Of all the ways to ring in the new year, jumping into frigid water while nursing a hangover seems like one of the least appealing options. Still, that hasn’t stopped thousands of people around the world from stripping down to their bathing suits on January 1st, and diving into the nearest ocean or lake.
A typical polar bear plunge event involves running into the water until you’re partially or completely submerged. And while enthusiasts say the icy dip spikes their adrenaline, some experts are decidedly less than thrilled about the ritual.
Plunging into cold water can actually be deadly, particularly for people with heart conditions, who might have a heart attack or drown, says Mike Tipton, PhD, a professor of human and applied physiology and a researcher at the Extreme Environments Laboratory at the University of Portsmouth in the United Kingdom. “Immersion in cold water is one of the greatest stressors we can place on our body,” he explains.
Although there’s no official definition of “cold water,” Tipton says experts place the number around 59 degrees Fahrenheit. But that temperature seems downright balmy compared some U.S. coastal waters in January. Along the northeastern seaboard, temps in the Atlantic Ocean and the Great Lakes range from the mid-30s to the low 40s. The water off the northwest Pacific coast only averages about 10 degrees warmer.
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So what does cold water do to your body?
You’re most at risk for heart problems during the first minute or so that you’re in the water, says Tipton. The reason: Plunging into the water activates a number of cold receptors that reside underneath the skin, kickstarting a process called “cold shock.” This can give you an adrenaline rush, but it makes you hyperventilate: Your heart rate increases, your blood pressure rises, and your breathing speeds up, says Tipton.
Next, you’ll involuntarily “gasp”—something that’s particularly dangerous if your head is submerged beneath the water. Tipton says that in water temperatures of about 50 degrees Fahrenheit, people can only manage to hold their breath for an average of 5 seconds before they reflexively open their mouths, sucking in something like 65 ounces of water, or the equivalent of a 2 liter bottle of soda. (Some context: Tipton says a person who weighs 150 pounds can drown after swallowing 1.5 liters of sea water.)
Putting your head under water also increases your chances of having an arrhythmia, or an irregular heartbeat, he says.
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Let’s say you decide to tread water for a little longer. In that case, even strong swimmers could have trouble making it to shore: Being in cold water—between 41 and 59 degrees—for anywhere from 1 to 15 minutes can trigger a type of nerve dysfunction, or paralysis, that limits your ability to swim, says Tipton.
After as little as 5 minutes, the deep muscles in the forearm can drop from about 98.6 degrees to 80.6 degrees. Within thirty minutes, the nerves and muscles nearest to the skin begin to cool, hampering your muscle strength, dexterity, and coordination, says Tipton. In that case, your arm muscles will likely freeze up first, followed by your leg muscles.
So yeah, on New Year’s Day, we’ll be chilling in the hot tub.
