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Passive tags are exactly that...they really don't transmit, they "reflect". the best way to describe it is imagine you're holding a signaling mirror, and when someone "interrogates" you with a flashlight, you flutter the mirror and send them a morse code serial number.
The tag operates off the energy it receives from the RFID reader transmitter. It causes the feedpoint impedance of it's antenna to change, causing energy to be reflected back to the reader, so the reader detects a the faint fluctuations in the reflections. There's no power source associated with the tag itself, and it doesn't store any energy.
If the amount of RF radiation you received during this process concerns you, is that cellphone in your pocket on?
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