How to survive a polar bear plunge (and why you shouldn't do one)

It sounds like a profoundly dumb idea: In the middle of winter, strip down into your bathing suit and take a running leap into the ice-cold ocean. Yet every year, hundreds of thrill-seekers around the world take part in this New Year's Day ritual. In fact, Coney Island has an entire club—the Polar Bear Club—devoted to the sport. They actually take the plunge multiple times a year.

For most people, the activity is an annual tradition done simply for the adrenaline rush. But that very thrill could turn dangerous or even deadly if you're not physically prepared for it. Here's what happens to your body immediately after the freezing water hits your skin:

The human body isn't exactly suited for freezing water. So as soon as you enter the ocean (or frozen lake, river, pool, etc.), you go into something called "cold-shock", which is an involuntary response—meaning you have no control over it—that causes you to inhale a giant gasping breath, says Joseph Herrera, the director of sports medicine at Mount Sinai Medical School. This can be incredibly dangerous; since you have no control over how and when it happens, it could occur when you are underwater, causing you to take water into your lungs. Depending on how much you inhale and how long it takes you to get to the surface, you could end up drowning.

The cold-shock response is followed by the diving reflex. Extremely cold water causes the blood vessels in the body to constrict. This helps to maintain heat on the outer part of the body, but also makes it harder for the heart to pump blood to internal organs. In the moment, "your body starts to force blood from the limbs to the heart and brain," says Herrera, to ensure those vital organs receive enough blood to survive.

The increased workload on the heart poses an obvious danger to people with underlying heart conditions. But healthy people—and even those who swim regularly—can also run into trouble when icy water triggers these reflexes.

Even so, countless thrill-seekers take the plunge for supposed health benefits every year. Herrera says that there can be some release of adrenaline in response to the shock of plunging into freezing cold water, so the rush is real. But most of the supposed health benefits of plunging into icy water aren't actually backed by science, he says. "It is thought that swimming in freezing temperatures will lead to the ability for the body to 'harden' and fight of disease, but this has not been proven."

For those individuals who simply can't live without the plunge, Herrera says there's one surefire way to minimize the danger: Practice. The body undergoes various adaptations if trained properly for cold-water swimming, he says. As your body gets used to longer and longer swims in colder and colder waters, for example, you'll likely develop an extra layer of fat to help keep you warm. The cold shock as well as the shiver response are both minimized as well. Herrera recommends going through this gradual training or skipping the plunge entirely. And on the day itself, instead of a true plunge, you should walk into the water slowly to minimize shock.

But are there any health benefits to taking a chilly dip? Herrera says swimming has numerous beneficial cardiovascular effects, but the only potential 'benefit' you get from doing it in cold water is that extra layer of fat.

So if you have a heart condition or haven't yet started training for this year's plunge, it's probably in your best interest to sit this one out. You'll just have to enjoy watching other folks freeze their butts off while you sit comfortably on the sidelines.