It was 41 degrees Celsius- as hot as a typical steam room - at Pure Yoga in Manhattan on a recent Saturday, but for the 16 women already perspiring through a series of plie squats in a ballet-based barre method workout, it wasn't blistering enough.

"We're turning it up," the instructor, Kate Albarelli, 31, announced in the sort of cheerful tone that would usually signal a time to rest. The women looked as delighted as if she'd given them one.

For exercisers like these, based mostly in New York and Los Angeles, only sweltering temperatures produce adequate workouts and "detoxing" sweats. So gyms and studios are trying to lure them with ever hotter, harder yoga classes, in addition to roasted versions of Pilates, kettlebells, group cycling and more.

"You don't waste three songs sitting around warming up - you can hit it hard from the start," said Mimi Benz, 31, an owner of the Sweat Shoppe, a hot group cycling studio in North Hollywood, California. "I'm not going to lie, it's intense."

Alexandra Cohen, 42, the supervising producer of a popular American daytime television talk show, said, "I don't have time for hours in the gym doing cardio and weights and then sitting in the steam room to detox."

She found Bikram yoga (a static sequence practiced in 41-degree temperatures) too slow, and hot power yoga (fast-flowing classes) too easy. So, twice a week she goes to the yoga teacher Carlos Rodriguez in New York for a grueling mix of the Brazilian martial art capoeira, explosive vinyasas, calisthenics and weights. For that hourlong workout, performed barefoot in a room hotter than the human body, Ms. Cohen lines up four exercise mats so she doesn't waste time when one becomes too sweat-slicked to use.

"A good day is when I have to literally wring my clothes out," she said. "Some people do crazy cleanses. I do hot-room workouts."

The workouts don't promise fat-melting or even weight loss; instead, the emphasis is on making the workouts seem more extreme.

Loren Bassett, 41, a demanding yoga teacher in Manhattan, lovingly calls some of her followers "insane." It's partly for them that she and her trainer, Cole McDonough, of David Barton Gym, developed Bassett's Boot Camp for Pure. The class is 75 minutes of alternating high-intensity cardio, crow poses and core work. Marie Claire magazine pronounced it "America's toughest workout"; spaces sell out weeks in advance.

Experts agree on the benefits, but only to a point. Douglas Casa, a kinesiology professor at the University of Connecticut, said that while there's no question that hot workouts are harder, any benefits peak at about 37 degrees. "Above that, you're just jeopardizing safety," said Dr. Casa, who is also the chief operating officer of the university's Korey Stringer Institute, named for the American football player who died of heat stroke in 2001.

Tracy Anderson, a trainer whose clients have included Gwyneth Paltrow and, at one point, Madonna, said her research put the optimum at 30 degrees and 65 percent humidity.

Dr. Casa judged that that was about right, "at least for sweat effect."

Vigorous hot workouts, he said, are only for the highly fit (and well hydrated) - and even then there are limits.

"If it's so hot you can't get a hard workout in, it defeats the purpose," said Dr. Casa. "You might as well work harder where it's cooler."

What about the detox effect? "That's a hoax," he said. "I don't think there's any inherent advantage to sweating more. Some people just like the feeling."

The New York Times