This article first appeared in Issue 1 of our new free digital magazine CURIOUS.
With the looming threat of a giant asteroid strike, Meryl Streep et al. jet off on a spaceship, their bodies cryonically frozen as they search for a new planet to colonize, and inevitably destroy as they did the last. In case you haven’t seen it, this is the ending to Netflix’s Don’t Look Up, the Oscar-nominated apocalyptic comedy released at the end of last year.
Of course, this is a work of science fiction and here at IFLScience, we prefer to deal with science fact. With that in mind, we ask: Can humans actually survive being frozen? Can we, like Meryl Streep, simply defrost our 22,740-year-chilled bodies and emerge on another planet like nothing ever happened (only to be savagely eaten by a dinosaur-like creature seconds later)?
While the latter question is firmly in the realms of sci-fi – the answer being a resounding no, definitely not – there are some examples of the human body surviving at pretty frosty temperatures.
In Minnesota in 1980, one woman spent six hours in the snow after crashing her car and searching fruitlessly for help. Doctors reported she was “frozen solid” and likened her body to “a piece of meat out of a deep freeze”. And yet, somehow, with the help of some warming pads, she lived to tell the tale.
In a similar story from 2016, a 25-year-old man did a 12-hour stint in the snow at temperatures of -20°C (-4°F). Despite doctors being unable to detect a pulse, they managed to “bring him back to life”, minus a few digits lost to frostbite.
Sounds pretty impressive, right? But did these people actually survive being frozen, as the headlines claim?
Can humans survive being frozen?
“The short answer is no, it is not possible, and very probably never will be possible,” Professor Gary Bryant, Associate Dean (Physics) at RMIT University, told IFLScience.
In fact, the human body – normally maintained at 37°C (98.6°F) – really can’t cope with much temperature change at all.
“In general terms, damage will start to occur to the human body if the internal temperature changes by more than a few degrees,” Bryant added.
“A fever of more than 40°C [°F] is very dangerous, and hypothermia begins to occur if the body temperature drops below 35°C [°F].”
And freezing, he added, “is a whole other issue.”
“There are no examples of people being brought back after being actually frozen.”
Why then, are there these reports of people recovering from being frozen solid? According to Bryant, these are simply examples of people being brought back from severe hypothermia. Arguably still pretty miraculous but not quite as death-defying as they initially seemed.
In medicine, there are some examples of low temperatures being used to prevent tissue damage – known as therapeutic hypothermia – but this happens at temperatures far above freezing.
“Controlled cooling of people during surgery is something that has been done (with cooling generally down to around 31-32 °C [°F]),” said Bryant. “But its use is limited (and the benefits vs risks disputed).”
In 2019, a human was put into emergency preservation and resuscitation (EPR) – what sci-fi fans might refer to as “suspended animation” – for the first time, essentially becoming a human tardigrade. The process involves being pumped full of salty water to chill the brain to a cool 10-15°C (50-59°F), buying surgeons up to two hours to complete potentially life-saving operations.
But is cryonics, the scientifically dubious speculative idea that humans can be put into low-temperature storage, preserved for future reanimation, the future? No.
How do humans survive extremely low temperatures?
To understand how humans survive at low temperatures, we first need to understand just how low we’re talking.
In the case of the 25-year-old “frozen” in a snow drift, “the… internal body temperature was unlikely to be below about 30°C [86°F],” Bryant told IFLScience.
Reports say his temperature didn’t register on a digital thermometer, but, Bryant says, this doesn’t mean it was sub-freezing.
“They were no doubt using a touchless (infrared scanner) these generally do not register temperatures below about 34°C [°F]. However, even if the skin temperature is 30°C [86°F] (such as my fingers on a cold day), the inside body temperature will still be around 37°C [98.6°F].”
As for how he survived, it seems he may have physics to thank, at least in part.
“From a physics point of view being trapped under the snow means that heat loss is quite slow (snow is a good insulator). Radiative losses would still occur, but over several hours these would only lower the body temperature by several degrees,” Bryant said.
“None of the internal vital organs would have come close to being frozen, even if there was severe frostbite on the outer layers.”
What happens in the body at these very low temperatures to keep us alive?
To answer that question, we can look to nature. It’s no secret that in the winter months, many animals retreat from the world and enter hibernation – often bulking up in preparation for the big sleep ahead (fat bears, we’re looking at you). But what happens during this months-long hiatus is less well known.
To survive at such cold temperatures, hibernating animals slow (but don’t stop) their metabolism – the process that converts oxygen and food into energy. For every degree Celsius drop in body temperature, metabolism drops by 5 to 7 percent. With a slower metabolic rate, animals in hibernation require less oxygen at lower temperatures and therefore slow their heart rates and breathing. This conserves energy when insufficient food is available.
In cases of extreme hypothermia, human bodies do much the same.
While this can be life-saving, it is still incredibly dangerous – as we know, breathing and having a heartbeat are essential when it comes to being alive. But, if someone suffering extreme hypothermia is discovered before the heart stops, then they stand a good chance of making it.
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Why can’t humans survive being frozen?
In one word: ice.
Humans cannot survive having any ice in our bodies. And seeing as ice formation and freezing go hand in hand, it is quite the roadblock to surviving being frozen.
“By the time the internal temperature gets to below 0°C [32°F], the cold itself will have already killed many cells and organs, but once ice forms, it is basically going to be lethal to most cells, and therefore to the human,” Bryant told IFLScience.
Unfortunately for anyone wanting to create a human popsicle, the adult human body is 60 percent water. Water is found in cells throughout the body: the brain and heart are nearly three-quarters water, the lungs are 83 percent water, and even our bones contain some. At extreme low temperatures, the water in these cells can freeze. As the water freezes, it expands, which can cause the cells to rupture. Lethal to both cell and human, as Bryant said.
Most cells in the human body, in fact most mammalian cells, cannot survive being frozen. However, there are some exceptions to this rule: red blood cells, sperm cells, egg cells, stem cells, and some cell lines for cancer research have all been, and are routinely, cryopreserved – the process by which cells or tissues are preserved at sub-zero temperatures.
What animals can survive being frozen?
Where humans fall short, animals come into their own. It may not be common, but there are a couple of animals who, unlike us, can survive being frozen.
Tardigrades, for example, are no stranger to sub-zero temperatures. The microscopic beasties have evolved, over hundreds of millions of years, to accumulate high levels of cryoprotectants – chemicals that prevent cells or tissues from becoming damaged when frozen – which enable them to survive freezing.
Frogs, meanwhile, are known to be able to survive being frozen for several months. Wood frogs are filled with natural antifreeze (sugars), which lowers the freezing point of water in their bodies and prevents ice formation. This allows them to survive extreme winter temperatures as they freeze and thaw with the environment – they can freeze up to 70 percent of their body water and survive.
Sadly for humans, the ability to survive freezing won’t be making the leap (frog pun intended) over to us. Even if we could engineer ourselves to have cryoprotectants, we wouldn’t be able to tolerate them.
We’re just going to have to crack out the thermals, wrap up in a blanket, and let the animal kingdom (and Meryl Streep) have the win on this one.