Airport security is the most awkward, confusing, and tedious part of every trip! After spending an hour or so marinating in a long line, you and your bags are scanned with who-knows-what kind of technology, zapped with who-knows-how much radiation, and possibly wiped down with a little swab for who-knows-what reason. Well, we can’t make airport security suck any less.
But we can explain exactly what’s being done to you and your bags … what you’re being exposed to … and how the science of security works these days. When you get to the security line, one of the first things you see is probably the bag scanner. The scanner uses X-rays to detect objects inside your bag, as well as how dense they are.
One side of the machine emits both low- and high-energy X-rays, which pass through your bag and hit detectors on the other side. When the X-rays pass through your bag and its contents, some of them get absorbed. Objects with lower density, like stuff that’s made of organic materials, will allow more of the lower-energy X-rays to pass through.
And things with higher density will absorb most of the low-energy X-rays, and allow some of the higher-energy rays to pass through. Based on the X-rays that reach the detector, the machine generates an image that shows all the different objects in your bag, colored based on their density. If something is colored orange, for example, that means it’s probably made of organic material — that is, something that contains carbon.
That’s important for security to know, because explosives tend to involve organic compounds. The security team analyzes the image for anything suspicious, like the outline of a gun or a bunch of organic material hidden inside a shoe. With all those X-rays scanning thousands of bags every day, you might think that security personnel would be exposed to a lot of radiation.
But the X-rays are confined to the machine, so the amount of radiation that workers are exposed to is so low that they aren’t even required to wear badges that monitor radiation exposure. While your bag goes through the X-ray machine, you’ve probably been asked to step through some kind of scanner yourself. Until a few years ago, that scanner was usually a metal detector.
Metal detectors work by generating a current in a coil of wire in short pulses, each of which briefly creates a magnetic field within the detector. When a metal object passes through the detector, this magnetic field creates another current in the metal, which in turn generates another magnetic field around the object. The interference caused by this magnetic field is what sounds the alarm.
But these days, you’re probably not asked to step through a metal detector. Instead, you walk into some big machine and raise your hands. This machine is either a backscatter X-ray or millimeter wave scanner.
If the machine just looks like a flat wall, it’s probably a backscatter X-ray scanner. These use very small amounts of weak X-rays that only penetrate your clothes, and don’t go through your skin. The machine detects the radiation reflected by your skin and anything else under your clothes, and generates an image that easily shows anything you’re trying to conceal.
The amount of X-ray radiation you’re exposed to in a backscatter scan is extremely low, a tiny fraction of the amount you’re about to be exposed to by flying on a plane. But still, this technology isn’t used very often, and it’s banned in many countries. Instead, millimeter wave scanners are much more common.
They’re the cylindrical machines that look like futuristic phone booths, and they use the same basic principles as backscatter scanners. But instead of X-rays, they emit microwaves, which are non-ionizing, meaning that they can’t damage your DNA, and you don’t need to worry about exposure. Since millimeter waves can penetrate through clothing, they can still show if you’re hiding something that you shouldn’t be.
Once you make it through the scanner though, you might still be in for some follow-up tests. If security finds anything in your bag that looks suspicious, they’ll probably swab you for traces of explosives. A security officer might swab your hands, your shoes, or your bag, then place the swab in yet another machine for analysis.
This is the Explosives Trace Detection machine, or ETD. The idea here is that, if you’ve been handling explosives, or if there were any inside your bag, the swab will pick up traces of suspicious compounds. To analyze the swab, the ETD uses a process called ion mobility spectrometry.
The machine gives the molecules on the swab an electric charge, turning them into ions. Then, a gas carries the ions through a tube. Different ions will take different amounts of time to pass through the tube, depending on things like their mass and charge.
The machine then identifies different compounds based on how long it took them to move through the tube. So, airport security might be tedious and annoying. But at least now you know what’s happening to you and your carry-on.
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