found on other site....

Reading tags on individual store items will be tricky, however, and spies will find it hard to track items with handheld readers or those mounted in doorways. A sheet of aluminum foil is many times thicker than it needs to be to shield an RFID signal, said ThingMagic's Reynolds. The signals are also disrupted by human flesh, which is made mostly of salt water: an RFID tag inside your fist cannot be read.
RFID tag systems, said Reynolds, also suffer from limited range. The readers -- due to FCC tag power limits and design limitations on tag antenna sizes -- work at distances up to about 20 meters. "In practice we see far less read range than that," he said.
Reynolds said he'd favor a "modest increase" in the receiver sensitivity limits set by the FCC.
But he pointed out that a laminated anti-static bag, for example, would not be readable by even a more potent system.