(FORTUNE Magazine) – In San Diego, garage-door openers froze up when residential neighborhoods were bombarded by stray naval radar. In Indiana, two hospitals banned cellular phones after suspecting that signals from the devices caused breathing machines and heart monitors to shut down. In Colorado, a paraplegic Korean war veteran was nearly killed when his motorized wheelchair, apparently driven haywire by nearby radio towers, went out of control and dumped him off a 23-foot cliff.

These are a few of the more dramatic results of overcrowding on parts of the U.S. radio spectrum. Cell phones, satellites, TV transmitters, weather radar, baby monitors, and remote controls for golf-course sprinklers are only a fraction of the millions of devices filling the air with ethereal emissions. As signals proliferate they sometimes clash, disrupting the workings of tools and toys run by microprocessors--a problem experts call electronic pollution. Companies that make products interference-proof say electronic pollution is costing industry billions of dollars a year.

For fear of electronic pollution, airlines order you to turn off your Walkman and laptop during takeoff and landing. Over the past decade NASA has documented more than 50 incidents in which computers, Nintendo sets, and other devices may have fouled up avionics gear. Some experts suspect onboard electronic interference as the cause of an unexplained computer failure that led to a 1991 Lauda Air crash in Thailand; 223 died.

Electronic pollution is a perfect example of what Princeton technology historian Edward Tenner describes in his new book, Why Things Bite Back: Technology and the Revenge of Unintended Consequences. Time after time, in fields ranging from farming to medicine to transportation, technological progress has created new, vexing problems that can be controlled only by what Tenner calls endless rituals of vigilance.

While the horror stories of electronic pollution are relatively rare, spectrum crowding is causing just the kind of costly, chronic headaches Tenner describes. According to vice president Michael D. Norton of Thermo Voltek, a booming Lowell, Massachusetts, firm that builds testing equipment, last year companies, and ultimately consumers, spent $17 billion worldwide to make products more resistant to electromagnetic interference. Underwriters Laboratories recently opened a new multimillion-dollar testing chamber the size of a two-story house that can handle large objects like cars and digital telephone switches.

Electronic pollution has grown exponentially in part because wireless communication is exploding. The number of U.S. cellular phone users, for example, grew by 1,500% between 1988 and 1995. Space on the U.S. radio spectrum is becoming so precious that the Federal Communications Commission may reap some $40 billion from its auctions of licenses to use slivers of the radio spectrum. At the same time, microprocessors have become ubiquitous. In cars, as many as two dozen chips regulate the transmission, brakes, airbags, and other components. Microprocessors also control CD players, clothes dryers, and self-flushing toilets.

How do radio waves and microprocessors clash? Besides processing information, chips and their associated circuitry inadvertently function as tiny radio broadcasting and receiving stations. The speed of microprocessors in portable computers is often measured in megahertz, the same unit of frequency used to identify FM radio stations. A 100-megahertz chip can process 100 million bits of information per second, and it can also "read" radio signals of that frequency.

If the chip is well shielded, everything works fine. But if a 100-megahertz radio signal finds its way inside, the chip can tune in to the equivalent of hash and go haywire. Or the chip may confuse a nearby wireless communication device by broadcasting its own 100-megahertz babble.

The problem gets more complicated because of harmonics, which are progressively weaker multiples of a given frequency. The chips in many portable CD players, for example, operate at 28 megahertz, with harmonics at 56, 84, 112, and so on--which makes them bad news on airplanes, where 112 megahertz is a navigation channel.

So far, foreign governments have been more active than Washington in controlling electronic pollution. Last year the European Union mandated that all electronic products be tested for their immunity to stray electromagnetic signals. Aside from some medical, avionic, and other specialized devices, the U.S. regulates only device emissions, figuring that the market will do a better job than government at weeding out products prone to interference. Until that happens, we'll have to endure more weird hiccups and glitches in the high-tech devices on which we have come to rely.