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The New Kind of Heat Humans Can’t Handle
The New Kind of Heat Humans Can’t Handle
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Phụ đề (533)
0:00
- Right now, my heart is beating
0:02
almost 100 beats per minute faster than normal.
0:04
I'm sweating fast enough
0:05
that I could lose up to a liter of fluid an hour.
0:08
If I stay in this room for too long without cooling down,
0:11
I could die.
0:13
I'm in the middle of an experiment
0:14
to test the human body's ability to deal with extreme heat.
0:17
This room is hot, like 100 degrees Fahrenheit.
0:21
I've been in temps like this before,
0:23
but that's not the worst part.
0:24
The killer is the humidity.
0:27
At this humidity level, my sweat just can't evaporate.
0:29
My body's cooling system just isn't functioning
0:31
like it's supposed to.
0:33
Conditions like these already happen regularly
0:36
for hundreds of millions of people on Earth.
0:38
And according to scientists,
0:39
it's coming for more of us faster than we can even guess.
0:43
I'm stepping into this heat research lab to show you
0:46
what happens to our bodies when we experience extreme heat,
0:50
how we built our world in ways
0:52
that doomed some people to deadly heat
0:55
and how we can fix it,
0:56
to how the one number we've been using
0:58
to tell us how hot was too hot is wrong.
1:02
And I'm already regretting it.
1:04
(bright music)
1:09
Hey smart people, Joe here.
1:11
The world is hotter than it used to be.
1:15
That's just a fact.
1:16
Average global temperatures
1:18
are about 1.2 degrees Celsius hotter over the past 50 years.
1:22
That might not sound like much,
1:24
but one in four heatwaves since the year 2000
1:27
would have been impossible without climate change.
1:30
More humans are experiencing extreme conditions
1:34
more often and for longer.
1:36
So just how dangerous is extreme heat?
1:40
If you look up data on how many people die
1:42
from extreme temperatures,
1:44
you actually find something curious.
1:48
Cold-related deaths outnumber deaths from heat
1:50
by a pretty good margin.
1:52
Or so it seems.
1:54
This data obscures something really crucial.
1:57
Most deaths that we attribute to temperature happen
2:00
in this "moderate cold" zone.
2:02
And that's for a simple reason.
2:04
It isn't that moderate cold
2:06
is secretly dangerous or something.
2:08
It's just those are the conditions people spend
2:10
the most time in.
2:12
The truth is, a person's risk of death
2:14
or injury goes up way faster in hotter temperatures
2:18
compared to cold.
2:19
In the US, heat and heatwaves are the leading cause
2:23
of weather-related death.
2:24
They've killed more people over the past 30 years
2:27
than hurricanes and tornadoes combined.
2:30
And in some places,
2:31
heatwaves are becoming literally unsurvivable.
2:34
At certain temperatures, the human body gets so hot
2:37
that it shuts down.
2:38
Then it's game over.
2:40
But what is it about heat
2:42
that can be so uniquely dangerous for our bodies?
2:46
And what can we do to make it safer
2:49
to live in a hotter world?
2:51
In 2001, All-Pro offensive lineman Corey Stringer
2:54
was working out in full pads
2:56
when the heat index hit 110 degrees.
2:58
He began to feel sick,
3:00
but by the time that trainers realized what was happening,
3:02
his body temperature had reached 108 degrees,
3:05
and he later died.
3:07
Corey's survivors established this research lab,
3:10
dedicated to preventing sudden death from heat.
3:13
And that's where I came to sweat it out
3:15
and learn from researchers like Becca
3:17
what happens to our bodies in extreme conditions.
3:21
- We were founded with the mission and goal
3:23
to not only enhance performance and health and safety
3:25
and prevent catastrophic injuries for athletes,
3:28
but also for soldiers and war fighters and laborers.
3:31
Heat is one of the top threats to all of those individuals.
3:35
- Now all of us spend at least part of our lives living,
3:38
working, or playing outside.
3:40
And as we crank up the world's heat,
3:42
those things become more dangerous.
3:45
- I think the surprising thing
3:46
that always catches people off guard
3:47
is that when you look at the top causes
3:49
of sudden death in sport, heat is in the top four.
3:52
And those four top causes, just four alone,
3:55
make up 90% of the deaths that we see.
3:57
- Extreme heat has extreme consequences on our bodies.
4:01
To figure out what those consequences are,
4:03
I've gotta get on a bike.
4:06
This bike will be my home for the day.
4:09
First we need to set a baseline, so we've set the room
4:11
to conditions you'd feel on a nice, comfortable spring day.
4:14
It's like perfect conditions.
4:16
I could do this for hours.
4:17
Easy peasy.
4:19
But let's take a look at what's going on in my body
4:21
when I start to get active.
4:23
When I start contracting my muscles to spin the pedals,
4:26
my cells start burning more energy.
4:28
Every cell in our bodies runs on a molecule called ATP.
4:31
It's the main energy source that keeps our muscles moving,
4:34
our brains working, and our hearts beating.
4:37
Your cells have a powerhouse in them
4:39
called the mitochondria.
4:40
They're like little factories that turn glucose
4:43
and oxygen into ATP, charging up our cells' batteries.
4:47
But to convert that energy into movement,
4:50
I have to chemically rip that ATP apart.
4:52
- We're actually not really that efficient
4:54
in terms of how we use our energy.
4:56
So anytime we have that muscle contraction,
4:59
about 80% of the byproduct of that contraction
5:01
is going to be heat.
5:03
- No matter if you're body maxing or bed rotting,
5:06
or whatever, our bodies are always making and burning ATP.
5:10
But when you're exercising, that burn rate goes way up,
5:14
and so does the amount of heat that you produce.
5:17
We're like little locomotives burning fuel
5:20
to create heat and then turn that into work.
5:23
And that is what's happening now.
5:25
I mean, the sweat's starting, the activity's up,
5:27
but it's comfortable, warm but comfortable.
5:32
A little dab.
5:33
The contraction of my skeletal muscles as I pedal
5:35
can burn 20 times more ATP
5:37
than if I was just vegging out on the couch.
5:39
And all of that heat is going to be absorbed
5:42
by my cells and blood,
5:44
raising my internal body temperature up
5:46
way past my ideal temperature.
5:48
That could get dangerous,
5:50
unless my body can figure out a way
5:51
to get rid of that extra heat and cool itself down.
5:54
- Your body not only has to fuel this exercise,
5:57
it also has to cool your body at the same time.
5:59
So the blood flow not only goes to your muscles,
6:01
it now goes to your skin,
6:03
because skin, sweat, and evaporation of that sweat
6:05
is the main mechanism we use to dissipate heat.
6:09
- [Joe] So right now,
6:10
nerve cells called thermoreceptors sense
6:12
that my internal heat is cranking up.
6:14
They're shooting a message to my brain's thermometer
6:17
that tells my body it's time to shed some heat.
6:20
I send unconscious signals to my blood vessels
6:23
telling him to expand and route warm blood toward my skin
6:26
to start the cooling process.
6:28
- Your body is made up of a large portion of water
6:31
and that water can have it
6:32
in all these different compartments.
6:33
So the water in your blood
6:35
is what is feeding your sweat to your skin.
6:39
- Our sweat is 99% water and 1% other stuff
6:43
like sodium, potassium, other trace minerals,
6:46
even stuff like proteins, steroids,
6:47
hormones, and waste products.
6:49
Sweat glands move this watery mix up to the surface
6:53
of my skin, to basically turn my body
6:55
into an air conditioner.
6:57
When that sweat hits the open air, the water can evaporate.
7:00
As water molecules make the jump from liquid to gas,
7:03
they need a boost to move faster.
7:06
And that comes from heat energy
7:07
that they take with them as they evaporate.
7:10
The heat that was in my body gets released
7:12
into the environment around me,
7:14
cooling me off in the process.
7:16
Oh, amazing.
7:19
I could do that all day.
7:20
Maybe not all day, but it's comfortable.
7:22
I can feel my body cooling off.
7:24
It feels great.
7:25
But what happens
7:27
when my body's evaporation cooling system can't work?
7:30
What makes this next test dangerous
7:31
isn't just biking in heat.
7:33
It's this number.
7:35
The wet bulb temperature.
7:37
It's a better way to determine
7:38
how dangerous extremely hot conditions really are.
7:42
Wet bulb temperatures give us a sense
7:43
of how much evaporation and cooling can happen
7:46
at a specific temperature and humidity level.
7:49
See when humidity is low, evaporative cooling makes
7:51
the temperature that you actually feel lower
7:54
than the temperature of the air around you.
7:56
Sweating is a pretty amazing trick,
7:58
but as the humidity goes up, the air is more saturated
8:01
with water, and less evaporation can happen,
8:03
so that means less cooling.
8:05
It's why 95 degrees in the desert feels a lot less awful
8:09
than 95 degrees in Austin.
8:11
For a long time, scientists believed
8:13
that 95 degrees Fahrenheit, with 100% humidity,
8:16
is the temperature at which
8:18
our internal air conditioning system stops functioning.
8:22
It's like our limit.
8:24
For years, we assumed that about 6 hours of activity
8:26
in those conditions was the limit
8:28
of what humans could survive.
8:30
But that changed in 2022.
8:32
Scientists found that with enough humidity,
8:35
our bodies lose the ability to cool down
8:37
at much lower temperatures.
8:40
Just 79 to 88 degrees Fahrenheit on the wet bulb scale.
8:44
That is way lower than we originally thought,
8:47
and it means that on an 80 degree day at 80% humidity,
8:50
even a young healthy adult could suffer a fatal heat stroke
8:54
in as little as six hours.
8:56
But calculating that danger threshold on paper is one thing.
8:58
What does it look like to put my body to the test
9:01
in the most extreme heat and humidity conditions?
9:05
That's what we're going to test next.
9:07
This is the before shot.
9:09
Look how dry I am, look how comfortable.
9:11
Let's see what the hot one feels like.
9:13
For this test, things are going to go a lot differently.
9:16
The temperature in this room is set to 100 degrees,
9:19
and we're pumping up the humidity to 60%.
9:21
So the wet bulb temperature is at about 99.5,
9:25
which means my body's heat regulating systems
9:28
aren't going to work.
9:29
That puts us right here, squarely in the danger zone.
9:33
Normally, in these conditions,
9:35
professional recommendation is stay inside.
9:38
Don't do this.
9:39
But I'm doing it anyway.
9:41
But I'll be under careful medical supervision,
9:43
ready to stop the test if I get too hot.
9:46
It doesn't take long
9:47
before this test starts going differently
9:49
than the first one.
9:51
We're holding a hair under 20 miles an hour.
9:54
You can feel the heat instantly.
9:56
We're only a couple minutes in.
9:57
I'm full on sweating.
9:59
There's no turning back now.
10:01
My brain is sending the same signals to my blood vessels
10:03
and sweat glands.
10:05
My body's trying to do what it evolved to do, use water
10:08
to get heat from inside my raging inferno of a body
10:12
to the outside so that I don't overheat.
10:15
This is a strange sensation
10:16
because the exertion is not that hard.
10:18
It's just so hot.
10:20
Like, even my palms are hot.
10:22
Because of all that humidity, my sweat can't evaporate.
10:26
There's sweat coming out of every pore of my body right now.
10:29
It's like there's a faucet hooked up to my forehead
10:32
and none of it is evaporating.
10:34
My body isn't cooling off,
10:36
so my core temperature starts to climb,
10:39
and my body keeps dumping out more water
10:41
in a futile effort to cool off.
10:44
I'm also losing electrolytes, critical for making sure
10:47
that your body systems stay in balance,
10:49
like the electrical signals that keep your heart pumping
10:52
and your muscles moving.
10:54
Oh hey, we're almost 30 minutes in.
10:55
Time flies when you're having fun.
10:57
I'm drinking water,
10:59
but at this point, my body's losing water
11:01
and electrolytes faster than I can absorb them back.
11:04
As your blood loses water volume, it thickens up.
11:08
That means your body has to work even harder
11:09
to pump that blood around and get oxygen
11:12
to your muscles and brain.
11:14
How long can I keep this up?
11:17
We're officially in the power through it zone
11:19
of the workout.
11:21
I'm definitely getting tired, but 10 more minutes,
11:23
I can do this.
11:25
As my internal body temperature keeps rising,
11:27
losing the water and electrolytes I need to function,
11:30
my body could soon start to shut down.
11:33
That means headaches, dizziness, cramps, vomiting.
11:37
Those symptoms can spread to your brain,
11:39
including confusion, delirium, loss of consciousness,
11:43
eventually even death.
11:45
But not today.
11:47
Oh my god.
11:48
My shirt is so heavy.
11:49
I think this is it.
11:51
This is as much sweat as I can physically make.
11:53
The athletic exertion, like how hard I was trying,
11:57
your body just starts freaking out.
11:58
Brain stops working.
12:00
I didn't know it was physically possible to sweat this much.
12:04
I did a reaction test after each session, cold and hot.
12:07
You can see the effects of dehydration
12:10
and elevated body temperature on my brain function.
12:13
I'm slower and less accurate.
12:16
I can't talk, this requires every bit of focus
12:18
I have in my entire brain.
12:19
I can feel the exhaustion setting in.
12:22
How much longer do I have?
12:23
Oh my God.
12:24
During the extreme heat test, I was losing sweat
12:27
at three liters an hour.
12:29
Even with what I drank, I lost more than a kilogram
12:32
of net weight in just water.
12:35
That's more than 1% of my body weight.
12:37
My heart rate got within a few beats of my absolute max.
12:42
And my body temperature
12:43
was just a few tenths of a degree away
12:45
from us having to pull the plug to avoid potential damage
12:48
to my organs and, you know, my big brain.
12:51
And it could have been even worse.
12:53
Because I'm actually well acclimated to heat.
12:57
- If you have a lot of sweat, it's a sign that you're fit,
12:59
it's also a sign that you're acclimatized
13:01
or used to working out in the heat,
13:03
and your body knows how to handle it.
13:04
So you want to sweat.
13:07
- I live in a hot place, Texas, and I'm really active.
13:11
So my body's cooling system knows how to respond already,
13:15
or at least try to respond.
13:17
That means I'm better suited to an experiment like this one
13:19
than someone who lives in somewhere
13:21
like Canada or Northern Europe,
13:23
places where conditions rarely
13:25
or never reach these extremes.
13:28
- When you become acclimatized to a hot environment,
13:30
your body will adapt and have this ability
13:32
to better prepare itself for that heat stress.
13:35
And thanks to research from scientists like these,
13:37
we're learning a lot about how we can help people adapt
13:41
to heat and stay safer.
13:43
The good news is our bodies can acclimate
13:45
to heat surprisingly quickly, if done carefully,
13:48
as little as 1-2 weeks of acclimation,
13:51
and our bodies can learn to better react
13:53
to extreme heat conditions.
13:55
- One of the really cool features of heat acclimatization
13:58
is that your body will retain and it learns
14:00
to reabsorb those electrolytes,
14:02
so you're not losing as much when you're acclimatized.
14:05
- Heat adaptation leads to some really weird body changes.
14:09
The plasma volume of your blood goes up 10 to 25%
14:13
so that you literally have more fluid available
14:16
to lose through sweating.
14:18
And you start sweating earlier and sweating more.
14:20
Your heart rate for the same effort drops,
14:23
and your core temperature rises more slowly.
14:26
These are all things that help you stay cooler longer.
14:29
All of this happens because humans have evolved
14:32
to be one of the best heat-shedding creatures
14:35
in the entire animal kingdom.
14:37
I mean, honestly, can you think of any other animal
14:39
that's as sweaty and hairless as us?
14:43
That's a really weird thing to say.
14:44
Anyway, when our ancestors moved from the forests
14:47
to the Savannah millions of years ago,
14:49
we needed to be able to survive with less shade,
14:52
and to travel longer distances to find our food.
14:55
And to do that, we put up to 5 million sweat glands
14:59
all over our skin, more than any other mammal
15:02
when it comes to sweat gland density.
15:04
Other large mammals, like wolves or big cats,
15:07
you ever notice how they have to pant
15:09
to cool down or else they overheat?
15:11
But humans can sweat up to 12 liters on a hot day.
15:15
That's more than 5 times the maximum sweating rate
15:18
of, say, a chimpanzee.
15:20
And speaking of chimps, we evolved another hot skill,
15:23
being naked.
15:24
Well, not right now,
15:25
but less fur meant more evaporation,
15:29
and less heat trapped against our skin.
15:31
Ancient humans' endurance and hairlessness
15:34
gave our ancestors the ability to track prey
15:37
for long distances, basically wearing them out
15:40
and making them easier to catch.
15:42
We're one of nature's ultimate endurance animals.
15:45
It took humans about 2 million years
15:47
to perfect this extraordinary cool-down system
15:50
that we have, but it's taken us less than 200 years
15:54
to create a world that's starting to outrun it.
15:57
- The awareness has definitely grown in sports medicine.
16:00
I think it's also, the conversation has had to grow
16:03
because of what we're seeing in terms of the climate.
16:05
We're seeing a greater number of heatwaves,
16:07
we're seeing greater duration of heatwaves.
16:08
So I think it's all culminating together.
16:11
- Dangerous wet bulb temperatures
16:13
are particularly common near the equator.
16:15
It's this perfect storm of humid and hot.
16:18
But dangerous heatwaves are becoming more common
16:21
in places where they didn't used to happen,
16:23
and where people aren't adapted.
16:25
Even in places where people are used to the heat,
16:27
temperatures are getting hot enough,
16:29
just like we saw in my test,
16:31
that our cooling system just can't keep up.
16:33
In 2015, extreme heatwaves across Pakistan and India,
16:37
places where you'd think people are already accustomed
16:39
to heat killed nearly 5,000 people,
16:42
as the heat climbed to nearly 120 degrees Fahrenheit.
16:46
Around the globe, extreme heat and humidity events
16:49
have doubled since 1979.
16:51
Even if we keep global warming to something
16:54
like 1.5 degrees Celsius in the near future,
16:57
nearly 20% of the world's population will be exposed
17:01
to potentially lethal heat and humidity.
17:04
But if we stay on the climate change path
17:05
that we're on right now,
17:07
it's estimated that more than 70% of people worldwide
17:11
could experience severe heatwaves by 2050.
17:14
By 2100, cities in the Persian Gulf could regularly exceed
17:18
the 35 degrees Celsius wet bulb danger threshold.
17:22
We're talking feels-like temperatures
17:24
hitting 165 degrees Fahrenheit.
17:28
I quite literally can't imagine that,
17:31
because those conditions
17:32
have never happened since humans have existed.
17:36
And with more people moving into cities,
17:38
the risks get even worse.
17:41
Urban areas can be up to seven degrees Fahrenheit hotter
17:44
than less densely built areas.
17:46
All that concrete and asphalt absorbs and emits extra heat,
17:51
creating a heat island.
17:53
Today, more than 4 billion people live in urban areas
17:56
with elevated heat.
17:57
Things like air conditioning can help keep people safe,
18:00
but hundreds of millions of people
18:01
on Earth still don't have access to that.
18:04
This is one of the most shocking parts of all of this.
18:07
When it comes to cities, heat risk isn't evenly distributed.
18:14
In the 1930s and 40s, the US government graded neighborhoods
18:16
based on how risky the investment was.
18:18
An A meant that the area was a safe investment,
18:21
and a D meant the area was high risk.
18:23
The end result was that areas
18:25
with more non-white residents were graded lower
18:27
and systematically denied access to mortgage loans
18:31
and home ownership.
18:32
This process, called redlining,
18:34
led to widespread segregation.
18:36
Today, formerly redlined areas are still suffering
18:39
many negative effects, including being more vulnerable
18:43
to extreme heat.
18:44
And a big part of that comes down to differences
18:47
in how much green space
18:49
and tree cover there is in those areas.
18:52
Using satellite imagery, researchers found
18:54
that redlined neighborhoods
18:55
have about 20% less tree coverage on average,
18:58
and in some cases as much as half the number of trees
19:01
compared to non-redlined areas.
19:04
As a result, redlined neighborhoods tend
19:06
to be significantly hotter still today.
19:08
We see this in city after city.
19:11
In sum, the temperature difference
19:13
between an A-rated neighborhood
19:14
and a redlined D neighborhood,
19:16
can be as much as 13 degrees Fahrenheit in the same city.
19:20
The good news is, this means something as basic
19:23
as planting more trees could help us survive
19:27
a hotter future.
19:29
Tree cover can lower the temperature of the land surface
19:32
by 10 to 20 degrees on a summer day,
19:35
and a row of trees on a city street
19:37
can lower summer air temperatures
19:38
by up to 2 degrees Celsius.
19:41
One study calculated that urban tree cover in the US
19:43
saves more than 1000 lives during heat waves every year,
19:47
and gives us something like $12 billion worth
19:51
of natural air conditioning.
19:52
While humans have evolved to be incredibly adaptable,
19:56
there are still limits to what our bodies can survive.
19:59
But thanks to the work of scientists like Becca,
20:01
we're figuring out ways to help humans live, work,
20:04
and play in a hotter world.
20:06
For instance, by planning outside activities around
20:09
the wet bulb temperature,
20:10
rather than just the regular old air temperature,
20:12
we can reduce heat-related deaths by a lot.
20:15
- Georgia has been a leader when it comes
20:17
to creating these policies.
20:19
They've shown that the data before and after
20:20
that 79% reduction when you use weather-based modifications
20:23
is from Georgia.
20:25
Georgia used to lead the way in heat stroke deaths
20:27
for high school athletes.
20:29
- We're also figuring out better ways to cool people down
20:31
when they do get too hot and working on wearable technology
20:34
that better measures our internal body temperatures.
20:37
That way, we can help people
20:39
before they reach those critical temps.
20:41
We can't change the fact
20:43
that our bodies are not made to live in a hotter world.
20:46
But what we can do is use these big brains
20:48
that we've evolved to help us keep adapting
20:51
to that changing world.
20:53
We can rethink how we build our cities,
20:55
develop new technologies
20:56
to help the people who live in them,
20:58
and push for changes that keep people safer
21:01
as the climate changes around us.
21:04
And if sweating my butt off in a lab helps with that,
21:07
then I'm all for it.
21:09
Stay curious.
21:10
Thank you so much for watching this video,
21:12
and an extra special thank you to everyone
21:14
who supports our show on Patreon,
21:16
including these fine folks at our top tier.
21:20
We could not make videos like this one without your help.
21:24
You helped send us up to that lab so that, well,
21:27
you could all watch me suffer.
21:29
I hope it was fun.
21:30
Videos like this one take a ton of effort and research,
21:33
and we think the world is a better place
21:35
with more content like this, for curious people,
21:38
developed by curious minds.
21:39
So if you'd like to help make more of this,
21:42
check out the link down in the description,
21:43
become a member of our Patreon,
21:45
and we'll see you in the next video.
21:47
I'm on the bike about to die.
21:49
Action pack, boom-boom!
21:50
Lots of cuts, bam-bam!
21:52
Ooh, it's so dangerous.
21:53
The best air conditioning I've ever felt in my life.
