Last time I checked, WHO's research claimed that EM radiation doesn't affect the human body at all. (EDIT: at least, the kind of radiation that mobile phones emit, thanks for correction. Btw, it is very nicely explained in the below posts.)

Still, they said better safe than sorry, so they made up a value which every device should stay below.

BTW, the SAR value of every phone is (and must be) publicly available from the manufacturers home page as well as the authority's home page which verified it.