Leaving aside the discussion of whether anybody suffers from
electromagnetic radiation exposure, here's the ultimate solution.
It's mu-metal, the gold standard for complete and total blocking
of electromagnetic fields.  One seldom sees it for sale in
small quantities.

http://www.goldmine-elec-products.com/prodinfo.asp?number=G16600A

Don't fold it or otherwise stress it.  It loses its blocking
properties when severely mechanically deformed.

I have no connection with the seller, except as a satisfied
customer.

Two or three of those sheets should be sufficient to line
a helmet.