EM spectrum exposure

Discussion in 'Wireless & RF Design' started by wayneh, May 21, 2015.

  1. wayneh

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    Sep 9, 2010
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    Anyone know where to find information and/or a good graphic on this topic?

    I'd like to understand what the natural exposure rates look like, and how they compare to man-made sources.

    So far (after a quick Googling) I've found only a few things that don't quite complete the story:
    • This useful plot shows what gets taken out by absorption by the atmosphere, but no information on what doses we get.
    Atmospheric_electromagnetic_opacity.png
    • This plot pretty well addresses solar EM, but has no information about man-made sources.
    NASA plot.png
    • This shows what the typical man-made sources are, but says nothing about typical doses.
    EM-Spectrum.png

    I get it that your exposure is going to depend on where and how you live, so there is no single value that is meaningful. But is there a summary somewhere to give an idea of typical exposure rates to the various frequencies?
     
  2. bertus

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  3. wayneh

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    Yes! That's where I found the images I posted. The answers I'm after may be lurking in the first few pages of hits but I didn't see them.

    I think what I'm after is an order-of-magnitude comparison of man-made sources to natural sources. For instance they say being on a mountain top or flying in a plane for a few hours is like getting a chest x-ray.

    As we move around in our daily lives, what are the "bright" sources of EM? Do cell towers give a big dose, or the microwave oven, or the smartphone in my pocket? Or the TV stations or my wireless router? I'm not worried in the least about any of these but I am curious how they would look if we could see EM at those frequencies.

    Here's one I can evaluate on my own: I listen to an AM radio station at 50kW that's ~100 miles away. The EM dose from that is like a dim candle from...some long distance. Of course there are many other sources much closer but just because I can detect it doesn't mean I'm "bathed" in EM as the alarmists would have you believe.
     
  4. bertus

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  5. alfacliff

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    people have a fear of cell towers, but the whole idea of cellphone is to have low power towers spaced close together. unless you plan on climbng on one and living there, dont worry about cell towers.
     
  6. MrChips

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  7. Kermit2

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    unless the power levels reach the point at which organic compounds become ionized I see no harm which they csn cause. picowatts and even microwatts of EM radiation are always absorbed and converted to heat by the ever present and unavoidable water molecules found in living tissue of both the animal and vegetable variety of this planet. Consider the microwave RF transmitters that 2 or 3 billion people place on their ear multiple times a day. Evidence is lacking any obvious cellular damage is occuring. If the radiation is not powerful enough to push an electron off of a molecule and "ionize" it, then that molecule will simply absorb the emitted frequency and increase in temperature slightly as a result.
     
  8. wayneh

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    As noted, I'm not worried about any particular EM exposure. I avoid it when it's easy to do so – like backing away from the microwave while it's running – but otherwise don't think about it.

    What I'm after is numbers, such as watts per square meter or watts per kg, for common sources of EM exposure. I've found one source that says cellphones are supposed to be limited to 2W/kg.

    I believe medical imaging is probably the highest intensity source most of us would ever encounter. In an MRI application, 6W/kg is a very high dose, according to this great paper on MRI exposure.

    Excerpt:
    As for energy absorption properties of the human body, electromagnetic frequency spectrum can be divided into four ranges 67]:

    1. from 100 kHz up to 20 MHz, the absorption in the trunk decreases rapidly with decreasing frequency and significant absorption may occur in the neck and legs;

    2. form 20 MHz up to 300 MHz, relatively high absorption can occur in the whole body, and to even higher values if partial body resonances are considered;

    3. from 300 MHz up to several GHz, significant local, non uniform absorption occurs;

    4. above 10 GHz, energy absorption occurs primarily at the body surface.

    It must be noted that electromagnetic waves normally utilized in MRI techniques are in the second range of absorption, at which high absorption occurs in the whole body.

    and later

    RF power deposition represents the greatest risk for patient safety in MRI exams. There is widespread agreement among scientists in considering that a local increase in temperature of 1°C in a healthy individual is absolutely free of risk. In MRI exams, an SAR of 8 W/kg could be used, but for short enough exposures so as not to produce a more than 1°C core body temperature rise.
     
    Last edited: May 22, 2015
  9. alfacliff

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  10. BR-549

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    wayneh,

    I don't think any comprehensive studies have been done on the whole spectrum.

    For that matter, what is the whole spectrum?

    I believe a spectrum analyzer is your only recourse for your locations.
     
  11. wayneh

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    Yeah, it seems that most studies identify man-made sources - microwaves, cellphones, etc. - and study just those sources. There isn't much information on the wider spectrum, like what you find for visible light from the sun, for instance.

    I think this is because the natural levels are so low that only the man-made sources need to be considered, like street lights in the darkness. Once you move away from these point sources, exposure is relatively so small, it's just not worth considering. It's noise.

    What I still have no feel for, for instance, is how holding a cellphone by your head compares to driving past a cell tower. Or how my kitchen microwave compares to passing one of those microwave relay towers. Where are the brightest lights - the most intense EM sources - in daily life?
     
  12. BR-549

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    That first picture of yours is pretty old.

    I'll bet if we had a good receiver for the whole spectrum, that curve would look a lot different.

    I would think that up to light, as said, heating would be the greatest effect.

    But with you walking around on earth, I would guess the brightest sources would be gamma.

    Gamma radioactive decay in earth and matter.

    Gamma from cosmic secondary showers.

    And every once and awhile, a cosmic bit could get down into you. It's not EM, but it will ionize the bleep out of you.

    Of course you could go down in your basement at the wrong time, and breath a few alpha particles.

    A few weeks of those inside your lungs, would do more damage than a lifetime of EM.
     
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