Heating of the Human Body and Radiation Wavelengths

[fog37]

Hello Everyone,

Sunlight is composed of UV, visible and infrared (IR) over a wavelength range from ~290nm to ~2500nm.

When we are exposed to sunlight and feel hot, is it because of the absorption of energy at the visible wavelengths and FIR, i.e . infrared wavelength much larger than 2500nm? I don't think radiation in the range 700nm-2500nm produces a heating sensation in the human body. Or does it?

Thanks

 

[jim mcnamara]

Let me try to get this thread going.

Humans sense external heat sources, sunlight and re-radiated longer wavelengths, ultimately through sensory neurons -
https://en.wikipedia.org/wiki/Thermoreceptor

produces a heating sensation

is vague to me. What you are really asking is: what is the absorption spectrum (I think) for human skin tissue surrounding Thermoreceptors?
doi:10.1117/1.JBO.17.9.090901 Optical properties of human skin
https://www.spiedigitallibrary.org/...man-skin/10.1117/1.JBO.17.9.090901.full?SSO=1

Hemoglobin and melanin are most active molecules in skin -- for absorbing light and therefore generating a sensory response. The link I provided seems good but is technical. Look for graphs, those are easy to understand.

 

[fog37]

Thank you jim mcmanara. I have read the article and looked at the graphs. Very interesting. That said, the graphs go up to 750nm which is near infrared (NIR).

I am still wondering how FIR, i.e. wavelengths longer than 2.5 micron, are absorbed by the skin and makes us feel hot or if they just penetrate below the epidermis. Most objects,under sunlight get very hot and their temperatures are such that their emission peak is in the 10micron or so. the human body emits strongly at 9 micron. Does that mean that is also efficiently and strongly absorbs energy at that same wavelength due to resonance?

 

[jim mcnamara]

We humans are homeothermic, we regulate our body temperature. We do this in several ways:
constrict epidermal blood flow to conserve body heat in low ambient temperature,
increase epidermal blood flow to remove heat via sweat and direct radiation in hot ambient temperature.

You are making more of this than it is worth. Yes, we absorb and re-radiate heat (why some mammal eating reptiles have Jacobsen's organs to detect heat signatures of mammalian prey).

Feeling hot or cold is a brain/neuron function. Nothing else.

If the entire human body throughout was at ambient temperature, humans would die off pretty quickly almost eveywhere. Humans start to lose consciousness when internal body temperatures are less than 94°F or greater than ~108°F. Long periods of time in vast areas of Earth experience temperatures outside that "golden zone".

 

[fog37]

Thank you.

You may be right. I guess I am just wondering, from a purely physics standpoint, which, between far infrared or near infrared radiation, in the same amount, is more effective at keeping as warm.

 

[Drakkith]

Thank you.

You may be right. I guess I am just wondering, from a purely physics standpoint, which, between far infrared or near infrared radiation, in the same amount, is more effective at keeping as warm.

It depends on how much of each wavelength your body/clothes absorb. If you absorb 700 nm light better than 2500 nm then the former would be more efficient at keeping you warm as long as the incoming energy at each wavelength is the same. However when dealing with sunlight there is more energy in the near IR band than the FIR band, so it likely plays a much larger role in warming you during the day.

Note that 'keeping us warm' is not the same as 'feels warm'. If lots of radiation is absorbed near your thermoreceptors, you may initially feel warmer than if the radiation was of a wavelength that was absorbed deeper in your body where you have little thermoreceptors. For example, microwaves and radio waves can penetrate your skin and heat your internal organs directly, without you ever feeling very warm. Absorbing 100 watts of IR radiation would heat you less than 500 watts of microwave radiation, but you may feel the former on your skin more than the latter.

 

[fog37]

Thanks Drakkith.

But I would say that shorter wavelength radiation, like UV and visible, are not very penetrating and get absorbed on the outer skin layers and produce the feeling of being warm. IR goes deeper and may still cause heating. But when we are outside under sunlight, we probably feel hot mainly because of the UV and visible...

 

[Drakkith]

IR goes deeper and may still cause heating.

Got anything to back this up?

 

[fog37]

I found this online, in relation to light therapy:

 

[Fervent Freyja]

I don't think radiation in the range 700nm-2500nm produces a heating sensation in the human body. Or does it?

In that range there will be no conscious heating sensation. However, bacteria are known to response to heating with the shorter radiofrequencies. The biofilm on our skin surface may experience thermal effects that we are not aware of. So, no heating sensation, but possibly thermal effects on the human body within that range.

 

[fog37]

So, do you think the heating sensation will be mostly due to far infrared, larger than 2500nm, and to UV radiation since it gets readily absorbed by the skin outer layers?

 

[Fervent Freyja]

So, do you think the heating sensation will be mostly due to far infrared, larger than 2500nm, and to UV radiation since it gets readily absorbed by the skin outer layers?

I think it's very difficult to set up experiments that separate them well enough to be able to tell.

And anyway, the heating sensation has numerous other parameters involved besides frequency!