Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Medical Heating of the Human Body and Radiation Wavelengths

  1. May 12, 2018 #1
    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?

  2. jcsd
  3. May 12, 2018 #2

    jim mcnamara

    User Avatar

    Staff: Mentor

    Let me try to get this thread going.

    Humans sense external heat sources, sunlight and re-radiated longer wavelengths, ultimately through sensory neurons -

    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

    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.
  4. May 13, 2018 #3
    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?
  5. May 13, 2018 #4

    jim mcnamara

    User Avatar

    Staff: Mentor

    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".
  6. May 13, 2018 #5
    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.
  7. May 18, 2018 at 8:59 PM #6


    User Avatar
    Staff Emeritus
    Science Advisor

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted