What Exactly am I Measuring with an EMF Phone App?

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You are measuring magnetic fields and not radio frequency/microwaves.  My EMF meter measures magnetic fields in Gauss (G) and milligauss (mG) instead of Tesla or micro Tesla (uT).  A conversion chart shows that
1 Gauss = 0.1 uT.  Some research indicates above 2 mG is linked to illnesses like leukemia.

How dependable are the reads that you find using a smartphone app?  The smartphone itself gives off a significant magnetic field and if struggling to connect with a cell tower far away it will emit an even higher field - which may be the case if you are in a rural area far from a tower.

It's hard to say for sure where your extremely high reads are coming from without doing an EMF assessment of your property.  

https://www.emfanalysis.com/wp-content/uploads/2021/05/Biological-Effects-of-AC-Magnetic-Fields.pdf

https://www.emfanalysis.com/how-to-create-a-healthy-home/   

https://bioinitiative.org/ 

Phone application measure constant magnetic field, not ELF magnetic field, not electric field and not RF.
It is basically measuring the earth magnetic field.
The measurement is not relevant to EMF exposure or handing EHS.
Get a real meter to do real measurments.

It looks like there is an AC version of the app:

https://play.google.com/store/apps/details?id=com.keuwl.acmagneticfieldmeter

They do include this caveat:

"Magnetic field sensors will vary in sampling speed, resolution and accuracy from one device to the next. For indication only.

...

This app is designed for indication and educational purposes only. Always use a suitable calibrated more accurate device appropriate for the situation when required."

If you're only trying to determine if a wiring change reduced the ELF field strength in your house, this might actually be good enough.