Uncomfortable heat and harmful heat

Factual Questions
1

Today, as I began to drive my car home after work, I happened to touch the dashboard and found it to be uncomfortably hot. (The car sat in the sun, not in the shade, on a 104 degree day, for eight hours or so.)

I noticed that while it was uncomfortable, it was not the kind of heat that leads you to automatically, reflexively draw away.

As a sort of an experiment, I again touched the dash, to see just how hot it felt. As I kept my hand in contact with the dash, the heat sensation became worse and worse. After several seconds, I felt with some urgency that I ought to remove my hand. So I did.

But at no point did the feeling come close to a burning feeling. I suspect that with some willpower, I could have touched the dash for an indefinitely long period of time. If it had been, instead, a red hot electric stove heating element, I think no amount of willpower could keep me in contact with it for more than a split second.

Nevertheless, though I think I could have kept in contact with the dash indefinitely, it is unmistakable that after several seconds, the sensation I was recieving from my hand was one that was telling me it would really, really like me to withdraw my hand from the dash as soon as possible.

I did no detectable damage to my skin. But I don’t know that there would have been no damage if I had kept my skin in contact with the dash for substantially longer.

So I have a question. Can a surface be hot enough to cause this kind of sense of urgency (as in, please, urgently, remove me from this surface, because it is hot!), yet not hot enough to cause any physical harm to the skin?

2

I’ve read news stories about people burning themselves on laptops, usually when it’s sitting on their lap. The stories state that not only can you be burned by a short contact with a high heat source but also by longer term contact with a lower temperature heat source.

Maybe someone who knows more about it can confirm if that’s true.

3

Here’s a chart that shows roughly how long you can be in contact with a given temperature of water before being scalded. It should be noted that air temperature is different, since water has both a higher heat capacity and higher rate of thermal transfer than air.

4

Yes, you would damage your hand if you kept it there long enough.

The exhaust pipes of my hot rod run right under my feet. The car is very small (Triumph Spitfire) and it will not clear a soda can laying on its side in the road, so the exhaust is almost contacting the metal floor pan. After driving it a few hours with my feet feeling sweaty but not burning hot, I felt the effects for some 4 days afterward. I put more layers of insulation under the carpet.

For the record, the pink/blue or silver styrofoam sheathing at Home Depot was far more effective than automotive type heat shielding, which did little.

5

You have two temperature related pain sensors in your skin, called the VR1 and VR2 pain receptors. VR1 kicks on at about 43 to 45 deg C for most folks, which is below the point where damage to your skin occurs. VR2 kicks in at about 50 or 51 deg C, where damage actually does occur. Between roughly 43 to maybe 48 deg C is the range where, exactly as described in the OP, you really, really, really want to move your hand away, but it won’t cause any physical damage to your skin.

Many years ago I worked in a neurobiology lab, designing equipment for a researcher who was trying to sort out exactly how our nerves and brain process pain signals. One of the machines I designed had a probe about the size of a dime that would go anywhere from about 1 deg C to 50 deg C, changing at about 10 deg C per second max, and locking on to the nearest 0.1 deg C. A lot of the test procedures specifically took advantage of that pain but no damage range for use on human subjects. When I first got the machine working (a few months after I started working at the lab), I would instantly pull my hand off of the machine at about 43 deg C. After I had played with the machine for a couple of months I could keep my hand on it at 46 or 47 deg for a long time. It wasn’t pleasant, but I had trained my body to ignore the “holy f*** get that off of me” response.

By the way, the VR1 pain receptor also happens to respond to capsaicin, the stuff in hot peppers that makes them “hot”. Birds and some other animals don’t have the VR1 pain receptor (apparently they get long fine with only the VR2) and therefore can eat hot peppers all day long and not feel “hot” from it.

6

Forgot to comment on this.

What burns you isn’t necessarily the temperature. It’s how much it heats up your skin. The two aren’t necessarily related. Some materials transfer heat more easily than others. Metals, for example, tend to transfer heat very quickly. Wood, for example, doesn’t transfer heat as easily. This is why it’s no problem to stick your tongue on a telephone pole in the winter, but not so advisable to do it with a metal pole.

If something is very hot, but has a slow rate of heat transfer, you can hold it a lot longer before it burns you than something that is not quite as hot, but has a much faster rate of heat transfer. You can hold a space shuttle tile that is heated red hot and it won’t burn your hand. If you tried to hold a piece of metal at the same temperature you’d be off to the emergency room to treat the burn after just a second or two of contact.

If you held the space shuttle tile long enough, eventually enough heat would transfer to your hand to burn you, but it would take a very long time.