But the other privacy concerns that civil libertarians raise about the RFID tags are much harder to answer and somewhat more nebulous. While a privacy-conscious library like Berkeley can take steps to ensure that no one knows what you're reading, it can't get away from RFID's assignment of a remotely readable, trackable number to every patron walking out the door with a book.

"What we're talking about is real privacy pollution. They're polluting the social environment with the unnecessary ability for someone to be tracked," says Tien from the Electronic Frontier Foundation. "The number is unique. And you don't necessarily have to know what it corresponds to. That allows the individuation of the person. They are not secure. And they are remotely readable."

How remotely is debatable, since librarians -- and manufacturers -- stress that the chips and readers are designed to interact only in a limited range, so you won't accidentally check out the books of the person in line behind you. But privacy advocates point out that the reader, not the chip, gives RFID its real power:

"Passive RFIDs are essentially like reflectors. They get their power from the reader. It emits a field. So it's from that field that the chip or tag gets the power," Tien says.

Privacy advocates paint a host of nefarious scenarios: How would you like to be unwittingly and remotely identified at a political rally, for instance, if not by name, then by the book in your backpack? If you're arrested hours later, would the book's RFID number be enough to prove you'd been active at the protest?

And one needn't hack into a library database to connect a specific number to a specific book. All you have to do is check out that book yourself and read the tag. So, if the FBI really wants to know who is reading "The Anarchist Cookbook" or the Koran, they need only check those books out of the library, check them back in, and then install a reader to capture that number on a street outside.

"You take the item out. You read the tag. And now you have a little database, capturing whenever someone else takes out this book," says Deirdre Mulligan, director of the Samuelson Law, Technology and Policy Clinic at UC-Berkeley's Boalt Law School, which worked with the Berkeley Public Library to develop its privacy guidelines around RFID.

But Mulligan doubts that a book in itself makes such a hot tracking device for monitoring political dissidents. The system just isn't very efficient -- people leave their library books at home or in their cars and give them to their kids. Still, she admits that the tags make surveillance via library books possible, if not probable: "It's not very likely, but there is always the outside chance."

Privacy advocates like the idea of tags that are rewritten with a new number every time a book is checked out. But, while technically possible, that's not what is going into the books on the shelves today.

Libraries aren't the only place that RFID is sparking privacy concerns. In California, a bill to set privacy standards for implementation of the technology in retail outlets recently failed in the state Assembly. Other states, such as Missouri and Utah have considered similar legislation.

But even people who have privacy concerns about RFID see why librarians want it. "It really does make inventory management wonderful," says Karen Schneider, director of the Librarians' Index to the Internet, who has written critically about RFID.

From the librarians' perspective, the promise of RFID in libraries does not stop at self-checkout. In theory, with a system that works smoothly, a librarian can just scan a wand up and down the stacks to see if all the books are in proper order, a laborious and time-consuming process to do manually. And librarians will be able to easily track which books in the noncirculating-reference rooms are being used and re-shelved frequently, so they'll know if it makes sense to invest in another Oxford English Dictionary or another subscription to the Journal of the American Medical Association.

Schneider, who herself has been a librarian, found that in discussing the privacy issues in committee at the American Library Association, many large library directors got very defensive. "I feel like it's a battle I've lost," she says. "A number of large libraries implemented RFID before it really got on the privacy scope of anybody. By the time big well-heeled libraries had gone out on a limb to implement this expensive new technology, it was too late. They have a lot invested in no one saying: 'Oh, you know those 3 million books you just chipped? There's an issue with that.'"

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