Science Questions

Why does warm water feel scalding to cold hands?

Sun, 13th Mar 2011

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Question

Joshua Speelman asked:

I live in Michigan in the USA and since I've had to shovel a lot of snow over the last week a question came to mind.  What is going on physiologically when I put my freezing cold hands under barely warm water but it feels near scalding to me?

 

Thanks and keep up the great podcast.

 

 

Joshua Speelman

 

Answer

Chris -   This is called adaptation and under normal circumstances, the nerve cells which are carrying sensory information from our peripheries into the core of the body, into the central nervous system, to inform the brain as to what the environment is like, they are very rapidly adapting.  What this means is that they are largely interested in how things are changing.  They're not so interested in the static situation.  That's because if you didn’t have something that quickly got bored, and stop signalling the absolute level of something, then you'd suffer from sensory overload.  So they tend to signal change.  Now if you have got your hands very, very cold, then there's very little increase in the temperature in your hands and therefore, the nerves that signal warmth are largely very inactive.  The ones that signal cold are firing off saying, “It’s cold and I'm getting colder.”  When you then put your hands indoors or under warmish water, the amount of firing in the warm fibres now goes through the roof because where they were previously not very responsive, they've gone from going, “I'm very, very cold so I'm not going to react” to suddenly, “Wow!  The temperature is increasing very, very rapidly.”  And it’s actually the change that they're interested in and so, they fire intensively, and that's why you experience this sensation that things are much, much hotter than they are.

Dave -   There’s also a lovely experiment you can do with this, completely confusing your body.  If you put one hand in a very hot water, one in a very, very cold then put them both in the middle, it’s entirely confusing.  One hand feels hot, one feels cold in the same water.

Chris -   It’s actually in our book, Crisp Packet Fireworks.  We actually explained how to do that experiment if you want to have a go.

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If I recall correctly, there was an explanation in a textbook I once had. It said that we have nerves to detect "cold" and nerves to detect "warm". In order to sense "hot", both nerve types need to be stimulated at once. So if the cold nerves are stimulated (by having cold hands) then stimulating the warm nerves with warm water will produce the sensation of hot. Supercryptid, Tue, 1st Mar 2011

Joshua Speelman asked the Naked Scientists: I live in Michigan in the USA and since I've had to shovel a lot of snow over the last week a question came to mind.  What is going on physiologically when I put my freezing cold hands under barely warm water but it feels near scalding to me? Thanks and keep up the great podcast. Joshua Speelman What do you think? Joshua Speelman , Tue, 15th Mar 2011

Your body does not sense absolute temperature, rather the temperature gradient through the skin, which is related to the rate of heat loss (since inside you are at 37.5°C).
It's also true that pain sensors kick in on an extreme temperature gradient - to indicate you may be doing damage. JMLCarter, Sun, 10th Apr 2011

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