Is it Safe to Swim in Cold Water?

All swimmers have their favorite water temperature. For me, 65 degrees is my sweet spot. It’s that just-right place that’s cool enough to keep me from overheating when I’m swimming hard and warm enough so I don’t feel too chilled when starting out.

If that temp sounds too cold to you, I should probably add that I’ve completed two official ice miles, events sanction by the International Ice Swimming Association that involve swimming a mile in open water that’s 41 degrees or colder, without the use of a wetsuit or any other heat-retaining gear. I’m also an annual participant in the Memphremagog Winter Swimming Festival that takes place every February in a 25-meter, 2-lane pool that’s literally carved into the frozen surface of Lake Memphremagog about 5 miles south of the Canadian border. Some might call me an extreme cold-water swimmer, but there are many people doing much more in even colder water than I do.

Still, for many swimmers, particularly those who are accustomed to pool swimming where the water typically ranges from about 78 to 83 degrees F, 65 feels downright frigid. And that’s OK—there’s nothing that says you have to get comfortable in water that makes you uncomfortable. Cold water isn’t for everyone, and in some instances it can be dangerous.

Cold Water Shock Response

In cases where you’re suddenly exposed to very cold water, you could be at risk of drowning or a cardiac event. This occurs because of a physiological phenomenon called cold water shock response.

Cold water shock occurs when the skin is suddenly cooled. It triggers gasping and hyperventilation as soon as you become immersed in cold water. Your blood pressure and heart rate also increase, which can trigger arrhythmias or cardiac arrest.

It’s this cold-water shock that makes falling through the thin ice on a pond so deadly; being dropped suddenly into very cold water when you’re not expecting it and gasping as you go down typically leads to inhaling water and panic. It doesn’t take much water in the lungs to initiate the drowning process.

At the same time, when your face hits the cold water, that triggers the mammalian dive reflex, another autonomic process that tamps down the nervous system’s response to stressors. This is why splashing cold water on your face is often a good way to calm down when you’ve gotten upsetting news.

But in that moment of when your body and face are both submerged in water that’s significantly colder than your normal body temperature, those two responses clash, causing a situation called autonomic conflict. That can also trigger heart arrhythmias. 

If you’re older or have an underlying condition, such as undiagnosed heart disease or high blood pressure, that can make entry to cold water even more risky. This is why most deaths during a triathlon happen in the beginning of the swim.

Swimming Toward Hypothermia

Cold incapacitation sets in next. This phase occurs as nerves and muscles just under the skin cool. It leads to fatigue and is responsible for that loss of good form you’ll no doubt feel as you get cold while you’re swimming.

It’s during this phase that the body constricts blood vessels in the extremities to redirect warm blood back to the core to help maintain organ function. This is why your hands and feet get so cold so quickly, even if your core seems to feel nice and toasty. This process is your body’s way of triaging the most important components—the loss of a finger or a toe is much easier for the organism to cope with than losing function of any of the vital organs.

If you stay in the cold long enough, your core will eventually cool, too. It can take a while, but eventually, the cold seeps through protective layers of fat and muscle and in about 20 to 30 minutes on average, depending on the person, the water temperature and other factors, hypothermia sets in.

Hypothermia occurs when your core body temperature falls below 35 degrees C (95 degrees F). Normal body temperature for the average adult typically sits around 37 degrees C or 98.6 degrees F. 

As you’re flirting with hypothermia, even after you’ve left the water, you’re not out of the woods as your body continues to cool and your shivering reflex kicks in. Afterdrop occurs when you’ve stopped exercising and your body is no longer generating heat from that activity. It leads to a drop in core temperature as your body beings trying to salvage the extremities by reopening blood vessels. The cold blood heads back to the core and that can trigger changes in heart rhythm and blood pressure.

Because of those risks, it’s important to handle rewarming carefully. If you’ve gotten very cold during a swim, it’s best not to jump right into a hot shower, as that can speed the release of cold blood in the extremities to the core and cause a drop in blood pressure that could make you pass out or trigger cardiac arrest. Instead, immediately remove your swimsuit; afterdrop can sneak up on you, so even if you don’t feel all that cold. Get dry and bundle up in warm clothes. Get inside a warm building or into a warm car. Sip a warm drink and allow your body some time to shiver. It’s the in-built system designed to bring your temperature back to normal.

Expanding Tolerance for Cold Water

If you’re interested in improving your tolerance for cold water, the good news is acclimation works. Over time, you can build up more tolerance for colder water by slowly increasing your exposure to it.

One of the best ways to get more comfortable with cold water is by “swimming down the temperature” from summer to fall. If you’d normally be out of open water by Labor Day, try swimming a few more times into the fall. Your swims will naturally be shorter in duration as the temperature drops, but you may find that you can build up your comfort level with frequent, short swims.

During that acclimation period, you’ll learn to better control your breathing in that initial cold shock response phase. It never completely goes away, but over time, the response does lessen.

While you may become more comfortable in cold water over time, hypothermia remains a possibility during every swim in cold water if you’re in for long enough. In time, you can learn to manage that, too. It’s best to swim with other experienced cold-water swimmers, especially when you’re first starting out, to learn how to build up to longer cold-water swims safely.

Along the way, you’ll likely find that how you respond to the cold can vary greatly from one day to the next depending on a range of factors from how well you slept the night before to how well hydrated you are. Learning how to respond when a swim isn’t going as well as hoped for and knowing when it’s time to get out and get warm is all part of a journey toward loving cold water that may offer myriad health benefits.