Touted as a way to reduce pain, speed injury recovery, and improve mood, cryotherapy involves exposing the body to temperatures up to minus 300 degrees Fahrenheit.

Celebrities and some athletes—LeBron James and Shaquille O’Neal in particular—are proponents of this treatment. However, a recent death has called the safety and efficacy of cryotherapy into question.

A woman named Chelsea Ake-Salvacion who worked at a Las Vegas cryotherapy center called Rejuvenice died during an after-hours session in a tank, where she was alone.  

She was found dead on October 20, 2015, by a friend and coworker. Ake-Salvacion’s uncle told The New York Times that the 24-year-old woman was found “rock-hard solid.”

The coroner’s office determined that Ake-Salvacion died from asphyxia from an oxygen-poor environment, KVVU reported on November 10, 2015. Cryotherapy environments can have oxygen levels around 5%—so low that someone can fall unconscious and die.   

While both of Rejuvenice’s locations were shut down following Ake-Salvacion’s death, the 2 centers have since reopened.

Rejuvenice’s website states that patients report improvements in stress, insomnia, rheumatism, muscle and joint pain, fibromyalgia, and psoriasis following cryotherapy.

It also boasts that cryotherapy can strengthen the immune system, improve blood circulation, accelerate tissue healing, boost metabolism, burn calories, produce an “instant anti-aging effect,” detoxify, and help rebuild serotonin, which can help with depression and anxiety.

Here are 6 must-know facts about cryotherapy: 

1. There are 2 ways to experience whole-body cryotherapy.

Those interested in cryotherapy can experience the treatment in 2 ways: alone (ideally under supervision outside the tank) or with a few other individuals.

The cryosauna device is a single-person tank that covers the individual’s body, but his or her head and neck are kept in the open air, rather than directly exposed to the freezing temperatures.

Cryochambers allow 3 people to experience the treatment together, and their entire bodies are exposed to the “hypercooled” air. Rejuvenice stated that this is especially popular with athletes and couples.

Typical attire for cryosaunas includes socks, slippers, gloves, and underwear, while the cryochambers typically require a few added items such as masks and earmuffs.

2. One study did find some evidence that whole-body cryotherapy can reduce inflammation.

In a 2011 PLOS One study, 11 endurance-trained men completed 2 trials, one of which involved passive recovery, while the other involved post-exercise whole-body cryotherapy.

The study participants completed a 48-minute treadmill exercise, then rested or underwent cryotherapy.

The researchers found that the cryotherapy reduced the inflammatory process, as evidenced by a decrease in pro-inflammatory cytokines activity and an increase in anti-inflammatory cytokines activity.

3. Another study shot down cryotherapy as a successful treatment for repairing muscle damage or reducing soreness.

A 2012 Scandinavian Journal of Medicine and Science in Sports study did not report similar findings.

The researchers randomly assigned 36 patients to either a group that received a pair of 3-minute treatments around minus 166 degrees Fahrenheit or a group that received treatments around 59 degrees Fahrenheit.

While the cryotherapy reduced tympanic temperature, it did not affect knee joint position sense, maximal voluntary isometric contraction, or force proprioception. Even after exercise, those who had undergone cryotherapy did not see a change in muscle soreness.

“WBC [whole-body cryotherapy] administered 24 hours after eccentric exercise is ineffective in alleviating muscle soreness or enhancing muscle force recovery,” the researchers concluded.

4. Cold baths and packs may be the way to go until further research is completed.

An analysis of 10 cryotherapy studies suggested that ice pack applications or cold baths offer a less expensive option that has comparable physiological and clinical effects to whole-body cryotherapy.

This 2014 review published in the American Journal of Sports Medicine noted that the majority of previous studies involved small numbers of athletes ages 35 or younger.

The researchers determined that there is “weak evidence” from the 10 studies to suggest that cryotherapy enhances antioxidant capacity or alters inflammatory pathways to aid athletes’ recovery.

“A series of small randomized studies found WBC [whole-body cryotherapy] offers improvements in subjective recovery and muscle soreness following metabolic or mechanical overload, but little benefit towards functional recovery,” the researchers concluded.

Although they found no adverse effects in the cryotherapy studies, the studies did not take “active surveillance of predefined adverse events.”

5. Cryotherapy is not regulated.

Cryotherapy has not been approved by the FDA, and no government body regulates its use. The FDA also does not recognize any medical benefits for cryotherapy.

An FDA spokeswoman told The New York Times that the administration was “concerned with devices that are potentially being sold without having met” the FDA’s requirements.

6. Ake-Salvacion is not the only one harmed by cryotherapy.

An American sprinter and Olympic champion named Justin Gatlin experienced frostbite on his feet after he went into a tank with sweaty socks.

In addition, a woman named Alix Gunn is suing CryoUSA after cryotherapy allegedly froze her arm, leading to third-degree burns, loss of use, and disfigurement, The New York Times reported.

Gunn’s lawyer alleges that his client was given wet gloves to use. However, the center is arguing that Gunn should have looked out for her own safety.

A trial has been set for January 2016.