[RFI] RFID tags in library books?

[Alan NV8A (ex. AB2OS)]

Read in the paper today that Kent District Library (W. Michigan) is 
going to put RFID tags in its 1,000,000 items.

Do RFID tags transmit all the time, or do they only respond to a 
"Who/Where are you?" signal? If these things transmit continuously and 
multiply on this scale, that amounts to a mass of RF at their operating 
frequency -- which is . . . ?

Alan NV8A