A new device that lets runners monitor their workouts could be used by others to track the wearers — including thieves seeking to know when people leave home, U.S. researchers warn.

The wireless device called the Nike+iPod fits into the runner's shoe and transmits a signal to an iPod nano, giving information about the speed and distance travelled.

But the same signal that reaches the iPod can also be received by others, according to computer scientists at the University of Washington who built low-cost devices that could determine whereabouts by using the Nike+iPod's transmissions.

"It is easy for someone to use the Nike+iPod as a tracking device," Scott Saponas, a doctoral student in computer science and lead author of a technical report posted online on Thursday, said in a press release.

"It's an example of how new gadgetry can erode our personal privacy."

Saponas and his fellow graduate students, as well as an assistant professor of computer science and engineering, were able to create five different tracking devices — one using Google Maps — for less than about $340 Cdn.

Apple and Nike have sold over 450,000 of the Nike+iPod since it hit the market in the summer.

The device has two parts. One is a chip the size of dinner mint that fits in the shoe and acts as a pedometer. The other fits into an iPod nano and stores the runner's information.

While items such as clothes or credit cards can contain Radio Frequency Identification (RFID) tags, most have a very limited range. But the researchers found the sensor in the shoe emits a detectable signal with a range of about 18 metres.

Such devices could be used by thieves to track when people leave their homes, the researchers said.

"There's a bigger issue here," said Yoshi Kohno, the senior author of the paper. "When people buy a toaster, they know it's probably not going to blow up when they plug it in.

"But when they buy a consumer device like the Nike+iPod kit, they have no idea whether the device might enable someone to violate their privacy. We need to change that."