How to block EMF from floor

I measured ELF in my work place, one of the rooms has highest, 10-20mG on floor, as the probe goes away from the floor, it decreases, finally at the face height, it's 2mG.
The next room of this affected room has AC unit and downstairs has big transformer for electricity.
I guess it's coming from the downstairs garage.
Is there any way to block EMF coming from the downstairs? Maybe putting some kind of  shielding on the ceiling at the downstairs?
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crcsupportAsked:
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aburrCommented:
EMF shielding can be quite simple. In your case, I would suggest putting a wire screen on the ceiling being sure to ground it.
copper screens would be best but they can be expensive. For best grounding, clamp long metal edgeing along at least two adjacent sides of the screen and ground the edging.
crcsupportAuthor Commented:
what is wire screen?
viki2000Commented:
A metal mesh:
https://www.google.com/search?q=wire+screen&biw=1440&bih=742&source=lnms&tbm=isch&sa=X&ei=R81kVd5QgaGwAZSZgcAP&sqi=2&ved=0CAcQ_AUoAg

You have to see if the transformer is open frame or with a case. Watch out for electricity danger!
If the transformer has a metal case, make sure is grounded AND the ground should have under 2 ohms.

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aburrCommented:
what is wire screen?
Metal window screen will do nicely
Dr. KlahnPrincipal Software EngineerCommented:
Side note:  20 milliGauss is on the order of 1/15th to 1/30th of the Earth's magnetic field (310 to 580 milliGauss at the surface.)

I would personally consider 20 milliGauss a trivial value unless there was extremely sensitive equipment involved.
crcsupportAuthor Commented:
DrKlahn,
How come my detector reads 0-1 mG on open space? I bought a cheap detector from ebay, it's named 'Cell Sensor'. it reads 1-50mG of ELF.

When I download and use android emf detector app, the range is around 300mG as you pointed.
I got confused what I am referring to. Is it because we are talking two different EMF, low frequency and high frequency?

Or  magnetic field is measured differently as electromagnetic field?
crcsupportAuthor Commented:
It seems as there are two magnetic fields, geo-magnetic field(MF) and electro-magnetic field(EMF) which carries low frequency radiation.

Below is from wiki.
http://en.wikipedia.org/wiki/Electric_power_transmission#Health_concerns



The mainstream scientific evidence suggests that low-power, low-frequency, electromagnetic radiation associated with household currents and high transmission power lines does not constitute a short or long term health hazard. Some studies, however, have found statistical correlations between various diseases and living or working near power lines. No adverse health effects have been substantiated for people not living close to powerlines.[20]

There are established biological effects for acute high level exposure to magnetic fields well above 100 µT (1 G). In a residential setting, there is "limited evidence of carcinogenicity in humans and less than sufficient evidence for carcinogenicity in experimental animals", in particular, childhood leukemia, associated with average exposure to residential power-frequency magnetic field above 0.3 µT (3 mG) to 0.4 µT (4 mG). These levels exceed average residential power-frequency magnetic fields in homes, which are about 0.07 µT (0.7 mG) in Europe and 0.11 µT (1.1 mG) in North America.[21][22]

The Earths natural geomagnetic field strength varies over the surface of the planet between 0.035 mT - 0.07 mT (35 µT - 70 µT or 0.35 G - 0.7 G) while the International Standard for the continuous exposure limit is set at 40 mT (40,000 µT or 400 G) for the general public.[21]
viki2000Commented:
I always wanted to know about a map with geomagnetic field strength to see how varies on different geographical regions of the globe. For instance as you move from equator to the north pole. Any idea?

By the way, measuring the ground resistance is not easy. You need a special instrument, a megger. You cannot do it with a simple multimeter. We use to do that regularly in industrial environment where big electrical motors generate all kind of noises and panels with automaton computers (PLC) had to be installed near by. The procedure of measurement requires  some electrodes, we used pipes, inserted in the ground at a specific distance between them and connected to the instrument as reference for the measurement grounding point.
Check out here some test procedure:
http://www.weschler.com/_upload/sitepdfs/techref/gettingdowntoearth.pdf
http://www.reeve.com/Documents/Ground%20Testing%20R1-1.pdf
http://www.chauvin-arnoux.com/sites/default/files/documents/dc_earth-ground_measurement_ed1.pdf

Easiest is to call a company which knows the procedure and have the instrument. They make the test for you and will give you a certificate, usually valid for 1 year.
A good grounding solves a lot of problems, not only EMF.
crcsupportAuthor Commented:
viki, the problem is eletromagnetic field as I researched. geomagnetic field often goes up to multi hundred levels of mG reading, but WHO recommends electromagnetic field to stay under 2mG in habitable space.
In my intuition without professional knowledge, magnetic field generated by eletricity matters to health.
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