HENDERSON (Nevada) • Ms Chelsea Ake-Salvacion felt she was on healthcare's cutting edge, working at a cryotherapy centre that promised to help clients burn off calories, reduce pain, strengthen immune systems and halt ageing by embedding them in freezing tanks for a few minutes at a time.

In her off hours, she engaged in the practice and dreamed of opening her own centre. But last week, the 24-year-old was found dead in one of the tanks, which can reach temperatures of minus 115 deg C.

After working an evening shift at the Rejuvenice spa in the Las Vegas suburb, she had stayed to give herself a cryotherapy session. Her body was found the next day.

The coroner's office said the cause of death had not yet been determined. But Ms Ake-Salvacion's uncle said the coroner had told him his niece's body was found "rock-hard solid".

"What she told me was that there is nothing dangerous about doing this," the uncle, Mr Albert Ake, 48, said.

"(And) that the only thing that could happen is that you're there a little too long and you get frost nip on your fingers."

The spa company's website said that its chambers are "equipped with numerous safety features" and that doors are never locked, allowing clients to exit at any time.

There are two forms of cryotherapy offered at Rejuvenice: a one-person cryochamber, which requires the client to wear earmuffs, a mask, gloves, slippers, socks and underwear; and a three-person cryosauna, which does not require the mask or earmuffs.

Office manager Hailey Cap said she and other workers at the spa would often use the cryochambers, but never alone. "We always had someone with us," she said.

The coroner's office told Mr Ake that his niece died just minutes after getting into the chamber.

One reason people do not try it solo, Ms Cap said, is that the nitrous gas used to chill the air can be debilitating.

A Las Vegas police spokesman said it did not appear that a crime had been committed.

NEW YORK TIMES