Could an attachment to cell-phones limit or neutralize electro-magnetic radiation? Assumption:  Cell-phones generate some degree of electro-magnetic radiation.  Whether it's harmful or not is not the question here.
Claim: Certain devices may "neutralize" this radiation.  (Ex: http://www.giawellness.com/2/products/terra-gia/cell-guard/)
Questions: 


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*Is such a claim even physically possible with what is known today (in stick-on form, as advertized)?  

*Presumably a full case made out of special material could limit it..  What effect would a suitable case have on the call quality and/or wifi connectivity?


(Related Question: Could cell-phone radiation cause cancer?)
 A: A small attachment? No, not possible.
Think of it this way: If EM waves are emitted from the phone's antenna at point A, the only way they can be stopped from reaching point B is if either


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*You place something directly between A and B that absorbs or reflects them, or

*You create an equal but opposite wave at B from somewhere else, destructively interfering with the signal from A so that there is no EM wave at B.


An attachment like the one linked can't shield all directions simultaneously, so option (1) doesn't work in general. With option (2), at best you can neutralize the wave at one point, but you can't generally neutralize it elsewhere at the same time. For the exact same reason there are noise cancelling headphones (neutralize sound at your eardrum) but not noise-cancelling devices fields that can make a whole airplane go silent. Something that makes destructive interference at one point will make constructive interference nearby.
Of course if you want to block EM waves, that's easy and you can do it for cheap. Surround your phone in metal (it can even have holes in it), creating a Faraday cage. It should be noted that shielding of EM waves also makes the phone not work - phones aren't magic, and that "radiation" is what they use to communicate. Block it and of course they stop working. If you want to see this effect on a large scale, just walk inside an earthquake-proof building that has lots of rebar (and doesn't have fancy cellular repeaters) - you'll see your signal strength die pretty rapidly.
