If it's 37°C outside and our internal body temperature is 37°C,
why are we hot?
Why do fans help us feel cooler?
This may sound obvious, but it's not. How does sweating help us
cool down, and I mean how does the fact that our body produces droplets of water
allow us to cool down? During the heat wave, you've sent me a lot of
heat-related questions you wanted me to address in a video.
Honestly I thought that the answers were obvious
until I looked into it and actually learned a few things.
I hope you will too. Here are the answers to three of your questions.
This video is
sponsored by NordVPN, more information about this at the end of the video.
Why do we get hot when it's 37°C outside when that's the human body's temperature?
Our internal temperature ranges between 36.1 and 37.8°C.
But that doesn't mean that the air should be at this temperature for us to feel comfortable.
In fact, for humans to be neither hot nor cold but just right, it is necessary for the difference between the heat
that we generate to live (linked to muscular activity, to digestion, etc)
and the heat we lose to our environment,
to be at 37°C. And it turns out that we generate so much heat that according to this article,
this balance is struck when the outside temperature
is around 21°C, not 37. Take this number with a grain of salt,
as each person produces more or less heat. Women especially
tend to be more comfortable in an environment slightly above 21°C.
The question of an optimal temperature
isn't simple; that will be the subject of a future video.
Temperature is often above 21°C in the summertime, which means that we produce too much heat compared to what we
release in the outside air; we risk overheating.
Ideally, we would produce as little heat as possible, hence the existence
of naps which allow us to remain at a low effort level,
or we could evacuate the extra heat.
And for this purpose, we have 4 four main tactics: convection, conduction, radiation and perspiration.
I won't go into all of them, but here's the first one.
Why does a fan makes us feel cool?
That's the question that seems simple but ultimately isn't.
The question is well formulated because
technically
a fan doesn't help cool down a room, it warms it up.
It's an electrical device that makes a current flow and releases heat.
So if you're not in the room
turn off the fan, it's only going to heat up the air.
On the other hand if you receive air from a fan, then you can have a perception of freshness thanks to
what you probably already know, convection. The air that's in contact with your skin gets
heated and moist from your sweat
and so your heat escapes from your body which cools you down, great.
But only to a certain extent. If the air is static, heat exchange
stops when it reaches equilibrium.
When there is a gust of wind or air from a fan, this hot and humid layer
is swept away and replaced by a new layer of air,
to which you once again
will transfer some of your body heat.
That's how you cool down with a fan,
by heating and humidifying air that is constantly getting renewed.
Except that if the air is extremely hot, warmer than your skin,
this effect is completely reversed.
If you keep sending hot air towards your skin, you'll be receiving air that's
warmer than your skin and heat exchange will work the wrong way around, with your body heating up.
The World Health Organization clearly distinguishes
the use of a fan when ambient temperature is below or above the temperature of the skin (about 35°C).
Above 35°C,
they recommend not to send air directly to your skin
and to drink plenty of water. Why drink a lot of water? Because by chasing air away
we increase the evaporation of our perspiration, so we need to
compensate by drinking a little more.
Yes, but several other research articles have appeared in recent years, which are in disagreement
with the recommendations of the WHO. And a 2012 systematic review
called Cochrane Review concluded that... they didn't know where they stood.
Although there is still no consensus on the use of a fan above 35°C,
which seems a little crazy,
some studies seem to agree on the matter of older people.
During heat waves and extreme humidity, the use of a fan
increases physiological stress in the elderly and not in young adults.
The reason for this being that the heat regulation capabilities of the elderly
are less good, they sweat less and have less ability to adjust the diameter of their blood vessels at the extremities
which is one of the physiological adjustments
that allows us to increase our heat loss.
So, when the air temperature exceeds 35°C, the elderly should
follow the WHO recommendations. But if you are not an elderly person,
what should you do when it's over 35°C? Well honestly, I don't know.
So I propose to work around the problem by using another way to lower your temperature even more efficiently,
much more efficiently than using a fan;
drink drink drink drink drink--water of course.
Why drink? Because it allows us to enjoy another cooling mechanism,
evaporation. How does perspiration help to evacuate body temperature?
Sweating is our most efficient cooling system is by far.
That's why you're told to drink non-stop during a heat wave.
You can lose up to two liters of water per hour, so you have to compensate that loss by drinking.
But how does sweating
reduce our internal heat?
Think about it for two seconds because it's really not so obvious.
It isn't perspiration itself that allows the body to cool down,
but evaporation.
If you wipe your sweat, if it runs to the ground or if it ends up in your clothes
these drops of sweat probably didn't contribute to cooling you down.
Only the evaporation of your sweat from the skin, meaning its
steam liquid transformation, allows you to lower your temperature.
So in the summertime, you should dress lightly to increase your surface
of evaporation, which most of you probably already do. Sweating from your pores
isn't what matters, it's the evaporation that counts because evaporation uses energy.
Energy that comes from our internal heat.
Our skin and the sweat drop are both at the same temperature
and you know what temperature really is, it reflects the thermal agitation of the molecules.
The more they jiggle,
the hotter it is. When it's hot, let's say the molecules are twerking
and when it's cold, they're doing Tai Chi.
And in the middle of the heat wave the molecules twerk
and breakdance all at once. In their liquid state, these molecules of water, since sweat is mostly water,
are all linked to each other.
They twerk while holding hands, if you will.
If they weren't linked, if they didn't 'hold hands', then these free molecules would be vapor.
This is what distinguishes a liquid from vapor. With thermal agitation,
some molecules will get so agitated that they'll break the bonds that hold them to the rest of the group
and then become steam. But in order to escape from the rest of the liquid,
they need to absorb energy from their environment. This energy is the heat they take away from us
and that's good. And if our skin's heat is used to evaporate our sweat
it's good that this heat is removed from our body.
We get cooler. Where it gets complicated is that for sweat to evaporate, it must have somewhere to go.
When the air is very humid, water has a lot more difficulty evaporating because the atmosphere is already saturated, and that's one of the reasons
why humid heat is much more difficult to tolerate than dry heat at equal temperatures.
Our perspiration evaporates very little and therefore cools us only a little.
These were the questions you asked.
If you also want to send me a question I will tell you how to do it right after this message.
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As I've been getting more and more suggestions and requests for video topics from you, I've created a
form to centralize all your requests.
The link is in the description. That doesn't mean that all future videos will be answers to your questions
especially because I already have a ton of topics to cover, but if I can cover a question in a video like this one,
why not. I hope you enjoyed this video, we only scratched the surface
of the subject, it's much more complex than it seems and we could do a whole series of videos just about this.
If you suffer from the heat, I've put the official recommendations on how best to protect yourself in the description.
I'll see you again next time and I wish you a nice summer. See you soon, bye.