- Right now, my heart is beating almost 100 beats per minute faster than normal.

I'm sweating fast enough that I could lose up to a liter of fluid an hour.

If I stay in this room for too long without cooling down, I could die.

I'm in the middle of an experiment to test the human body's ability to deal with extreme heat.

This room is hot, like 100 degrees Fahrenheit.

I've been in temps like this before, but that's not the worst part.

The killer is the humidity.

At this humidity level, my sweat just can't evaporate.

My body's cooling system just isn't functioning like it's supposed to.

Conditions like these already happen regularly for hundreds of millions of people on Earth.

And according to scientists, it's coming for more of us faster than we can even guess.

I'm stepping into this heat research lab to show you what happens to our bodies when we experience extreme heat, how we built our world in ways that doomed some people to deadly heat and how we can fix it, to how the one number we've been using to tell us how hot was too hot is wrong.

And I'm already regretting it.

(bright music) Hey smart people, Joe here.

The world is hotter than it used to be.

That's just a fact.

Average global temperatures are about 1.2 degrees Celsius hotter over the past 50 years.

That might not sound like much, but one in four heatwaves since the year 2000 would have been impossible without climate change.

More humans are experiencing extreme conditions more often and for longer.

So just how dangerous is extreme heat?

If you look up data on how many people die from extreme temperatures, you actually find something curious.

Cold-related deaths outnumber deaths from heat by a pretty good margin.

Or so it seems.

This data obscures something really crucial.

Most deaths that we attribute to temperature happen in this "moderate cold" zone.

And that's for a simple reason.

It isn't that moderate cold is secretly dangerous or something.

It's just those are the conditions people spend the most time in.

The truth is, a person's risk of death or injury goes up way faster in hotter temperatures compared to cold.

In the US, heat and heatwaves are the leading cause of weather-related death.

They've killed more people over the past 30 years than hurricanes and tornadoes combined.

And in some places, heatwaves are becoming literally unsurvivable.

At certain temperatures, the human body gets so hot that it shuts down.

Then it's game over.

But what is it about heat that can be so uniquely dangerous for our bodies?

And what can we do to make it safer to live in a hotter world?

In 2001, All-Pro offensive lineman Corey Stringer was working out in full pads when the heat index hit 110 degrees.

He began to feel sick, but by the time that trainers realized what was happening, his body temperature had reached 108 degrees, and he later died.

Corey's survivors established this research lab, dedicated to preventing sudden death from heat.

And that's where I came to sweat it out and learn from researchers like Becca what happens to our bodies in extreme conditions.

- We were founded with the mission and goal to not only enhance performance and health and safety and prevent catastrophic injuries for athletes, but also for soldiers and war fighters and laborers.

Heat is one of the top threats to all of those individuals.

- Now all of us spend at least part of our lives living, working, or playing outside.

And as we crank up the world's heat, those things become more dangerous.

- I think the surprising thing that always catches people off guard is that when you look at the top causes of sudden death in sport, heat is in the top four.

And those four top causes, just four alone, make up 90% of the deaths that we see.

- Extreme heat has extreme consequences on our bodies.

To figure out what those consequences are, I've gotta get on a bike.

This bike will be my home for the day.

First we need to set a baseline, so we've set the room to conditions you'd feel on a nice, comfortable spring day.

It's like perfect conditions.

I could do this for hours.

Easy peasy.

But let's take a look at what's going on in my body when I start to get active.

When I start contracting my muscles to spin the pedals, my cells start burning more energy.

Every cell in our bodies runs on a molecule called ATP.

It's the main energy source that keeps our muscles moving, our brains working, and our hearts beating.

Your cells have a powerhouse in them called the mitochondria.

They're like little factories that turn glucose and oxygen into ATP, charging up our cells' batteries.

But to convert that energy into movement, I have to chemically rip that ATP apart.

- We're actually not really that efficient in terms of how we use our energy.

So anytime we have that muscle contraction, about 80% of the byproduct of that contraction is going to be heat.

- No matter if you're body maxing or bed rotting, or whatever, our bodies are always making and burning ATP.

But when you're exercising, that burn rate goes way up, and so does the amount of heat that you produce.

We're like little locomotives burning fuel to create heat and then turn that into work.

And that is what's happening now.

I mean, the sweat's starting, the activity's up, but it's comfortable, warm but comfortable.

A little dab.

The contraction of my skeletal muscles as I pedal can burn 20 times more ATP than if I was just vegging out on the couch.

And all of that heat is going to be absorbed by my cells and blood, raising my internal body temperature up way past my ideal temperature.

That could get dangerous, unless my body can figure out a way to get rid of that extra heat and cool itself down.

- Your body not only has to fuel this exercise, it also has to cool your body at the same time.

So the blood flow not only goes to your muscles, it now goes to your skin, because skin, sweat, and evaporation of that sweat is the main mechanism we use to dissipate heat.

- [Joe] So right now, nerve cells called thermoreceptors sense that my internal heat is cranking up.

They're shooting a message to my brain's thermometer that tells my body it's time to shed some heat.

I send unconscious signals to my blood vessels telling him to expand and route warm blood toward my skin to start the cooling process.

- Your body is made up of a large portion of water and that water can have it in all these different compartments.

So the water in your blood is what is feeding your sweat to your skin.

- Our sweat is 99% water and 1% other stuff like sodium, potassium, other trace minerals, even stuff like proteins, steroids, hormones, and waste products.

Sweat glands move this watery mix up to the surface of my skin, to basically turn my body into an air conditioner.

When that sweat hits the open air, the water can evaporate.

As water molecules make the jump from liquid to gas, they need a boost to move faster.

And that comes from heat energy that they take with them as they evaporate.

The heat that was in my body gets released into the environment around me, cooling me off in the process.

Oh, amazing.

I could do that all day.

Maybe not all day, but it's comfortable.

I can feel my body cooling off.

It feels great.

But what happens when my body's evaporation cooling system can't work?

What makes this next test dangerous isn't just biking in heat.

It's this number.

The wet bulb temperature.

It's a better way to determine how dangerous extremely hot conditions really are.

Wet bulb temperatures give us a sense of how much evaporation and cooling can happen at a specific temperature and humidity level.

See when humidity is low, evaporative cooling makes the temperature that you actually feel lower than the temperature of the air around you.

Sweating is a pretty amazing trick, but as the humidity goes up, the air is more saturated with water, and less evaporation can happen, so that means less cooling.

It's why 95 degrees in the desert feels a lot less awful than 95 degrees in Austin.

For a long time, scientists believed that 95 degrees Fahrenheit, with 100% humidity, is the temperature at which our internal air conditioning system stops functioning.

It's like our limit.

For years, we assumed that about 6 hours of activity in those conditions was the limit of what humans could survive.

But that changed in 2022.

Scientists found that with enough humidity, our bodies lose the ability to cool down at much lower temperatures.

Just 79 to 88 degrees Fahrenheit on the wet bulb scale.

That is way lower than we originally thought, and it means that on an 80 degree day at 80% humidity, even a young healthy adult could suffer a fatal heat stroke in as little as six hours.

But calculating that danger threshold on paper is one thing.

What does it look like to put my body to the test in the most extreme heat and humidity conditions?

That's what we're going to test next.

This is the before shot.

Look how dry I am, look how comfortable.

Let's see what the hot one feels like.

For this test, things are going to go a lot differently.

The temperature in this room is set to 100 degrees, and we're pumping up the humidity to 60%.

So the wet bulb temperature is at about 99.5, which means my body's heat regulating systems aren't going to work.

That puts us right here, squarely in the danger zone.

Normally, in these conditions, professional recommendation is stay inside.

Don't do this.

But I'm doing it anyway.

But I'll be under careful medical supervision, ready to stop the test if I get too hot.

It doesn't take long before this test starts going differently than the first one.

We're holding a hair under 20 miles an hour.

You can feel the heat instantly.

We're only a couple minutes in.

I'm full on sweating.

There's no turning back now.

My brain is sending the same signals to my blood vessels and sweat glands.

My body's trying to do what it evolved to do, use water to get heat from inside my raging inferno of a body to the outside so that I don't overheat.

This is a strange sensation because the exertion is not that hard.

It's just so hot.

Like, even my palms are hot.

Because of all that humidity, my sweat can't evaporate.

There's sweat coming out of every pore of my body right now.

It's like there's a faucet hooked up to my forehead and none of it is evaporating.

My body isn't cooling off, so my core temperature starts to climb, and my body keeps dumping out more water in a futile effort to cool off.

I'm also losing electrolytes, critical for making sure that your body systems stay in balance, like the electrical signals that keep your heart pumping and your muscles moving.

Oh hey, we're almost 30 minutes in.

Time flies when you're having fun.

I'm drinking water, but at this point, my body's losing water and electrolytes faster than I can absorb them back.

As your blood loses water volume, it thickens up.

That means your body has to work even harder to pump that blood around and get oxygen to your muscles and brain.

How long can I keep this up?

We're officially in the power through it zone of the workout.

I'm definitely getting tired, but 10 more minutes, I can do this.

As my internal body temperature keeps rising, losing the water and electrolytes I need to function, my body could soon start to shut down.

That means headaches, dizziness, cramps, vomiting.

Those symptoms can spread to your brain, including confusion, delirium, loss of consciousness, eventually even death.

But not today.

Oh my god.

My shirt is so heavy.

I think this is it.

This is as much sweat as I can physically make.

The athletic exertion, like how hard I was trying, your body just starts freaking out.

Brain stops working.

I didn't know it was physically possible to sweat this much.

I did a reaction test after each session, cold and hot.

You can see the effects of dehydration and elevated body temperature on my brain function.

I'm slower and less accurate.

I can't talk, this requires every bit of focus I have in my entire brain.

I can feel the exhaustion setting in.

How much longer do I have?

Oh my God.

During the extreme heat test, I was losing sweat at three liters an hour.

Even with what I drank, I lost more than a kilogram of net weight in just water.

That's more than 1% of my body weight.

My heart rate got within a few beats of my absolute max.

And my body temperature was just a few tenths of a degree away from us having to pull the plug to avoid potential damage to my organs and, you know, my big brain.

And it could have been even worse.

Because I'm actually well acclimated to heat.

- If you have a lot of sweat, it's a sign that you're fit, it's also a sign that you're acclimatized or used to working out in the heat, and your body knows how to handle it.

So you want to sweat.

- I live in a hot place, Texas, and I'm really active.

So my body's cooling system knows how to respond already, or at least try to respond.

That means I'm better suited to an experiment like this one than someone who lives in somewhere like Canada or Northern Europe, places where conditions rarely or never reach these extremes.

- When you become acclimatized to a hot environment, your body will adapt and have this ability to better prepare itself for that heat stress.

And thanks to research from scientists like these, we're learning a lot about how we can help people adapt to heat and stay safer.

The good news is our bodies can acclimate to heat surprisingly quickly, if done carefully, as little as 1-2 weeks of acclimation, and our bodies can learn to better react to extreme heat conditions.

- One of the really cool features of heat acclimatization is that your body will retain and it learns to reabsorb those electrolytes, so you're not losing as much when you're acclimatized.

- Heat adaptation leads to some really weird body changes.

The plasma volume of your blood goes up 10 to 25% so that you literally have more fluid available to lose through sweating.

And you start sweating earlier and sweating more.

Your heart rate for the same effort drops, and your core temperature rises more slowly.

These are all things that help you stay cooler longer.

All of this happens because humans have evolved to be one of the best heat-shedding creatures in the entire animal kingdom.

I mean, honestly, can you think of any other animal that's as sweaty and hairless as us?

That's a really weird thing to say.

Anyway, when our ancestors moved from the forests to the Savannah millions of years ago, we needed to be able to survive with less shade, and to travel longer distances to find our food.

And to do that, we put up to 5 million sweat glands all over our skin, more than any other mammal when it comes to sweat gland density.

Other large mammals, like wolves or big cats, you ever notice how they have to pant to cool down or else they overheat?

But humans can sweat up to 12 liters on a hot day.

That's more than 5 times the maximum sweating rate of, say, a chimpanzee.

And speaking of chimps, we evolved another hot skill, being naked.

Well, not right now, but less fur meant more evaporation, and less heat trapped against our skin.

Ancient humans' endurance and hairlessness gave our ancestors the ability to track prey for long distances, basically wearing them out and making them easier to catch.

We're one of nature's ultimate endurance animals.

It took humans about 2 million years to perfect this extraordinary cool-down system that we have, but it's taken us less than 200 years to create a world that's starting to outrun it.

- The awareness has definitely grown in sports medicine.

I think it's also, the conversation has had to grow because of what we're seeing in terms of the climate.

We're seeing a greater number of heatwaves, we're seeing greater duration of heatwaves.

So I think it's all culminating together.

- Dangerous wet bulb temperatures are particularly common near the equator.

It's this perfect storm of humid and hot.

But dangerous heatwaves are becoming more common in places where they didn't used to happen, and where people aren't adapted.

Even in places where people are used to the heat, temperatures are getting hot enough, just like we saw in my test, that our cooling system just can't keep up.

In 2015, extreme heatwaves across Pakistan and India, places where you'd think people are already accustomed to heat killed nearly 5,000 people, as the heat climbed to nearly 120 degrees Fahrenheit.

Around the globe, extreme heat and humidity events have doubled since 1979.

Even if we keep global warming to something like 1.5 degrees Celsius in the near future, nearly 20% of the world's population will be exposed to potentially lethal heat and humidity.

But if we stay on the climate change path that we're on right now, it's estimated that more than 70% of people worldwide could experience severe heatwaves by 2050.

By 2100, cities in the Persian Gulf could regularly exceed the 35 degrees Celsius wet bulb danger threshold.

We're talking feels-like temperatures hitting 165 degrees Fahrenheit.

I quite literally can't imagine that, because those conditions have never happened since humans have existed.

And with more people moving into cities, the risks get even worse.

Urban areas can be up to seven degrees Fahrenheit hotter than less densely built areas.

All that concrete and asphalt absorbs and emits extra heat, creating a heat island.

Today, more than 4 billion people live in urban areas with elevated heat.

Things like air conditioning can help keep people safe, but hundreds of millions of people on Earth still don't have access to that.

This is one of the most shocking parts of all of this.

When it comes to cities, heat risk isn't evenly distributed.

In the 1930s and 40s, the US government graded neighborhoods based on how risky the investment was.

An A meant that the area was a safe investment, and a D meant the area was high risk.

The end result was that areas with more non-white residents were graded lower and systematically denied access to mortgage loans and home ownership.

This process, called redlining, led to widespread segregation.

Today, formerly redlined areas are still suffering many negative effects, including being more vulnerable to extreme heat.

And a big part of that comes down to differences in how much green space and tree cover there is in those areas.

Using satellite imagery, researchers found that redlined neighborhoods have about 20% less tree coverage on average, and in some cases as much as half the number of trees compared to non-redlined areas.

As a result, redlined neighborhoods tend to be significantly hotter still today.

We see this in city after city.

In sum, the temperature difference between an A-rated neighborhood and a redlined D neighborhood, can be as much as 13 degrees Fahrenheit in the same city.

The good news is, this means something as basic as planting more trees could help us survive a hotter future.

Tree cover can lower the temperature of the land surface by 10 to 20 degrees on a summer day, and a row of trees on a city street can lower summer air temperatures by up to 2 degrees Celsius.

One study calculated that urban tree cover in the US saves more than 1000 lives during heat waves every year, and gives us something like $12 billion worth of natural air conditioning.

While humans have evolved to be incredibly adaptable, there are still limits to what our bodies can survive.

But thanks to the work of scientists like Becca, we're figuring out ways to help humans live, work, and play in a hotter world.

For instance, by planning outside activities around the wet bulb temperature, rather than just the regular old air temperature, we can reduce heat-related deaths by a lot.

- Georgia has been a leader when it comes to creating these policies.

They've shown that the data before and after that 79% reduction when you use weather-based modifications is from Georgia.

Georgia used to lead the way in heat stroke deaths for high school athletes.

- We're also figuring out better ways to cool people down when they do get too hot and working on wearable technology that better measures our internal body temperatures.

That way, we can help people before they reach those critical temps.

We can't change the fact that our bodies are not made to live in a hotter world.

But what we can do is use these big brains that we've evolved to help us keep adapting to that changing world.

We can rethink how we build our cities, develop new technologies to help the people who live in them, and push for changes that keep people safer as the climate changes around us.

And if sweating my butt off in a lab helps with that, then I'm all for it.

Stay curious.

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We could not make videos like this one without your help.

You helped send us up to that lab so that, well, you could all watch me suffer.

I hope it was fun.

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I'm on the bike about to die.

Action pack, boom-boom!

Lots of cuts, bam-bam!

Ooh, it's so dangerous.

The best air conditioning I've ever felt in my life.