Speaker A
It is hot right now. Genuinely, historically, record-breakingly hot. And if you are sitting in a room somewhere watching this and it's uncomfortable, you probably have a fan running. Maybe you even have two.
Explains how fans can be deadly in extreme heat and humidity due to wet-bulb temperature limits on human cooling.
Ask about this video. Answers come from its transcript only — with the timestamp, so you can check them.
Generated from the transcript and can be wrong — check the timestamp.
Key Takeaways
- Fans help cool only when evaporation is effective; in extreme heat and humidity, they can be harmful.
- Wet-bulb temperature is a critical measure of heat stress and human survivability.
- 35°C wet-bulb temperature marks the physiological limit for human cooling under ideal conditions.
- Extreme wet-bulb temperatures are already occurring and will increase with climate change.
- Populations exposed to outdoor heat and humidity are at greatest risk of heat-related illness and death.
What the video covers
- Fans cool by accelerating evaporation of sweat, which removes heat from the body.
- In high humidity, evaporation slows down, making sweating ineffective for cooling.
- Wet-bulb temperature measures how well evaporation can cool the body under current conditions.
- A wet-bulb temperature of 35°C is the absolute limit where the human body can no longer cool itself.
- Above this threshold, fans can worsen heat stress by blowing hot, humid air onto the skin.
- Extreme heat and humidity events near this threshold have already occurred in parts of the world.
- Vulnerable populations include outdoor workers, elderly, and those without air conditioning.
- The 35°C wet-bulb limit is optimistic; dangerous heat stress can begin at lower wet-bulb temperatures.
- Climate change is increasing the frequency and severity of these dangerous heat conditions.
- Understanding wet-bulb temperature is crucial for recognizing the limits of human survivability in heat.
Chapters
- 00:00Introduction to extreme heat and fans
- 00:32What is wet-bulb temperature?
- 01:04How the body cools itself
- 01:39Evaporation and humidity effects
- 02:21Measuring wet-bulb temperature
- 03:08Wet-bulb temperature threshold for survival
- 04:22Why fans can be deadly in extreme heat
- 05:11Real-world occurrences and climate implications
- 07:18Vulnerable populations and risks
- 08:43Summary and final warnings about fans
Full Transcript — Download SRT & Markdown
Speaker A
But here's something that nobody tells you. Under the right conditions, that fan is making things worse.
Speaker A
Not a little worse, potentially fatally worse. Blowing hot air across your skin when the temperature crosses a certain threshold doesn't cool you down. It speeds up the process, and that'll kill you.
Speaker A
And right now, in parts of this planet, those conditions already exist. This is the story of wet-bulb temperature, a number most people have never heard of.
Speaker A
The threshold the human body cannot negotiate with, and a climate reality that is arriving faster than almost anyone expected. But to understand why, you first need to understand what your body is actually doing when it's hot.
Speaker A
And to understand why a fan can kill you, you first need to understand how your body cools itself.
Speaker A
Because most people think the answer is sweating. It's not. Not exactly. Sweating is just the delivery system.
Speaker A
The actual cooling mechanism is evaporation. When sweat sits on your skin and evaporates, turning liquid into vapor, it carries away energy with it.
Speaker A
Thermal energy, heat. That's the process your body is depending on every single second when you're hot. Without evaporation, sweat is just wet. It does nothing. This is why humid days feel so much worse than dry ones at the same
Speaker A
temperature. In dry air, sweat evaporates quickly. You might not even notice you're sweating because it's gone before it builds up. In humid air, on the other hand, the atmosphere is already packed with moisture, and evaporation slows right down. You feel
Speaker A
drenched. You feel like the heat is suffocating you because in a very real sense, your cooling system is being limited. Your body produces heat constantly, just from being alive, from your heart beating to your cells burning fuel. At rest, you're generating roughly
Speaker A
80 watts of heat. Do anything physical, and that number climbs fast. Exercise, and you can hit 6, 7, 800 watts. And all of that heat has to go somewhere.
Speaker A
Evaporation is the primary exit route. And when that exit route closes, something has to give. And scientists found a way to measure exactly when that system starts to fail. It's called wet bulb temperature. Most people have never even heard of it. Meteorologists have
Speaker A
been measuring this problem for over 200 years using something called a wet bulb thermometer. It's almost comically simple. You take a regular thermometer, wrap its tip in a damp cloth, and expose it to the air. As water evaporates from
Speaker A
the cloth, it cools down the thermometer. The reading you get, the wet bulb temperature, tells you exactly how efficiently evaporation is working in that moment. In dry air, the wet bulb temperature is much lower than the actual air temperature, because the
Speaker A
water evaporates easily, and it cools the thermometer significantly. In humid air, that gap closes. The wet bulb temperature creeps toward the actual air temperature, and if the air becomes fully saturated, 100% humidity, there's no evaporation at all. The wet bulb
Speaker A
temperature and the air temperature become identical. And this number, the wet bulb temperature, is arguably one of the most important climate metrics that most people have never even heard of.
Speaker A
Because it doesn't just tell you how hot it is, it tells you how hot your body experiences the heat. It tells you whether your cooling system can function or not.
Speaker A
And there is a specific wet bulb temperature beyond which the human body simply cannot cope. So, what is that number? And what does it actually mean for your body?
Speaker A
35°C wet bulb. That is the threshold. Above it, the physics stop working in your favor. Your body temperature is approximately 37°.
Speaker A
For heat to leave your body into the surrounding air, the surrounding air needs to be able to accept it. At wet bulb temperature of 35°, the air is so hot and so saturated with moisture that evaporation from your skin becomes
Speaker A
essentially zero. Your body keeps generating heat. It keeps trying to sweat, but there is nowhere for the heat or the sweat to go. Core temperatures begin to rise. And unlike a fever, where the body deliberately rises the temperature of the body to fight
Speaker A
infection, that rise is uncontrolled. Heatstroke, organ failure, and eventually death. And this is where the fan comes in. At moderate temperatures, a fan helps by moving air across your skin and accelerating evaporation. That's the whole point of a fan. But when the air
Speaker A
temperature rises above your skin temperature, roughly 35°, a fan stops helping with evaporation and starts doing something else. It's now blowing air that is hotter than you onto your skin. You are essentially absorbing heat from the moving air. The fan is now a
Speaker A
convection heater aimed at your body. Add high humidity to that, close to the wet bulb threshold, and you have a worst-case scenario. Evaporation is already failing. The fan cannot restart it, but it can increase the rate at which hot air transfers its heat to you.
Speaker A
Under those conditions, the fan does not save you. It accelerates the process that will kill you.
Speaker A
This is not a theoretical concern. This is documented. Studies have shown that in elderly people whose cardiovascular systems struggle to compensate, a fan in extreme heat and humid conditions measurably increase core temperatures, rather than reducing it. Now, here is
Speaker A
the part that should genuinely alarm you. Scientists studying historical climate data believed, until fairly recently, that a sustained wet-bulb temperature of 35° was essentially unprecedented in the modern era.
Speaker A
Something to worry about in the future. A threshold humanity had not yet crossed. They were wrong.
Speaker A
A 2020 study published in Science Advances analyzed decades of weather station data and had found that the wet-bulb temperature of 35° or at least very close to it had already occurred in South Asia, in the Persian Gulf, in
Speaker A
parts of the American Midwest, and in the Gulf Coast. These events were short, hours not days, but they happened. And they happened more frequently than anyone had predicted. The Persian Gulf city of Bandar Mashhar in Iran recorded a wet-bulb temperature of approximately
Speaker A
34.6° in 2015. Combined with an air temperature of 46°. Conditions that sit at the absolute edge of human survivability. People live there. People were outside. And each year, the frequency of these extreme heat and humidity events increases.
Speaker A
Parts of South Asia, home to billions of people, many of whom work outdoors, many of whom have no access to air conditioning, are projected to experience regular sustained wet-bulb temperatures that cross the survival threshold within decades, not centuries.
Speaker A
Decades. This isn't a distant problem. It's an arriving one. But here's the catch. 35° wet-bulb is actually the optimistic number. Now, here's the thing about the 35° wet-bulb figure. It represents the absolute physiological limit for a young, healthy
Speaker A
adult at rest in a laboratory under ideal conditions. Real life isn't that. Studies suggest that for most people, accounting for age, health, physical effort, and whether they have shade or shelter, dangerous heat stress begins at wet-bulb temperature closer to 28°.
Speaker A
For elderly people, that number drops even further. For someone doing manual labor outdoors, the physical effort alone means their body is generating far more heat to shed.
Speaker A
So, the threshold where they start losing the battle arrives much sooner. This matters enormously when you look at who is most at risk. It is not people sitting in air-conditioned offices watching the thermometer. It is agricultural workers in India and
Speaker A
Pakistan. It is construction workers in the Gulf. It is elderly people in European apartments during a surprise heat wave, which is exactly what killed over 70,000 people across Europe in 2003.
Speaker A
The number 35° wet-bulb is technically where survival b
Speaker A
So, back to the fan. The reason that a fan can make you die faster isn't because fans are dangerous. It's because the conditions in which fans stop working and start harming are conditions that now exist on this planet.
Speaker A
Conditions that are becoming more and more common, and conditions that most people have no framework for understanding because almost nobody talks about wet-bulb temperature. We talk about air temperature. We talk about heat index, feel like temperatures, and warnings to stay
Speaker A
hydrated. But the actual physical limit, the number that tells you whether your body's cooling system can function at all, rarely makes the news. 35° wet-bulb, that's the wall.
Speaker A
Below it, your body can fight. Above it, no amount of water, no amount of shade, no fan, nothing changes the outcome. The physics have closed the door. And the honest, uncomfortable truth is this.
Speaker A
Parts of the world already touching that wall. More parts will reach it and billions of people living in those regions have no way to get behind the only thing that reliable works above the threshold.
Speaker A
Air conditioning, which for billions of people in parts of the earth already crossing this threshold isn't an option.
Speaker A
Next time it's hot, and right now it is very hot, and you reach for that fan, now you know what it can and cannot do and why sometimes the thing that you reach for to save yourself is the thing
Speaker A
that makes it worse.
Topics:wet-bulb temperatureheat stressfan safetyevaporation coolingheatwaveclimate changeheatstrokehumidityhuman survivabilityextreme heat
