Air-conditioning takes on a whole new meaning when you take a Bikram yoga class. As about 40 of us, each clutching a huge bottle of mineral water, filed into the exercise studio, the heat hit us like a wall. The room was a sweltering 43 C - intentionally so.
Once assembled on mats and towels, novices and experts of all ages are taken through a gruelling 90-minute series of 26 yoga postures which are designed to work all the body's organs and systems. You are encouraged to reach your own "personal edge", which sounds scary but is far from the outdated exhortation to "feel the burn" from aerobics classes of old.
The idea of Bikram is to feel intensity of effort at the same time as relaxation. You get a rest after each posture, and while the teachers do constantly shout encouragement, they also tell you to sit out anything that is too difficult, and drink if you feel dizzy.
By the end of the session, everyone was extremely hot, sweating profusely and tired. But we felt mentally good and, paradoxically, invigorated.
Michèle Pernetta is the woman who brought Bikram yoga to the UK. She has a toned body and an aura of calm, and explains that she came to the method after martial-arts injuries to her knees refused to heal.
"My doctor offered surgery, which I didn't want. Then I heard about Bikram Choudhury, a world-champion weightlifter who had crushed his knees in an accident when he was 20. Consigned to a wheelchair, he went back to doing yoga, which he had started as a three-year-old.
"Working with his Indian yoga master, he put together his unique series of postures. They are based on classical yoga, but doing them in sequence in one session gives the whole body a thorough, enlivening and healing workout."
Bikram perfected the postures by combining Eastern doctrine with three years' research with western doctors and scientists at Tokyo University hospital, and went on to open schools throughout the east. Pernetta was back to health within nine months of starting Bikram's classes in Los Angeles, where he now lives, and after studying with him for four years was determined to import his method to Britain.
Since she opened the UK's first Bikram school, the Yoga College of India in Kentish Town, north London, in January 2000, followed two years later by Bikram Yoga West, others have sprung up across the country. Pernetta is soon to branch out inIslington, London, and Brighton.
But does it do you any good? Quigs Dehan has been going to Pernetta's classes for two years. Always very sporty, Dehan says that she had hit a physical low point: "I was 48, had three children, and a history of lower-back trouble." She had begun to find her exercise regime too harsh, thought it might be exacerbating her back problems, and had reluctantly hung up her gym kit.
Coming across an article about Bikram's method coincided with her daughter seeing a programme about it on television, and she decided to give it a go. "I worked very steadily and carefully. One session a week for months so as not to strain anything, but now I'm totally hooked. I go four times a week. I've lost two-and-a-half stone, my back has strengthened beyond belief and my sense of well-being is phenomenal - I want to shout it from the rooftops."
Dehan's enthusiasm is matched by some medical evidence suggesting that the Bikram method may improve digestion, reduce blood pressure and cholesterol levels, balance hormones and help to rid the body of toxins. Doctors from Los Anegles Cedars Sinai Hospital apparently refer patients to him instead of offering them surgery. Bikram has also worked on projects to study the effects of yoga on stress and heart disease as well as with Aids sufferers in San Francisco. Bikram says their immune systems are boosted by the technique.
But isn't there a risk that exercising at such high temperatures will make us so flexible we may simply snap? Dr Michael Wright, consultant rheumatologist and musculoskeletal physician says: "I can't see that it would be harmful. It would be best to start slowly and build up gradually, acclimatise, but you can't really overstretch joints and muscles." Wright agrees with other experts, however, who have warned that while young people will probably be fine, excessive sweating and dehydration can be dangerous for elderly people or those who are at risk of blood clots.
While Bikram's method is gaining credence in western medical circles, Pernetta says that one of the reasons she loves it is that it takes yoga back to its roots. "Many western yoga teachers are wonderful, but who ever said yoga had to be all wrapped up in chanting and incense? Bikram is one of only a few Indian yoga masters teaching in the west and his method is the genuine article - it's spiritual, yes, but also fiery, vital and fun."
But what about the heated room? A bit gimmicky? Not at all, Pernetta explains: "Yoga originated in India. It's hot there. The heat is just there to support you. It's the best working temperature for muscles and joints, so it keeps them safe during exercise and you don't lose the heat to a cold room. All ages can work in this method without risk of injury.
"You'll come to a class and push and sweat and laugh and groan. Men love it here because they say it's not precious, they don't have to chant at the beginning and have a long nap at the end."
And indeed, numerous high-powered men are, or were, among Bikram's devotees. Richard Nixon is apparently the only person to have done it in private with the man himself, but other converts include Michael Jackson and the late George Harrison. Closer to home, a visit to one of Pernetta's two London studios could find you swapping bodily fluids (the sweat literally pours out) with the fashion designer Alexander McQueen or Goldie, the musician and actor. A Spice Girl or two might pop in, as do a host of other stars of stage and screen.
Although I must confess to a growing urge to go back (maybe it's like giving birth - you forget the pain), I was privately more reassured to know that my partners in grime (our own sweat - the studio itself is amazingly clean) probably included a GP or two, a physiotherapist and possibly a surgeon.
For more information visit Pernetta's site www.bikramyoga.co.uk. A drop-in 90-minute session costs £10.50: a 10-class card costs £85. www.bikramyoga.com contains a list of all affiliated schools worldwide.