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 a woman named Chelsea Ake-Salvacion, 24, died in one in Nevada in October, it focused attention on the fast-growing but little-known 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 around the country, some go further, saying that the chambers prevent osteoporosis, treat asthma, increase libido and kick-start rapid weight loss. Celebrity athletes like 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 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 does not recognize any medical benefits from cryotherapy chambers, nor does it regulate the devices. The death of Ms. Ake-Salvacion has many questioning the safety of the procedure and the claims of those who praise it.