I assume that they are passive (not active) devices and respond only when in or near a card reader of some kind.
This being the case, they will not emit any form of radiation until "energised".
I do not think that they will pose any risk, especially at only 12 MHz.
The anti-theft devices attached to clothing in department stores and similar devices placed in books are also passive devices, which respond only when energised in the appropriate electro-magnetic field.
Interesting topic. I must do some more research on such devices to find out what current developments are.
There is some (fairly dated) information at , but not at first glance, specifically concerning the devices you mention.
Bookmarks