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Dear Matthew Alice:

Every time I go through the airport metal detector, the buzzer goes off and I am taken aside and checked by a hand detector. I am ordered to empty my pockets, which never contain anything more than a tissue or two. The last time I was there, I was wearing a sweat suit, sneakers, and no jewelry of any kind. That blankety-blank thing still buzzed. I see scads of people wearing jewelry and the thing doesn't buzz for them. What gives? Am I emitting some kind of signal? Am I made of something other than flesh and blood?

-- Alice Healing, San Diego

The elves have gone to get your medical chart to check that last point. In the meantime, here's what we have, after grilling three manufacturers of the walk-through-style metal detectors and making them empty their pockets into a little tray. The idiot's guide to detectors says they work by sending out radio signals that surround you with an electromagnetic field. When the field strikes that .38 special in your pocket, there is generated a tiny electrical current, which produces a radio signal, which is picked up by the detector's antenna. The signal is amplified and analyzed, then, maybe, the alarm goes off.

Detectors can be adjusted to many levels of sensitivity, depending on whether you're looking for something like a needle (very high sensitivity, generally used in prisons) or perhaps, at a department store, a small home appliance hidden in someone's hat. Detectors can also be set to zero in on certain metals and be less reactive to others. Steel, stainless steel, and aluminum are very popular suspicious metals at airports. Gold and silver will get you a free pass and maybe an extra martini on the flight. The machines can also be adjusted to find something the size of a gun but ignore small things like a car key or quarters.

The FAA sets standards for airport detectors, and they're supposed to be tested daily. The settings are low enough to keep watches and pens from setting off the machines but high enough to spot a handgun. And every detector-person we talked to said it is only metal that triggers the alarm. Do you wear shoes with a metal plate in the instep? Many boots, men's shoes, and most women's high heels have metal in them. Bobby pins? Rivets in your jeans? Many pierced-and-studded body parts? If the machine is more sensitive than necessary, even a bra with a metal underwire will do the trick. Do you somehow bump the machine as you pass through? This will occasionally set off some detectors.

But here's the harebrained theory we've all been waiting for: the "body capacitance" speculation, our ability to retain a tiny electric charge because of our faintly saline body fluids. Oddly enough, salt water looks like stainless steel to a metal detector, says one engineer. But he also says that the quality of pass-through detectors varies a lot, and he claims there is plenty of badly designed equipment in the field, prone to false alarms for a variety of reasons. He guesses you really do have some small bit of detectable metal on your person, and the machine is malfunctioning just enough to pick it up when it should ignore it.

But if you think you have troubles now, just wait until the engineers put the finishing touches on the vapor detectors that sniff air samples as we pass through. Coming soon to an airport near us?

And You Spend All Day Dodging Magnets

Got a quick reply to our wrestling match with Alice Healing's question about constantly setting off the airport's metal detector. Heather Campbell e's us, to wit: "I've heard of a man who suffered from a condition in which his body stored excessive amounts of iron; he regularly set off metal detectors. [It is] hemochromatosis, a rare and dangerous disorder."

According to my medical sources and two people who actually have hemochromatosis, you're a little off the beam. You're right, it's (usually) a genetic condition in which the body stores large amounts of iron in the liver and elsewhere. But people with the condition don't have little rusty chunks of metal stowed around their bodies. They don't clank when they walk. I'm told by two people who have hemochromatosis that this story wanders by from time to time, and to their knowledge it's completely untrue. Their doctors concur. They flew all the time before they were diagnosed and treated, and they never set off any metal detectors. So the story is just a very arcane urban myth.

Victoria's Secret

Hey, Mahatma: The woman going through the metal detector with the false alarms [5/11] is wearing an underwire bra. I've set them off in a computer chip factory.... I did have to "prove" to a female security officer that I really was wearing the alleged bra.

-- Yet another female engineer, San Diego

In the original answer we mentioned unmentionables as a possible, if unlikely, explanation. We were met with some skepticism. So imagine how happy we were to receive this real-life tale of extreme personal and professional humiliation, proof that your underwear can set off a hypersensitive metal detector.

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