If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

LeBron Asks: Why does sweating cool you down?

LeBron asks Sal why sweating helps cool the body down. Created by LeBron James and Sal Khan.

Want to join the conversation?

  • piceratops ultimate style avatar for user ∫∫ Greg Boyle  dG dB
    Is sweating too much or too little ever dangerous? If the sweat stops evaporating could I get sick or even die?
    (283 votes)
    Default Khan Academy avatar avatar for user
    • leaf red style avatar for user ville
      You can become dehydrated if you sweat a lot and that can be dangerous, even fatal if you become severely dehydrated. You can also become over hydrated if you drink too much water, which can be fatal at large quantities. Balance is key!
      (313 votes)
  • purple pi purple style avatar for user hfrassrand
    Why does sweat smell bad if it is just water?
    (110 votes)
    Default Khan Academy avatar avatar for user
    • leaf blue style avatar for user Pearl Alex
      Sweat itself doesnt smell! Body odour comes from bacteria in your skin that breaks down and feeds on sweat secretions coming from your sweat glands..
      I don't mean to gross anyone out but for the purpose of science I shall also mention that the warm and damp conditions in our armpits is also great environment for bacteria to thrive and feed hence that smell
      (250 votes)
  • aqualine ultimate style avatar for user Thomas Doan
    This might sound silly: Why don't your skin molecules just "pop off" due to kinetic energy? What properties of water molecules vs dermal molecules make it evaporate?
    (18 votes)
    Default Khan Academy avatar avatar for user
    • hopper cool style avatar for user Arwick
      Assuming that you mean molecules present on the dermal surface with ''skin molecules'': they don't ''pop off'' when the adhesive force of the droplet to the skin surface is stronger than the kinetic energy; if the momentum caused by movement is high enough droplets will be displaced or get ''flung into space''.

      However, droplets of sweat in normal conditions (non-excessive sweating, i.e. during exercise) have a low amount of liquid mass. Therefore adhesion will be stronger than kinetic force most of the time. Which makes evaporation (influenced by temperature and airflow) a more energetically favored mode of action.
      (8 votes)
  • hopper jumping style avatar for user Chase Hammes
    Cant sweating be a bad thing?
    (15 votes)
    Default Khan Academy avatar avatar for user
  • hopper happy style avatar for user Unknown_Phoenix
    Is sweating a real way to cool down?
    (10 votes)
    Default Khan Academy avatar avatar for user
    • mr pink red style avatar for user Jeffrey Baum
      yes. due to evaporative cooling. the sweat gets vaporized from the skin into the atmosphere, by taking in heat from the body. (endothermic reaction to vaporize the sweat). thus the body cools down as it loses heat. this is why on a humid day you get hot and sticky, since the sweat cannot evaporate into the already saturated air. therefore, it remains on your skin and you dont cool off
      (2 votes)
  • piceratops tree style avatar for user Mithran Venugopal
    can sweat get into your eye? is that dangerous?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user Lucía C.
    There´s something that´s been bugging me: why do molecules move? Is it simply due to the intermolecular connections? Say, if we were to isolate a single molecule, would it rotate and vibrate because of it´s own energy? Would that be caused by the energy the electrons and protons have? If so, what even is negative and positive energy, really? sobs
    (2 votes)
    Default Khan Academy avatar avatar for user
    • old spice man green style avatar for user Matt B
      Molecules move because they have energy. For similar reasons you want to be active when you eat sugar (energy), you too will want to move. The energy in a molecule is mostly in the bonds between atoms and these will vibrate/rotate/move
      (14 votes)
  • blobby blue style avatar for user Rover
    Does skin ever reabsorb sweat?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • hopper cool style avatar for user Kavin Puri
    Why does the body bother putting salt into its sweat? Wouldn't that be deleterious to the body since it's wasting salt?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user Joshua
      Sodium also causes the cells in our body to lose water via osmosis. This is why too much sodium will get us dehydrated. Sodium is found in most foods we eat, so sweating out some sodium isn't harmful. Excess sodium also gets excreted via urine.
      (2 votes)
  • leaf orange style avatar for user Lázaro Krantz
    Well, though I accept your answer, I dont think le bron asked the question to know why he cooled down whilst having sweat. Sal anweres the question disregarding the movement of the subject. The way I experience the cooling down effect that sweat has ,is when I am moving against the wind. It seems to me that Sal forgot about talking about air in this video. The way that I found to explain the observation is simply that air must cool water much faster than it does to us. Given that the water is on our skin for a great duration of time, it has a much greater chance of achieving "temperature equilibrium" with us, that the air would. Is this a correct explanation?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • starky ultimate style avatar for user ++§ Αλεκσανδαρ
      Water has huge heat capacity and it actually cools slower than other substances. I haven't really paid much attention to what Sal says in this video, but from what i saw i'd say his answer is correct and doesn't miss anything major.

      However, when a wind blows, that helps water on the surface of your skin (sweat?) to evaporate faster. The body can then make more sweat and cool down faster.
      (1 vote)

Video transcript

Why does sweating cool you down? That is an excellent question LeBron, and to answer it, let's zoom in on a little droplet of sweat. And sweat is mostly water, so when we zoom in, and we've really zoomed in, even more than I've drawn over here. When we really zoom in, we'll start seeing mainly these water molecules. And the water molecules, just to be a little more accurate, I've drawn the oxygen in blue, and then the hydrogens that are bonded with that oxygen I've done in white. We all know that water is sometimes refered to as H2 -- that's for the two hydrogens -- H20. So each of these are a molecule of H2O, or a molecule of water. What I've drawn down here, and this is an oversimplification of the molecules of your skin, but just for simplification, these are molecules of your skin. really, the parts of the skin cells. These aren't even the skin cells themselves, these are the molecules that make up the skin cells. And right over here, these are molecules of sweat, or it's really just molecules of water. So the question of why does sweat cool you down could really be restated as: Why does having water on the surface of your skin actually cool you down? And to answer that, or to think about that question, we have to think about what it means to have temperature, or what temperature even really means. Temperature, what we perceive as temperature, is really just the motion of the molecules of something. So higher temperature means that they're moving around more. So high temperature they're moving around more, and low temperature they're moving around less. And they can move around in different ways, they can have translational motion, which is they're actually moving around. They could be vibrating. They could be rotating in some way. And in general, on average, the more of this motional energy, often referred to as kinetic energy, that these molecules have on average, the higher the perceived temperature would be. Now, how does having this water here cool down the skin? And well, first of all, why is the skin warming up? Because the muscles are doing all of this work. They're releasing heat. That heat is being transferred to the skin. But then how does having this water here help? Well, the skin has a certain temperature, a certain kinetic energy, or motional energy. But when we say that it doesn't mean that all of these molecules have the exact same motion. The temperature is the average motion. Some of these are bumping around at a faster speed. Or vibrating at a faster speed, or rotating at a faster speed Some are doing it at a slower speed. But as these bump around, they're going to bump into these water molecules and get them moving around. They would probably be moving around a little bit to begin with, but then the warmer this is, the more energy here, they'll bump into these molecules. So let's say this guy bumps into that, then he'll bump over there , so that energy, this bumping energy, or this kinetic energy, well, some of it will be transferred, or you could even say some of that temperature, some of that heat will be transferred to these water molecules. But the important thing to remember is this is a really kind of crazy thing, they're all bumping into eachother and rotating in all sorts of crazy ways. They will have an average kinetic energy, which we perceive as temperature, but this one might be going really, really, really fast in that direction, while this one might be going really, really, really slow, this one might be going really really really fast in that direction this one might be going really slow in this direction So the thing to think about is, given that you have all of this variation in the energy of each of these particles, which of these are most likely to escape, to actually evaporate? And to think about evaporation, you just have to think about that most water molecules or the water molecules that are in that droplet they do have an attraction to eachother, we call those hydrogen bonds. They do have an attraction to eachother, that's why a droplet kind of sticks together. But if one of these molecules is moving fast enough and if it's moving in the right direction it has a higher probability of being able to escape, being able to actually escape that droplet. And the process of these molecules actually escaping, that's what we refer to as evaporation. If a molecule has enough energy it will escape this, escape the bonds of the other water molecules, and just evaporate into the air. But we still haven't fully answered our question. So let's say that this is one that has evaporated, it has fully escaped. Why would that actually cool down this entire system? Why would it cool down the droplet and essentially give it more capability to accept more energy from the skin? Well, we just said the ones that have the highest energy are the ones that are most likely to escape, the ones that have the highest kinetic energy. So if you have a bunch of stuff, some are fast, some are slow, some are vibrating a lot, vibrating less but the ones that have a high kinetic energy are the most likely to escape, what happens when they escape? Well then the average kinetic energy will go down. Or you could say the temperature would go down, which is really just the average amount of motion or kinetic energy that's in this droplet. If the really fast ones, the ones with a lot of energy, are leaving, then the ones are left over, on average, are going to have a lower kinetic energy, or a lower temperature. And so that is what is cooling you down at a molecular level.