Fitness trackers — such as the FitBit, Jawbone UP24 and Nike+ FuelBand — collect data about your activity levels and sleep patterns, then send the information to your computer or smartphone via low-level radio-frequency waves.

“We have no information whatsoever on the long-term health effects of wearable fitness-tracking devices,” says Devra Davis, PhD, MPH, an epidemiologist and founder of the Environmental Health Trust. “The lack of proof of harm is not evidence of safety. You have to decide for yourself if that’s a risk you want to take.”

The non-ionizing waves emitted by wireless trackers are similar to those of cell phones, which have been classified as “possibly carcinogenic to humans” by the World Health Organization’s International Agency for Research on Cancer. The National Cancer Institute recommends limiting cell-phone calls that involve phone-to-head contact.

But unlike cell phones, activity trackers are meant to be worn on the body around the clock.