A new breed of electronic billboards that can alter their text and graphics every hour monitors the radio-listening habits of passing motorists. According to the New York Times, the billboards' sensors detect "radiation leakage that is emitted when antennae are tuned to a given radio station." Is your Blaupunkt car stereo a Chernobyl waiting to happen?
Though the phrase "radiation leakage" may conjure up some rather nasty images, the phenomenon is relatively benign. The signals emitted by radio towers are, in essence, a form of radiation, commonly referred to as radio frequency radiation. These electromagnetic waves are picked up by your car's antenna and then converted into recognizable noise—music, talk shows, and the like—by the tuner.
Cheap car-radio antennae—known as "monopoles"—are not particularly efficient mechanisms, and thus not all of the RFR is converted into useful content. Antennae leak a significant amount back into the surroundings, and this is what is picked up by the billboards' sensors. Because radio stations broadcast relatively strong narrow-band signals, those sensors can differentiate the passer-by's radio choices from the countless other waves that emanate throughout the universe. NASA's SETI program, which is searching for signs of extraterrestrial intelligence by looking for narrow-band signals, operates according to similar principles.
None of this means that you should fear your car antenna as a health hazard. RFR is a low-frequency type of radiation, and is thus "non-ionizing"—that is, it will not alter the molecules that form your body. There is some scientific debate as to whether long-term, low-level exposure to RFR and other low-frequency electromagnetic waves can cause health problems. But the controversy centers on radio towers and other transmitters, not car-radio antennae.