EPCGlobal has published guidelines for how to ensure and maintain safe levels of exposure to electro-magnetic fields. It seems pretty clear that the 13.56 MhZ library RFID is generally emitting EMF well below levels of concern, but you may want to read this report for further data.
http://www.epcglobalinc.org/public/bestpractice/EPCglobal_Best_Practices_January_2007.pdf