The Food and Drug Administration does not recognize any medical benefits from cryotherapy chambers, nor does it regulate the devices.

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DENVER —

You step into an upright cylindrical capsule, padded on the inside and open on top. The floor grinds upward, and your head pops out. There’s a whoosh. And suddenly you are encased in gas, below the neck, in a tank that is minus-300 degrees Fahrenheit, colder than the coldest naturally occurring temperature recorded on Earth.

These are cryotherapy chambers. When Chelsea Ake-Salvacion, 24, died in one in Nevada in October, it focused attention on the fast-growing practice of full-body cryotherapy, which enthusiasts bill as a path to pain reduction, injury recovery and mood enhancement. Among the many cryotherapy centers opening nationwide, some go further, saying the chambers prevent osteoporosis, treat asthma, increase libido and kick-start rapid weight loss. Celebrity athletes such as LeBron James and Shaquille O’Neal have been among their earliest proponents in the United States.

In interviews, cryotherapy operators — some prefer the term “cryotherapists” — call the practice extremely safe, noting that people have applied cold to injuries for thousands of years and that serious athletes routinely take ice baths.

But the Food and Drug Administration (FDA) does not recognize any medical benefits from cryotherapy chambers, nor does it regulate the devices. The death of Ake-Salvacion has many questioning the safety of the procedure and the claims of those who praise it.

Nevada officials Monday expanded their investigation into the industry, saying they would examine employee safety and whether the machines are safe for the public.

“There are no regulations for this industry right now,” said Steve George, the top administrator at Nevada’s Division of Industrial Relations, which is working with the state’s health department on the investigation.

Deborah Kotz, an FDA spokeswoman, would not comment on the legality of specific claims but said the agency was “concerned with devices that are potentially being sold without having met” requirements.

In Texas, a trial is set for January in a lawsuit brought by Alix Gunn, who says a cryotherapy center froze her arm, leaving her with third-degree burns, loss of use and disfigurement. Her lawyer says Gunn was given wet gloves to wear during the treatment.

In response, CryoUSA said Gunn did not properly look out for her safety and added that she had signed a waiver, releasing the company from liability.

In Britain in September, the Cochrane Library, a health-research database, published a review of studies of whole-body cryotherapy, all of which included small sample sizes, and found “insufficient evidence” that the practice reduces muscle soreness or improves recovery times in athletes when compared with rest.

Proponents say the sudden drop in temperature helps the body and mind by releasing endorphins and prompting vigorous blood flow.

Humans have used cold as therapy since ancient times. But it was not until the 1970s that a Japanese doctor introduced the concept of immersing the body in chambers cooled to far below zero. The doctor, Toshima Yamauchi, sought to soothe rheumatoid-arthritis patients; modern technology allowed him to apply extremely low temperatures.

The idea was called whole-body cryotherapy, to distinguish it from localized cryotherapy, an accepted medical practice that involves applying small amounts of liquid nitrogen to the body to destroy abnormal tissue, such as cancer or warts. Yamauchi’s remedy caught on in Europe, became trendy in former Soviet bloc countries like Poland, and in recent years jumped to the United States.

By last month, when Ake-Salvacion died, whole-body cryotherapy centers had recently opened in Alabama, California, Colorado, Minnesota, North Carolina, Ohio and elsewhere, with clients raving about their experiences on Facebook. “LOVE LOVE LOVE!” wrote Julie Antuzzi-Rueffert, who visited a center in Colorado Springs and compared the cold to a pleasant blast of mountain air.

In Manhattan, KryoLife sees 80 clients a day, with most paying $90 for a three-minute session. Owner Joanna Fryben said the practice was so popular that she would soon expand to seven centers, including a cryotruck: a therapeutic freezer on wheels.

“I’m in New York City,” said Fryben, who has a degree in linguistics and translation. “You probably can imagine how many A-types we have. We have open arguments about them being overachievers; they all want to go in over three minutes.”

She has to tell them no: “These are cryogenic temperatures. Eight minutes in these temperatures can be fatal.”

Little is known about how long a person can survive without clothes at these temperatures. Cryotherapy centers cool their chambers to between minus-160 and minus-300 degrees , far lower than the lowest naturally occurring temperature cited on Earth: minus-135.8 degrees F, recorded in Antarctica using satellite in 2013.

Humans have a core body temperature of about 98.6 degrees. At about 81 degrees, the heart fails. The question is how long it takes to move between the two.

Three minutes, at least, is bearable, because it would take longer for our blood and internal organs to freeze. “We’re big chunks of meat,” said Gordon Giesbrecht, a professor at the University of Manitoba whose work studying humans in extreme environments has earned him the nickname Professor Popsicle. “It takes a long time to warm up or cool down a big chunk of meat.”

Frostbite, however, can set in quickly. “At negative-70 degrees, unprotected skin can freeze in as little as two minutes,” he said. “At negative-300; nobody has done these experiments.”

Ake-Salvacion worked at the Rejuvenice spa and was a strong believer in the benefits of cryotherapy, said her uncle, Albert Ake. On Oct. 19, she entered a tank at Rejuvenice alone, after hours.

The next morning, a colleague found her frozen solid in one of the chambers, in the fetal position with her cellphone nearby, said a lawyer for the Ake family. A coroner has not determined the cause of death. But Harris said it was possible Ake-Salvacion dropped her phone in the chamber, reached down and became trapped in a space with too much nitrogen and not enough oxygen, causing her to pass out and eventually freeze.

Nitrogen is in the air we breathe, but when nitrogen replaces oxygen, the situation becomes dangerous, doctors say. “Here’s a supposed ‘benign’ treatment that can kill you,” Harris said. “Maybe we need to regulate it. I mean, we regulate a barber.”

Kevin Goujon, an owner of Rejuvenice, declined to comment.