The day Phil Mackenzie decided to expose his almost naked body to gas colder than the lowest natural temperature ever recorded on Earth started like any other day. The professional rugby player woke up and headed to the playing field in Manchester, England, for his usual grueling workout. He ran passing and kicking drills. He was repeatedly tackled. He lifted weights. By the end of practice he was exhausted. Usually Mackenzie would head back to the locker room and soothe his sore body with a hot shower. On this day, however, an enclosed pod resembling a massive standing tanning bed beckoned from the nearby parking lot. Mackenzie and a couple of his teammates stepped inside. Frigid gas started to swirl around them.

Mackenzie had wanted to try this procedure, called whole-body cryotherapy, specifically to ease his achy joints. But he says that after receiving multiple two-minute sessions spread out over several days he saw other benefits, too. “I felt refreshed right away. My sleep was better,” he recalls. Soon the treatments became routine: Mackenzie would go four times a week to chill out amid the icy vapors, wearing nothing but his spandex shorts, gloves, socks, slippers and headband to protect against frostbite. Most of his teammates adopted the regimen, too. In fact, there was usually a line for the pod after practice.

Mackenzie and his fellow rugby players are hardly the only devotees of cryotherapy. Star athletes, including Kobe Bryant and LeBron James, have turned to it. Reportedly, Hollywood A-listers such as Daniel Craig and Jennifer Aniston have, too. The market for these devices is beginning to burgeon in the U.S., with sports teams snapping them up to condition their players, and spas and wellness centers installing them for clients looking to relax, lose weight and fight signs of aging. One large U.S. distributor of whole-body cryotherapy machines, Dallas-based CryoUSA, says it has installed more than 200 units across the country since 2011, half of them in 2015. The company expects that the 2016 tally will show an even sharper uptick in sales.

Yet the science behind these devices is decidedly lackluster. In July the U.S. Food and Drug Administration issued a warning stating that there is no evidence these technologies help to ease muscle aches, insomnia or anxiety or provide any other medical benefit. Instead, it said, they may cause frostbite, burns, eye damage or even asphyxiation. In a statement to Scientific American the agency added, “The FDA has not approved or cleared any whole-body cryotherapy devices, and we do not have the necessary evidence to substantiate any medical claims being made for these devices.” The agency based its warning on its own informal review of published literature and generally recognized hazards associated with exposure to the gas that creates the cold conditions in the treatment chamber. Adding insult to injury, cryotherapy is pricey. A package of five, two-minute sessions can cost several hundred dollars.

A Chill in the Air

The notion of supercooling the entire body for therapeutic reasons got its start in Japan during the late 1970s, when it was touted as a potential way to relieve joint pain in patients with multiple sclerosis or rheumatoid arthritis. It then gained traction in western Europe in the 1990s. Only recently, in the past decade, has it risen to prominence in the U.S. and Australia. As the practice has spread, the list of ailments that it can supposedly address has exploded. According to the latest marketing claims, it can treat not only pain but conditions ranging from asthma to Alzheimer’s disease.

The logic of whole-body cryotherapy stems from the widely accepted science underlying standard-issue cold therapy, which uses ice packs and ice-water baths to treat acute soft-tissue injuries. Doctors will typically recommend icing as part of a care regimen for a sprained or strained ankle, for example. Clinical studies have found that applying ice to an injury site for some five to 15 minutes can lower skin temperature to less than 55 degrees Fahrenheit, which slows and thus dulls pain signals from affected nerves. Ice may help in another way, too. Animal studies suggest that it combats inflammation after injury by decreasing the number of white blood cells moving to the injury site, among other mechanisms, says Chris Bleakley, a sports medicine researcher at Ulster University in Northern Ireland. (Prolonged inflammation can extend pain, decrease range of motion and impair the blood flow around the damaged area.)

But whether cryotherapy can actually produce those same benefits is uncertain at best. Unlike run-of-the-mill cold therapy, it uses gasified liquid nitrogen to cool the air around recipients who stand in an enclosed chamber to temperatures below –200 degrees F. Although the gas temperature is much colder than ice, the cold from ice applied directly to the body has a better chance of penetrating through layers of skin and fat to reach the target soft tissue than does icy gas that swirls around the skin but is not pressed against it, making chilling of deeper parts of the body harder to achieve.

Indeed, a 2014 analysis of preexisting ice, cold-water and whole-body cryotherapy studies, carried out by Bleakley and by other researchers, found that ice packs delivered the biggest reductions in skin temperature and intramuscular temperature: a 10-minute ice-pack application cooled skin between 32 and 47 degrees F, for example. Three minutes of whole-body cryotherapy, however—the average time manufacturers recommend to protect user safety—resulted in a lesser reduction, ranging between six and 35 degrees F. 

Because whole-body cryotherapy is not as effective at cooling intramuscular temperatures, it is unlikely to slow pain signals as effectively as ice does or to cool soft tissues enough to quell inflammation, Bleakley says.

Other studies compound these doubts. In the gold standard approach to evaluating efficacy of a given therapy, participants are randomly designated to receive the treatment in question, a different one or none at all. To date, researchers have conducted four such randomized control trials of whole-body cryotherapy. In an exhaustive examination of those studies, exercise physiologist Joe Costello of the University of Portsmouth in England, along with Bleakley and others, found no significant benefit to the treatment. “There is insufficient evidence to prove whether whole-body cryotherapy reduces muscle soreness or improves recovery after exercise compared to ... no intervention,” he states.

Those four trials, as well as Costello’s assessment of them, are not the final word. They were very small, totaling just 64 subjects. And because all but four of the subjects were men, with an average age in their early 20s, it is impossible to say whether the putative panacea might affect women or older people differently.

Unanswered Questions

The shortcomings of these trials are emblematic of the poor state of the science of whole-body cryotherapy. Most studies of the treatment involve “very small numbers” of participants and have “methodological flaws” such as the lack of a control group, Bleakley says. “Sports scientists really need to pick up this area and align it with the quality of studies in wider medicine,” he asserts.

As for the effects of whole-body cryotherapy on all the other ailments it can purportedly address beyond athletic injuries, the science is virtually nonexistent. The claims have not been subjected to the rigors of a randomized trial. Nor do researchers have definitive answers about whether exposure to gasified liquid nitrogen produces beneficial effects on heart rate, blood pressure or metabolism—effects that, if they occurred, might help ease anxiety, treat migraines or fuel weight loss, among other aims.

Mark Murdock, managing partner at CryoUSA, does not dispute that whole-body cryotherapy lacks evidence for many of the uses claimed for it. The company promotes the devices for reducing pain and inflammation and increasing energy, but in his view, that use provides “comfort,” not medical assistance. He adds that medical claims, such as that the devices can drive weight loss, are “crazy.” He also says he supports the FDA’s decision to release the warning it issued in July and thinks the agency should ultimately step in to regulate the industry and curb such assertions.

Not only are the supposed benefits of cryotherapy chambers unproved but scientists also lack a clear understanding of any risks they might pose. No studies have focused on adverse effects. And not all whole-body cryotherapy is created equal: treatments vary in duration, temperature and which body parts are spared contact with the subzero vapors. How long a person is exposed, at what temperature and under what conditions matter for safety, says Naresh Rao, the USA Water Polo Olympic team’s physician.

Nevertheless, the notion of treating what ails us with a stint inside a glorified freezer has a powerful allure. Recipients report positive effects anecdotally but the lack of evidence to support these claims suggests they may simply stem from belief in the treatment—the placebo effect. Rao, who is also a doctor of osteopathic medicine (a field that supplements traditional medical care with holistic treatments), says that although he would not choose cryotherapy as first-line treatment for injured athletes, he supports his patients who want to use it—even if the benefits are subjective at best. Yet, he notes, “I do think it needs to be medically regulated. I wouldn’t say it’s ready for a consumer coming off the street.” People with heart issues or uncontrolled hypertension, for example, should not seek out cryotherapy, he warns, because sudden exposure to such cold temperatures could trigger heart attacks or other serious health complications in these individuals.

Some researchers are still hoping for good news about cryotherapy’s efficacy. Rebeccah Rodriguez, a Science Board member of the President’s Council on Fitness, Sports & Nutrition, an osteopathic physician for the San Diego Breakers rugby team, is among them. She plans to start a study in 2017 focused on evaluating cryotherapy chambers for facilitating recuperation from concussions. And a research team in Marseilles is conducting a preliminary study to assess whether whole-body cryotherapy has anti-inflammatory effects that could make it a viable alternative to popping traditional nonsteroidal anti-inflammatory drugs (known as NSAIDs).

“There is much work to be done,” Ulster’s Bleakley says. Only large randomized controlled studies can gauge the efficacy of whole-body cryotherapy—and arm consumers with the cold, hard facts.