h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Apparently you can't get sunburnt after 4pm (or before 11am). But I've been sitting out in the sun this afternoon (4.30 ish) and my skin *feels* like it's burning i.e. it's very hot and the sensation on my skin is almost but not quite too uncomfortable.

My question is about the difference between radiation burns (which I assume is what sunburn from UV light is) and heat burn (which I'm guessing is what I feel today). What I want to know is if the heat burn damages the skin, if it increases the risk of melanoma, and if any of this has relevance to vitamin D production (which is why I was sitting in the sun even though it was probably too hot).

Erm, I don't know who told you that you can only get sunburn within a certain range of time, but they're wrong.

Never heard of the phrase "heat burn" being used to describe what you are suffering from, which is actually sunburn, and of course can have all the effects you stated.

Actually it was a pharmacy student who first told me that you can't get sunburnt after 4pm. That was 20 years ago, I didn't believe her at the time. But it does seem to be standard advice in NZ now, that in order to avoid sunburn stay out of prolonged direct sun before 4pm (and after 11am). Obviously the sun doesn't turn off it's burning rays exactly at that 4pm But there obviously is a difference in burn potential at 5pm vs midday:

http://www.sunsmart.org.nz/uv-radiation--index/ultraviolet-radiation-(uvr).aspx

I burn fairly easily. I've never been burnt after 4pm unless I've also had exposure during the day.


As for today, I could feel the heat while in the sun (in the same way that I would if I was sitting too close to a heater element), but now later there is no redness and no pain. So technically I guess there were changes to the skin cells but it wasn't like a sunburn that involves redness and pain. That takes me back to my original question about damage and melanoma.

Believe me, I know what I'm talking about when talking about sunburn... I'm a redhead - I've even managed to get sunburnt on a cloudy day!


What you talk about sunburn and 'heatburn caused by the sun'... You're actually talking about different degrees of severity of the same thing, with exactly the same types of effect on your body, just in different quantities.


And as with anything we do to our body, pain is nature's way of telling you you're doing too much.

Damage to cells is damage to cells and it tends to be repeated/chronic damage that can eventually lead to cancer.

Cells are remarkably good at repairing themselves but keep doing the damage and eventually something goes wrong and one manifestation is that the cell's lifecycle can go wrong and you have a cancerous cell.

It doesn't matter whether that damage is caused by heat, ionising radiation, non-ionising radiation, oxidative stress or by physical foreign bodies such as viral invasion or physical stress from solids such as silica (e.g. asbestos). *Any* form of long term repeated cell damaging stress can eventually lead to cancer.

>>I've even managed to get sunburnt on a cloudy day!<< Whisky

Yes, but that is relatively easy to do and is different than what happens in the late afternoon. Believe me, I live in a country directly under the hole in the ozone layer and with one of the highest skin cancer rates in the world

So, yes you can get burnt on a cloudy day, because UV rays go through cloud. However at the end of the day the UV rays are less (something to do with the angle of the sun to the earth?). so you burn less quickly at 4pm than you do at midday.


>>
What you talk about sunburn and 'heatburn caused by the sun'... You're actually talking about different degrees of severity of the same thing, with exactly the same types of effect on your body, just in different quantities.
<<

I'm not sure I agree, see my response to Orcus below.



>>
It doesn't matter whether that damage is caused by heat, ionising radiation, non-ionising radiation, oxidative stress or by physical foreign bodies such as viral invasion or physical stress from solids such as silica (e.g. asbestos). *Any* form of long term repeated cell damaging stress can eventually lead to cancer.
<< Orcus

Let's just stick with burns for a minute. If I sit in front of the heater and my skin gets red are you saying that that is as damaging as if I sit in the sun and my skin gets red. I think there is a qualitative difference. UV burns deeper maybe?? Certainly sunburn lasts a lot longer than my skin overheating from being next to a heater. And sunburn will peel the skin in a way that sitting in front of a heater won't.




btw, I didn't get sunburnt from that half hour in the sun late afternoon. When I went out of the sun the redness went away fairly quickly.

There is a difference between between heatburn and UV burn but my point really was in the exposure time and rate and in trying to demystify what goes on when a cell turns into a cancer cell.

Repeated damage of *any kind* can cause it and I think if you heat burned yourself as often as you exposed yourself to UV light it's pretty likely you'd get the same if not more increase in the likelihood of cells going wrong in a cancerous fashion. The reason I say that is that heat burn causes greater and more extensive damage to cells than UV which is more subtle (and therefore the reason why we don't notice it so much).

Bottom line, heat burn is more immediate and obvious so we avoid it. Same as we try to avoid walking under a bus.
UV damage happens whilst you're unaware. *That* is its danger, it's more insidious.

UV dammage happens when you're dark haired and walking about in central UK on a cloudly day in the rain, its always happening whenever ther eis light from the sun... which unless your caving is all the time. In some places in the world, and at some times of the year/day, its likely to be more strong, or less strong, but it'll be there basically all the time to varying degrees... By 'heat burn' you're meaning infared? I really don't think the sun is that* subtle at breaking up the types of radiation your recieving from it, they're will be X rays, infa red, UV all in it, thorugh of course the intensity and duration of each can vary...

Well it wasn't me who was distinguishing them - You don't receive X-rays from the sun though, the ionosphere and van Allen belts shield us from the most damaging ionising radiation like X-rays, gamma rays and solar wind particles.

All of them? I must be mistaken then, I thought some still made it through...

>>There is a difference between between heatburn and UV burn but my point really was in the exposure time and rate and in trying to demystify what goes on when a cell turns into a cancer cell.
<<

Yes, thanks Orcus. I agree. But it's still not clear how damaging the heatburn is. I know if I sit in the sun at midday for 10 minutes I'll get sunburnt and that will up my chances of melanoma. But 30 mins at 4.30, giving me not sunburn but heat and redness on the skin, I have no idea what that means in real terms re damage. The reason I am asking is because there is alot of concern about vitamin D now, and how much sun you need to get adequate production. The message in NZ is to stay out of the sun between 11am and 4pm for prolonged times, but to get sun on exposed skin otherwise (for the vit D).

Obviously UV is there all the time the sun is up but it varies in strength. I assume heatburn does to but don't know how to quantify it in terms of potential damage.

I would say that's likely good advice they're giving out in that case.

Infra-red or 'heat' radiation is likely to do very little damage in comparison to UV in like for like dose as it is of lower energy.
UV is more dangerous 'per photon' if you like because it actually interacts with molecules by shifting their electrons around and so can disrupt, destroy and generally damage them.

Infra-red (IR) can only cause molecules to vibrate more rapidly which whilst potentially damaging is nowhere near the potential destruction of UV.

In fact having thought a bit more about what your scenario, the redness and heat burn you are getting in the evenings is almost certainly not due to IR but probably from UV anyway. The infra-red heat from the sun is barely hot enough to kill and cook cells in the center of the world's hottest deserts so I doubt it's a problem in the evening in NZ, hot as it may be.

Which brings us back to the original question: is there some effect where, late in the day, we may not be getting much in the way of UV rays?

Light has to refract through the atmosphere go get to us, and different wavelengths refract at different angles. It make sense that we would go dark on certain wavelengths before others. I would make the guess that the UV wavelengths might go before visible light, because the Sun looks red as it is settings, but then that could be down to something else.

Now, that's all a bit tenuous. Anyone who knows more about what they're talking about feel like confirming or correcting me?

On a total tangent..




<>

Is this true for *all* molecules or just most of them? I guess what I mean is there some basic energy level needed to move electrons that UV has and IR doesn't, and which is true no matter what the molecule is and whether it's an ionic or covalent bond?

>Is this true for *all* molecules or just most of them?<

All of them yes.

>I guess what I mean is there some basic energy level needed to move electrons that UV has and IR doesn't, and which is true no matter what the molecule is and whether it's an ionic or covalent bond? <

Yes.
The energy to move electrons between discrete molecular energy levels is far too high for infra-red light to get anywhere near achieving. Really low energy electronic transitions can occur at visible regions of light but not in the IR.

Where electrons are spread out of the entire solid in what we call bands (i.e. metals) then you can move them around with thermal energy, metals absorb pretty much any wavelength from radio to UV, even X-rays.
But we're not made of metal.

The molecular motions that are excited by various freqencies of light are (in increasing energy per photon):

Radiowave ---> Magnetic resonance (e.g. MRI in hospitals)

Microwave ---> Molecular rotations

Infra-red ---> Molecular vibrations

UV/visible ---> Electronic transitions

X-ray ---> Core electrons of atoms

Gamma-ray ---> Nuclear transitions (i.e. raising the energies in the atomic nucleus)

Cheers Orcus. You da man

But..

If you heat something with IR til it decomposes, ie a chemical bond breaks, what then?

Would that mean it's the atomic motion rather than the IR doing it?

That just doesn't happen. You can't treat a chemical bond like a paperclip and assume it's going to break if you bend it enough.

Only ionizing radiation breaks molecules apart. And only UV, X-Rays and Gamma have got that sort of oomph behind them.

B

Indeed, what you suggest Pedro would be like trying to boil the Mediterranean with a tealight as your heatsource. There's just not enough energy there.

Infrared radiation doesn't have enough energy to break bonds - but it is true that if you heat something strongly enough then molecular shapes can break down.
Boiling an egg is a good example - the heat of the water changes the protein of the white such that it aggregates and solidifies.

Also thermal motion causes molecules to bump into one another and when that happens with enough energy electrons can flow to make new chemical bonds and a chemical reaction has taken place.

Sitting in the warm afternoon sun in NZ is unlikely to do much of that very quickly though. Certainly not much above what happens anyway with your body's metabolism.

I think you misunderstand what heating something is also Pedro.

Heat is transferred mostly through conduction and convection - unless a radiation source is clucking intense that aint going to cut it (and the sun is intense but we're 93 million miles from it).

Conduction and convection heat transfer occurs through molecules flying around in space and through vibrational and rotational movements too (a bit like bumper cars ).
As objects cool they emit IR.

But if you try heating something by shining IR radiation on it you're in for a LOOOOONG wait.
(Try shining your TV remote on your hand and see how hot you get )

Ok then, to take an extreme example of what I meant.

If I heat a diamond* (in a vacuum etc) to 1 million degrees, presumably it turns into plasma or something.

Let's say this is done by a ridiculously large amount of IR lasers all pointing at it (would IR lasers make a difference?).

The diamond decomposes due to the high temperature.

What I meant was is this because of
a) the heat received? (Doesn't sound like it from what you've said.)
b) the atoms basically bouncing off each other so hard decomposition occurs.






*or something else if diamond's funny that way.