Basic Protection from EM Fields
Electromagnetic (EM) fields are important to life and we are living in a "natural" magnetic field of Earth--and it does not harm our health.
When birds migrate 1000 miles (without getting lost), they use natural magnetic field lines to maintain course--allowing them to arrive at the very same destination each year.
Additional (unnatural) EM fields can have adverse biologic effects on living beings, so it is important to find a critical threshold of additional EM radiation and use it as a cutoff in order to preserve our health.
It doesn't matter whether EM fields are measured with regard to the electric field strength (Volts per meter) or with regard to magnetic flux density (Tesla or Gauss)--a health threshold could be fashioned in terms of either one or the other.
EM radiation comes in different wavelengths, with some wavelengths capable of causing harm at even lower field strength--i.e., even lower "Volts per meter" (V/m) or even lower "milliGauss" (mG) than others.
A good cutoff looks at the most-dangerous wavelengths, and then tells you the minimum EM field strength (at those wavelengths) which would cause you harm.
One organization which puts out information on cutoff levels is the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [1].
This outfit warns that general public exposure of just under 0.95 milliGauss magnetic (or ~27.5 V/m electric) of the more-dangerous wavelengths would be a good cutoff level for exposure.
This cutoff (27.5 V/m) for general public exposure is confirmed by other researchers, as well [2]. Evidence is mounting that exposure to the EM fields from wireless phones and devices increases your risk for cancer, especially brain cancer [3].
It looks like we in the general public could protect ourselves if we all agreed to each individually limit our personal exposure to under 0.9 mG (or under 27 V/m).
But how?
You can buy EF field detectors and even download apps to your phone or tablet. To double-check that they are working, you can compare readings to the natural magnetic field of Earth--using just a compass and a magnet (and some trigonometry!).
Here is a diagram showing how to check devices and apps to see if they're doing what they claim they are doing:
Once you find the circle which (at any point along the circumference of that circle) results in a 45-degree offset from N, then you have found the distance where a magnet is matching the horizontal component of Earth's magnetic field (the component which affects compass needles).
The horizontal component of Earth's magnetic field is approximately 250 mG.
The trigonometric solution is that the tangent of the angle, theta, is equal to opposite side over adjacent side--with regard to the imaginary right triangle formed when the "45-degree offset" compass needle is taken as the hypotenuse (and original needle position is taken as the adjacent side).
Tangent (45 degrees) = 1, meaning that the adjacent side (the Earth's field) and the opposite side (the magnet's field) are equal.
Unit Conversion
If your app or device puts field strength in microTesla instead of milliGauss (mG), multiply the reading by 10 to get mG. With one of the apps available, here is what the screen looks like when you calibrate (or test it) with a magnet:
The main thing to think about is your distance from electronic devices capable of creating EM fields, especially with regard to your total time of exposure. Do not sleep right next to your phone. Walls do not cut EM field strength, only distance does.
You can "trap" all of the EM fields that your phone or tablet makes (and receives) by wrapping it in aluminum foil (eg, a Faraday bag). You can test this by streaming audio online as you wrap the device until it no longer receives any signals (the music will stop).
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Footnote 1: When averaged over time, you're supposed to be exposed to less than 1 Volt/meter (V/m) of radiofrequency or microwave radiation--though it is okay if the peak reading spikes up to 1 V/m every now and again, as long as it is not sustained (or an average of all readings).
In spite of this, some unfortunate people are being exposed to a time-averaged intensity of 3 V/m (over 3 times the intensity which harms you!).
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Footnote 2: Power lines (very low-frequency, very long wave EM fields)
To get the field strength at distance from a power line, double the current (in amps) and divide by distance away (in meters)--giving you the milliGauss. A high-voltage transmission line can have 1000 amps. To get the field strength at 100 meters (just over a football field) away, it'd be:
2 * I (amps) 2 * (1000) 2000
------------------- = ------------------- = -------------------- = 20 milliGauss
r (meters) (100) 100
Note how, at just 10 meters away (standing directly below a line suspended 10 meters up), you'll be exposed to a whopping 200 mG!
Neighborhood "pole-to-pole" distribution lines usually only have about 40 amps of current, so the numerator above will only be 80 instead of 2000. At 80 meters away, you will get 1 mG. At 40 meters away you will get 2 mG.
2 * (40 amps) 80
----------------------------- = ------------- = 1 milliGauss
(80 meters away) 80
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Sources
[1] International Commission on Non-Ionizing Radiation Protection. [https://www.icnirp.org/cms/upload/publications/ICNIRPemfgdl.pdf]
[2] Stanford University. [http://www-group.slac.stanford.edu/esh/eshmanual/references/nirReqExpLimits.pdf]
[3] Carlberg M1, Hardell L1. (2017). Evaluation of Mobile Phone and Cordless Phone Use and Glioma Risk Using the Bradford Hill Viewpoints from 1965 on Association or Causation. Biomed Res Int. 2017:9218486 [abstract: https://www.ncbi.nlm.nih.gov/pubmed/28401165]
Image Attribution
Page URL: https://commons.wikimedia.org/wiki/File:Horseshoe_magnet_by_Zureks.jpg
Attribution: Zureks [CC0]