On the afternoon of Sept. 21, 2013, temperatures dropped suddenly and sharply on the shores of Lake Tahoe, plummeting from 72 degrees F to 34 degrees. Snow began to fall from the sky, gradually increasing in both size and density. The early-season snowstorm was a marvel to vacationers who were strolling alongside Kings Beach or through Squaw Valley, site of the 1960 Olympic Games. But there was another athletic challenge set to take place there the very next day: the inaugural Ironman Lake Tahoe. If any of the athletes racing preferred chilly conditions, they weren’t disappointed; the morning of the race dawned breezy and cold, with a layer of fog rising from Lake Tahoe—a visual representation of heat leaving the water. And it was warmer in the water than it was on land—a rarity in triathlon racing.

While conditions improved as the sun rose and athletes exited the water and rushed to the change tent for dry clothes, that initial cold was too much for many of the competitors. Around 20% of the field did not finish, with many citing hypothermia as their reason for quitting. While that race became infamous for the cold weather, the effects it had on the field weren’t unique. In many races with high DNF rates, cold temperatures and hypothermia are often a culprit, sending athletes to throw in the towel and take refuge in the med tent.

Understanding precisely how and when hypothermia occurs and finding ways to delay or circumvent this process has long been an active area of interest for researchers, doctors, and various groups ranging all the way from the Nazis to the U.S. military. Today, research continues into how to extend human limits in the face of cold, damp conditions—and triathletes, especially those preparing for races in cold conditions, are paying close attention.

The chilly road to hypothermia

For the average adult, 37 degrees C or 98.6 degrees F is considered normal body temperature. Medically, hypothermia is defined as a core body temperature of less than 35 degrees C, or 95 degrees Fahrenheit. Just a few degrees dip in core body temperature can make a world of difference in how you feel and your ability to perform not just athletically, but in basic human functioning.

But hypothermia is not an instantaneous phenomenon. Well before you achieve clinical hypothermia, a cascade of events and physiological responses take place, said Heather Massey, senior lecturer in clinical exercise physiology and member of the extreme environments laboratory and clinical health and rehabilitation research team at the University of Portsmouth.

“Most of the problems and most of the risk actually occurs way before you become hypothermic,” she said.

There are four phases related to hypothermia, each one describing a spectrum of experiences:

Phase 1: Cold shock response

“Cold shock response is when you get in [cold water], take a deep breath in and then hyperventilate,” Massey explains. Blood pressure and heart rate increases when this occurs, which could trigger arrhythmias or cardiac arrest. Panic can also set in, and as such, it’s during this initial phase where many drownings occur, especially among individuals who have fallen through the ice and aren’t expecting to suddenly find themselves submerged. But even those who willingly enter the water experience this response: If you’re in the midst of a mass swim start and another athlete swims over you as you’re gasping for breath, the resulting panic could also be problematic. “That’s the drowning process started,” Massey said.

The act of submerging both the head and the body at the same time also increases risk in this initial phase in terms of how the autonomic nervous system, which regulate breathing heart rate and other unconscious bodily functions, responds to the inputs it receives.

“The body is being accelerated when it goes into cold water. Your heart rate increases, breathing increases, blood pressure increases."

But when you put your head in cold water, the nervous system puts the brakes on all those processes—submerging the face in cold water has a calming effect, and in fact, it’s this well-known reflex is why many people splash cold water on their faces to calm down when faced with distressing news or other stressful situations.

In terms of swimming in cold water, Massey said “you get those two things clashing, and we think it causes something called autonomic conflict. That can kick out heart arrhythmias.”

Add to that situation the pressure of competition or an athlete who’s older or has an undiagnosed condition, such as high blood pressure, heart disease, or SIPE (Swimming-Induced Pulmonary Edema) and the risk for a life-threatening cardiac event increases. This is why the majority of deaths that happen in triathlons occur during the swim, and particularly the beginning of the swim. “It’s way before anybody’s gotten hypothermic,” Masey explained, “but is related to cold water all the same.”

RELATED: Solving the Mystery of Swim Deaths

Phase 2: Cold incapacitation

While the initial cold shock response period is all about cooling of the skin and the rapid response of the autonomic nervous system, the second phase of cold water immersion is about cooling the nerves and muscles just under the skin, which leads to fatigue.

“When we get muscle temperatures of about 27 degrees C [80.6 degrees F] we will notice a significant increase in fatigue within our muscles.” This is when your body position in the water starts to droop and your coordination to move your arms begins to falter. Even the strongest swimmers are not immune to these changes. Massey and her team have observed this during experiments in an endless pool in the lab. In extreme cases, that fatigue can lead to swim failure and, potentially, drowning.

During this phase, the body also shunts warm blood from the extremities to the core in an effort to protect the vital organs—the heart, brain, and lungs—from cold exposure. In the grand scheme, fingers and toes are not nearly as important to the overall survival of the organism as keeping the brain and heart warm and functional. This evolutionary adaption means your hands and feet will likely cool faster and become painful sooner than the core does.

Phase 3: Cooling the deep body

It takes time for the cold to penetrate the layers of fat and muscle that protect the core, and how quickly it happens depends on the size and shape of the athlete as well as the temperature of the water.

“You’ve got to cool the outside before you can cool the middle bit,” said Massey. Therefore, “we don’t normally see people becoming hypothermic until 20 or 30 minutes in the swim as a bare minimum.”

It’s also not a uniform experience for all people. Tall, lean people with less subcutaneous body fat cool more quickly than shorter individuals with more insulating fat. Other factors are also involved, including how much heat a person is generating through exercise, the temperate of the water, and how quickly the environment is wicking away that heat.

Though body fat can insulate an athlete, Massey warns that getting ready for a cold event isn’t as simple as just loading up some fat and hoping for the best. “You need the right amount of body fat for the right amount of fitness. You need the combination of both.”

Phase 4: Circum-rescue collapse

Circum-rescue collapse is a sudden loss of consciousness post rescue from cold water immersion. Initially thought to be related to afterdrop, (explained in more detail below) it’s believed now to be related to arterial blood pressure. Struggling to maintain proper heart function, blood pressure, and core temperature under the stressful conditions of cold water immersion, the body goes into shock. This shock can take multiple forms, from fainting to cardiac arrest.

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When danger strikes

Physiologist Gordon Giesbrecht, who operates the laboratory for exercise and environmental medicine at the University of Manitoba in Winnipeg, notes that while it takes a while to become hypothermic, issues related to being too cold can sneak up on you. “Normally, I tell people to watch out for extended periods of shivering,” he said. “If you’re shivering for 15 minutes, that’s a sign that something’s wrong. But the problem is, when you’re racing, you’re producing heat. This heat production can mask the fact that you’re losing more heat than you’re generating.”

This can lead to what Giesbrecht, who was anointed “Professor Popsicle” in a 2003 Outside article, calls insidious onset of hypothermia: “You have these people who are doing their thing, and then either suddenly, or in a very short period of time, their condition deteriorates to where they’re weaving around on the road or falling down and can’t get up or just being unconscious.”

“You have these people who are doing their thing, and then either suddenly, or in a very short period of time, their condition deteriorates to where they’re weaving around on the road or falling down and can’t get up or just being unconscious.”

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Hypothermia during a race

Because water conducts heat away from the body 25 times faster than air, the swim is the portion of the race where hypothermia is most likely to become a problem. But it’s not impossible to have difficulties during the bike or run, depending on the conditions. Weather can change suddenly and dramatically, as athletes have discovered at multiple iterations of the full and 70.3 Ironman in St. George, where freak winds and temperature drops made for hazardous conditions. A recent tragedy in the Gansu province of China, where 21 runners participating in a 60-mile ultramarathon died when a sudden hail storm brought freezing temperatures across the course, is another prime example of how cold can always be a threat, even when one doesn’t expect it to be.

For triathletes, transitioning from the swim to the bike can be a dicey moment, because while your core temperature might not have reached the level of clinical hypothermia, your muscles may be cold enough to make it difficult to handle the bike safely. Though pedaling hard can help build heat in the muscles that will eventually warm you, the wind chill as you go whizzing along presents an additional challenge, making that transition all the more important.

“As long as you’re wet, you’ll have evaporative heat loss,” Giesbrecht said. “Drying off as thoroughly as you can in T1 will lead to less evaporative cooling once you hop on the bike. In a chilly race, it’s worth it to take the extra minute or two to get completely dry before you rush out of transition.”

Once you get moving again on the bike, you should be able to generate enough heat to counteract any cooling you experienced during the swim. But just as it takes a while to cool down, it may take some time before you’re back up to a normal core temperature. In one study Massey conducted, one test subject needed a full 40 kilometers on the bike to regain a normal core temperature, but most people they’ve tested have warmed up significantly quicker than that.

When you’ve removed yourself from the immediacy of a potentially hypothermia-inducing situation, the danger is only half past. That’s because afterdrop looms.

The afterdrop

Afterdrop transpires when you’ve stopped exercising and generating heat. This loss of heat generation from exercise leads to a drop in core temperature.

At the same time, the body begins the process of trying to salvage the limbs it was willing to sacrifice while you were immersed in the cold water and begins shunting warm blood from the core out to the limbs, creating an exchange that brings cold blood from the extremities into the core. That exchange of cold blood from the periphery with warm blood from the core can lead to unsafe changes in heart rhythm and blood pressure.

Thus, rewarming must be handled carefully; rather than jumping in a hot shower or hot tub, those with hypothermia should avoid submersion in warm water as that can speed up afterdrop and lead to cardiac arrest. This is why so many races wrap athletes in mylar “space blankets” after crossing the finish line. Body heat is trapped in the blanket, allowing the body to gradually adjust and lessening the chance of afterdrop.

The acclimation paradox

Giesbrecht notes that while you can train to become more tolerant of the cold, there’s a paradox associated with toughening up. The recent interest in winter and ice swimming (swimming varying distances in water colder than 45 degrees F) has led to an anecdotally-noted uptick in people who are able to tolerate the cold to the point of collapse.

“It’s not perfectly scientifically shown, but there seems to be a tendency that people who are at more risk of collapsing in the water are people with more experience. They believe that because they’re tougher, they can put up with more,” Giesbrecht said. Someone who has trained to disregard that discomfort and push on can swim themselves into unconsciousness and potentially death. Someone with less experience, on the other hand, wouldn’t be able to tolerate the cold beyond a certain point and will stop.

This contradiction was tragically illustrated in 1959 when Jason Zirganos, arguably the hardiest cold water swimmer in the world at the time, died while attempting to swim across the North Channel between Northern Ireland and Scotland.

After six hours of swimming in frigid conditions without the protection of a wetsuit, Zirganos lost consciousness and turned blue. He was pulled into the boat where the doctor on board used a penknife to open his chest and conduct open heart massage to try to revive him. (Such invasive surgical intervention was common practice at the time, prior to the development of the modern CPR compression protocol.)

Resuscitation efforts failed and Zigranos was pronounced dead of hypothermia-related cardiac arrest.

The irony of Zirganos’ death is that this potential problem had been noted by a leading physiologist several years prior. Zirganos had been a key subject in a case study, “The Physiology of Channel Swimmers” published in The Lancet in 1955 that demonstrated that he did not perceive cold the way control subjects did but that his core body temp fell faster than expected based on his physiology.

Hypothermia isn’t all bad

Clearly, in swimming and triathlon, hypothermia can be a real threat, especially in spring and fall events when water and air temperatures are low. But hypothermia isn’t not always a bad thing. Increasingly, therapeutic hypothermia—purposefully cooling a patient’s body after a trauma—is being used to help patients who’ve suffered cardiac arrest, stroke, or other ischemic injuries recover.

Dr. Benjamin Abella, director of the center for resuscitation science and vice-chair for research and professor of emergency medicine at the University of Pennsylvania in Philadelphia, explained that numerous studies conducted in animal models over the past few decades have established that therapeutic hypothermia can be profoundly beneficial.

“A lot of the early work was done in cardiac arrest, and it was found that when you lower the core body temperature of animals, you reduce a lot of the injury mechanisms, such as brain swelling and inflammation after a cardiac arrest,” Abella said.

In a sense, therapeutic hypothermia is a whole-body means of applying ice to an injury, just like you would do if you twisted an ankle. The general rule of thumb is that the patient will be cooled with a special device to a core body temperature of 33 degrees C [91.4 degrees F] and held there for about 24 hours. It’s important not to cool the person too far, as that an cause cardiac arrest, Abella said. “It’s not cryogenics. We’re not freezing people. We’re just cooling them, to give the body time to recover.” After the recovery period, the patient is gently brought back to a normal temperature and hopefully a full recovery.

Not every patient is a candidate for therapeutic hypothermia, and its application is often limited by resource availability and the speed with which a patient can get appropriate care. But Abella believes this area of medicine is growing as scientists learn more about how to apply and manage cold for better health outcomes.

Other researchers are investigating the power of cold to achieve other health gains. The pandemic pushed many triathletes outside into colder water for diversion and fitness, and as such, there’s been a boom in cold water swimming. Many proponents say cold water immersion boosts health in several ways, most notably boosting mental health and potentially delaying the onset of dementia or cognitive decline.

“There’s a lot of information out in the media at the moment that purports there’s a lot of science behind it where actually, there’s lots of experiential work,” Massey said. “Lots of people are blogging about it, writing books, talking to the newspapers, but actually, when you come to find the scientific papers, they’re very, very limited in this area.”

She hopes her lab’s ongoing work will provide the needed data to back up the growing consensus that swimming in cold water—though not everyone’s idea of fun—can actually be a good thing if managed appropriately.

Signs of Hypothermia

You should, of course, always be aware of the signs of impending hypothermia if you’re entering cold water or wet and chill conditions (such as cold rainy bike rides). Even before you reach a clinical state of hypothermia, the below symptoms indicate you’re colder than your body would like to be.

SIGNS OF HYPOTHERMIA
Shivering and teeth chattering. Giesbrecht explains that shivering is nothing more than muscles contracting and relaxing: “It does no work but it produces heat.”
"The claw." When your hands cramp up and you can’t move your fingers freely because they’re too stiff form cold, that’s the dreaded claw.
A drop in stroke rate in swimming or a sudden decline in cadence or power output in the bike or run.
Dizziness, nausea, or headache.
Grogginess, sleepiness, or disorientation such as wandering off the course and being unable to respond appropriately to directions.
Inability to answer simple questions or a loss of logic and reason.
Slurred speech.

Mitigating your risk of hypothermia

The good news: there are a few things you can do to keep yourself warmer when competing in chilly conditions.

• Train. First among these is training. Getting acclimated to cold water can habituate you to the experience of cold shock so that you can understand what to expect. “We know that five or six immersions in cold water for about three to five minutes each will actually halve that cold shock response,” Massey said. Giesbrecht adds that “you can certainly get to the point where you put up with [cold] better.” Becoming more fit so you can increase the amount of heat you generate while exercising is one way of trying to outrace hypothermia.
• Get wet. On race day, getting wet prior to the start to get over that initial cold shock response before you begin racing. This is why athletes at the Norseman Xtri are sprayed down with cold hoses before jumping off the boat into the water. Starting away from the scrum or at the back of the pack can also reduce anxiety leading to a less pronounced cold shock response.
• Dress right. A well-fitting wetsuit can help mitigate the cooling effects of water and significantly extend the time you can say in cold water. But Massey notes that the key is the wetsuit must fit properly. “If it’s not fitted correctly, you’re going to experience flushing of water in and out of the wetsuit and all that does is remove the heat from the body.”
• Add accessories. If the race permits it, you can also consider wearing neoprene gloves, booties ,or a hood to prevent heat loss from these highly vascularized areas. “Those things are going to provide marginal gains,” Massey said, noting that any gain, however small, is still a gain.
• Plug your ears. Many swimmers also swear by earplugs, which prevent cold water from infiltrating the inner ear. Cold water in the ear can make you feel colder and lead to dizziness. “The more insulation you can get, the better,” Massey said.
• Pace yourself. Pacing yourself can also be a strategy for preserving heat. Starting out with a conservative pace you can hold for the duration of the distance is best; if you go out too quickly and need to slow down, you’ll be producing less heat in the muscles as you get deeper into the swim, which can hasten muscle cooling.
• Fuel and hydrate. Making sure you’re properly fueled and hydrated is also important. Your body needs energy and water to keep all its systems firing, which becomes even more critical during endurance exercise. Massey recommends having a small meal with lots of “slow-burning carbs” such as oatmeal prior to the race to give you energy. Topping up your nutritional needs throughout the race as needed can also help you keep warm in cold conditions. And while consuming warm feeds during the event may not offer much actual heat gain, it feels nice and will distract you from feeling cold in the moment.

RELATED: How to Prepare for a Cold-Water Triathlon