An RFID tag can be small enough to be fitted to an ant's back and often use little to no energy, activating only when they are close to a reader. They have been used for years to keep track of livestock, authenticate ID badges, manage inventories or pay bridge and highway tolls.
"I've been involved with RFID for 16 years, and this is the very first year that things are exploding," he said.
Privacy organizations have long criticized the use of RFID chips in documents and items that could be used to track people's movements, determine their identities or make inferences about their habits.
Shortly after the Wal-Mart announcement, privacy organizations said the tags remained active after shoppers took them home and disposed of them, and could be used by marketers or criminals to find out what a person recently bought - including purchases buyers prefer to keep private.
But privacy advocates worry about how retailers themselves might use the information and about the possibility of cross-referencing data from costumers' loyalty cards and RFID-equipped purchases that could effectively identify and track how often shoppers go into the store and where they spend their time. Once information is collected, it's not always clear how it will end up being used or abused," she said