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Lewis Pugh swims in the coldest waters on the planet in only a Speedo, swim cap, and goggles. The 47-year-old Brit has front-crawled across the North Pole and breast-stroked through a glacial lake at the foot of Mount Everest—all to raise awareness for climate change. As a result, he’s no stranger to frozen fingers and hypothermia.

“People think that because I have done it so many times that I have some kind of genetic advantage,” Pugh says. “But your body will never get used to it.”

On his latest trip to the Antarctic Peninsula earlier this month, Pugh swam for 18 minutes, covering his target 0.6 miles off the icy 32-degree islet of Half Moon Island. Shortly after the swim, on a stopover in Santiago, Chile, en route to Cape Town, Pugh and his medical team talked to �����ԹϺ��� about what happens to the human body in freezing water, and how Pugh is able to pull it off.

The Dangers

To swim nearly two-thirds of a mile in open, freezing water is to put your body on the edge of life and death, Pugh says. Parts of your body begin shutting down, and it can happen so quickly that you don’t even realize it. In fact, a few English Channel swimmers have , despite not presenting any alarming symptoms—and that’s in water that’s 60 degrees.

Pugh likens the sensation to that of burning skin. Your skin cells slowly freeze and burst, causing your fingers to swell. And the longer you are in the water, the more the pain you feel. Hypothermia creeps in slowly.

It’s not just the cold water than can kill you, says Roger Melvill, a South African neurosurgeon and mountaineer who acted as Pugh’s trip doctor. During Pugh’s swim, Melvill rode in a support boat nearby and watched for leopard seals. These predators will grab a person by the leg and pull them under. Sighting one would mean the end of the swim. Sharp floating ice is another concern—for obvious reason—Melvill says.

Air temperature needs to be monitored, too. On previous swims, Pugh has resorted to using a breast stroke action because it’s actually warmer if he keeps his arms submerged, Melvill says. During a in October the air temperature reached -35 degrees.

Preparing the Body and Mind

Pugh spent the journey to Half Moon Island alone in his cabin, calmly focusing his mind. It’s a meditation-style practice he’s developed to reduce anxiety before a dangerous swim. British psychologist Lee Harrison, who studies how humans experience pain, believes Pugh is on to something. “By practicing mindfulness (concentrating on the task at hand) or alternatively by using distraction techniques, it’s possible to diminish the threat that causes stress,” he wrote in an email to �����ԹϺ���.

Once Pugh has his feet in the snow at the water’s edge, it is very important that his team avoids expressing any kind of negative emotion. “Fear is contagious,” Pugh says. In the final moments before a swim, Pugh will typically put in ear buds and listen to music—anything from Puff Daddy to opera arias. But on this trip, he just concentrated on “justice,” he says. “Justice for future world children” who will grapple with a warming planet, he says. “Justice for marine animals and justice for the future of the Antarctic.” Then he dove in.

Surviving the Swim

The act is a mental battle as much as a physical one. “Once in the water, there is a good wolf and bad wolf in your head,” Pugh says. “The wolf you feed is the wolf that wins.”

Harrison agrees. “A fearful brain will typically increase the perception of pain, whereas a distracted or mindful brain will inhibit this information.” 

Once in the water, Pugh focuses on controlling his breathing. At the start of , in 2010, Pugh began breathing heavily, gulping water and then vomiting, prompting him to pull out and reschedule. “Hyperventilation linked to fear and panic is thought to be one of the main causes of drowning,” says Michael Ramage, a surgeon who researches muscle function and has spent 15 months working in the Antarctic. “This happens very rapidly, even if the individual can swim and is rescued early.”

During his recent Half Moon Island swim, Pugh’s fingers started changing color, from pink to white. When exposed to cold temperatures, blood vessels on the skin’s surface narrow in a process called vasoconstriction. The body is trying to reduce the amount of blood that becomes chilled, instead pooling it in the organs, the abdomen, and fat found deep underneath the skin. The process stops cold blood from returning to the heart. But “there’s a point where you are going to have to stop,” Melvill says, “otherwise there’s no coming back.”

Partway through the swim, Pugh began dropping his head further below the water’s surface, indicating increased physical strain. A member of his support team, Dawid Mocke, paddled close to check on him. It was a delicate moment. “Should Dawid tell me the time?” Pugh said after the swim. “Or should he tell me the distance?” He explained this conundrum: “If I am exhausted and he tells me I’m only half-way, that would break me. Alternatively, if he tells me, ‘Fantastic, you have done ten minutes,’ that might boost me. It’s a constantly developing situation and we only sometimes get it right.”

How to Recover

After the swim, Pugh struggled to haul himself into the rescue Zodiac. He was slurring his speech and his movements were uncoordinated. Clambering over the boat’s rib was excruciating, and caused massive bruising on the partially frozen soft tissue around his shins and ribs. “I was both physically and mentally finished,” Pugh said later. His trip doctor had no doubt that the swimmer was hypothermic.

Support team members covered Pugh with blankets and wrapped his face, then ferried Pugh to a nearby ship for a lukewarm shower. The short ride over was slow, as any sudden movements could have thrown Pugh into a lethal heart rhythm called ventricular tachycardia. “From this there is no way back,” Melvill says.

Melvill continued to attend to Pugh during an hour-long shower. As Pugh’s constricted blood vessels began to open, warm blood returned to his blue fingers. Ramage warns that this moment can be the most dangerous for a cold water swimmer. A cold-water survivor’s body temperature can drop by more than a degree as chilled blood returns in greater volume from the extremities to the heart, a phenomenon known as “afterdrop.” The consequences can be fatal.

After showering, Pugh’s body temperature warmed and Melvill relaxed. The bruising on the swimmer’s legs and ribs was serious, but Pugh is never out of the action for long. Just three days later, Pugh swam a short distance around a giant Antarctic iceberg on the same day that two of his team members staged an impromptu wedding on a nearby island. Aerial photos later revealed the ice block to be in .

Asked what plans Pugh has for future swims, he says “the swims are just to carry the environmental message about melting ice and the fragility of the polar regions. I have a three-year fight to get six more Marine Protected Areas agreed in the Antarctic before 2020. There is no time to lose.”