002, researchers raised the body temperatures of nine test subjects and then tilted them upright to the point at which they'd faint. They conducted the same experiment again, but cooled participants' skin before and during the tilting, and found those subjects were able to tolerate standing.

In the case of fainting, Dr. Cheung says, "I would get them in a cooler room and … have cold towels to really cool their skin down."

Instructors say the body can tolerate the extreme heat of Bikram and Moksha yoga because sweating is its "natural air conditioner." It's not sweat itself, but evaporation of moisture off the skin, that cools the body down, Dr. Cheung explains. "If you're in a hot and humid environment, your ability to lose heat from sweating is hugely decreased because the air is already saturated," he says.

At Moksha Yoga Uptown in Toronto, manager Jen Blanko says variations in class size make it difficult to regulate humidity - which is supposed to be at 40 per cent. "If we have a class of 10 people, it's one thing. If it's 55 people, it's a totally different environment in there," she says.

At times the room's extensive exhaust system isn't enough, and instructors have to crack a window open, she says. But Ms. Blanko believes that on the odd occasion that a student has fainted, it has been the student's mistake - not eating before class or not properly hydrating. However, the studio does remove students from the room if they've fainted.

If a student mentions blood-pressure issues or heart disease, Ms. Blanko and her colleagues suggest they check with their doctors before they sign up for a class. Moksha Yoga Uptown's website notes that women in their first trimester of pregnancy should avoid the class if they haven't been practising hot yoga for at least six months. It also says children should not participate until their sweat glands have developed.

Dr. Cheung says that if patients feel dizzy in class, they should rest. "There's a fine line between what is discomfort and what is pain," he says. "This is supposed to be about health."

Hot yoga tips

- Don't starve yourself before class, but don't scarf down your lunch just before you go into the hot room. Avoid eating for two to three hours before class in order to give your body time to digest what you consumed earlier. If you forgot to eat, you can have something small (half a muffin or a banana).

- Avoid drinking coffee before class (it will dehydrate you).

- Drink plenty of water before and during class to make up for all the fluid your body will lose through sweat.

- You'll lose electrolytes (sodium and potassium, mostly) as you sweat, so add a pinch of salt, sugar and a squeeze of lemon (natural sources of electrolytes) to the water bottle you take to class. Coconut water and electrolyte tablets (which can be dissolved in water) are other good alternatives.

- If you have a history of irregular blood pressure or heart disease, talk to your doctor before you sign up for a class.

Sources: Bikram Yoga Bloor, Moksha Yoga Uptown, Bikram Yoga Hamilton Dundas

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