ALI ROGIN: It's the middle of winter in the Northern hemisphere and it might not feel like the season for jumping in the water, but more and more people are finding an icy plunge is invigorating.

William Brangham takes a look at the world of winter swimming and its potential benefits and risks.

WILLIAM BRANGHAM (voice-over): Even though the temperatures are plunging, so are these swimmers, a brave group in the northern Chinese city of Harbin.

Taking a subzero jump one by one into the icy water.

CHEN XIA (through translator): I felt prickling all over my body.

The water here is about 10 degrees Celsius lower than it is in my hometown, but it still made me feel blissful.

WILLIAM BRANGHAM (voice-over): These hardy souls are part of the city's winter swimming team, which boasts hundreds of members.

They say they train throughout the year to develop the physical strength and mental fortitude needed to dive into pools carved from ice during the city's annual winter festival.

YU XIAOFENG (through translator): Once these challenges are overcome, you will feel the endless joy brought by winter swimming.

WILLIAM BRANGHAM (voice-over): Endless joy or crazy obsession?

In the Czech Republic, locals there take to the cold waters of the Vltava River in early January to mark the end of the Christmas season.

SASA SELCAN (through translator): Well, I've stuck with it.

It's a bit of an addiction now.

KAMIL NEMECEK (through translator): Once you've tried it, you have to keep doing it.

WILLIAM BRANGHAM (voice-over): In the Danish capital, Copenhagen, it's a tourist draw to brave the icy water.

ORLANDO: It's crazy.

It's really refreshing.

I think it's an experience that every single person has to have once in their lifetimes.

EMILIANO: Here in winter.

ORLANDO: Yes, in winter.

And I love it.

It's really cold, but we love it.

WILLIAM BRANGHAM (voice-over): Devotees of winter swimming are convinced these bone chilling dips boost their health and happiness.

LIU SHURONG (through translator): Back in the day, I used to get leg cramps and ever since I started winter swimming, it has stopped.

YOU DECANG (through translator): I'm 76 years old, but I've never got a cold because I swim in winter and I feel very well.

It brings me good health.

WILLIAM BRANGHAM (voice-over): Online advocates say the benefits are many, helping with everything from migraines to arthritis and even weight loss.

WOMAN: It boosts the immune system.

MAN: It really improves your mental health.

WOMAN: Great blood circulation helps you sleep better.

WILLIAM BRANGHAM (voice-over): But not everyone is convinced.

HEATHER MASSEY, University of Portsmouth: Some of the claims that you'll see that splashed over the web, we don't actually have good, robust evidence to be able to support those claims at the moment.

WILLIAM BRANGHAM (voice-over): Heather Massey studies cold water immersion and swimming at the University of Portsmouth in England.

HEATHER MASSEY: The level of science is far more powerful from the side of it having negative effects.

WILILAM BRANGHAM (voice-over): Massey says the initial plunge into icy water is shocking and the body reacts instantly.

Shortness of breath, higher blood pressure, even impaired cognitive function.

HEATHER MASSEY: Definitely make sure you're well fit and healthy before you go in.

If you've got any underlying health problems or you're not feeling great going in cold water is not going to help.

WILLIAM BRANGHAM (voice-over): She says even healthy people should proceed with caution.

There can be very real risks of hypothermia and heart attacks.

But Massey is quick to add there could be health benefits, and she and other scientists are looking into what they might be.

HEATHER MASSEY: It may be that it's a placebo effect.

But if it's a placebo effect and it works well, it's still in effect.

WILLIAM BRANGHAM (voice-over): Evidence or not, these swimmers can't seem to stay away from their icy plunges and the thrills that come with it.

RITA EGG: It's so cold.

I was freezing.

I'm saying if I can do this, I can do anything in the whole world.

That's my feeling.

WILLIAM BRANGHAM (voice-over): For PBS News Weekend, I'm William Brangham.