Airport scanners detect a wide range of concealed items, from metal weapons and plastic explosives to liquids, ceramics, and even traces of chemical residue on your hands. Modern airports use several different screening technologies, each designed to catch what the others might miss. Here’s how each one works and what it’s actually looking for.
Walk-Through Metal Detectors
The archway you walk through is the oldest and simplest screening tool. It generates a low-frequency electromagnetic field (up to about 50,000 Hz) between the panels on either side. When metal passes through that field, it creates a disturbance the machine registers as an alert. This catches knives, firearms, and other metallic objects reliably.
The limitation is right in the name: metal detectors only detect metal. Plastic explosives, ceramic knives, liquids, and other non-metallic threats pass through without triggering an alarm. That’s why most airports now pair metal detectors with more advanced body scanners or use them as a quick preliminary screen.
Full-Body Millimeter Wave Scanners
The booth-style machines you step into at most U.S. airports are millimeter wave scanners. Your body naturally emits millimeter wave energy, but metals, plastics, ceramics, liquids, and gels emit far less of it and reflect ambient energy differently. The scanner reads those energy differences to build an image revealing the location of anything concealed under your clothing, whether it’s metallic or not.
These scanners detect a broad category of threats that metal detectors miss: plastic explosives, ceramic weapons, composite materials, hidden liquids, and gels. They’re also the machines most likely to flag harmless items. In testing, 39 percent of all passengers triggered false alarms, most caused by sweat, buttons, or folds in clothing. Millimeter waves reflect off water, so perspiration alone can look like a concealed object. Cargo pant pockets, kangaroo-style hoodie pockets, and layered clothing are frequent culprits.
If you get flagged, a TSA officer will typically do a brief pat-down of the specific area highlighted on the screen. It doesn’t mean the machine found something dangerous. It means it found something it couldn’t identify.
Privacy Protections
Early versions of these scanners produced detailed images of each passenger’s body, which raised significant privacy concerns. Current machines use Automated Target Recognition software that replaces the image of your actual body with a generic outline. Anomalies show up as highlighted boxes on that generic figure. The screen is located right next to you at the checkpoint, not in a separate room, and no individualized body image is generated or stored.
CT Scanners for Carry-On Bags
The conveyor belt your carry-on rides through increasingly uses computed tomography, the same basic technology behind medical CT scans. These machines rotate an X-ray source around your bag to create a detailed 3D image. TSA officers can spin and zoom that image on a touchscreen, examining your bag’s contents from any angle without opening it.
CT scanners are particularly good at identifying explosives because they can measure the density of individual objects inside your bag. They also distinguish between types of liquids, which is why airports equipped with CT scanners have begun relaxing the old 3.4-ounce liquid rule in some countries. The 3D imaging makes it easier to spot threat items hidden among dense, cluttered belongings like electronics, toiletries, and snacks.
Older X-ray machines, still in use at many airports, produce flat 2D images and rely more heavily on the operator’s judgment. They use color coding to indicate material categories: organic materials (food, paper, plastics) typically appear orange, metals show up as blue or green, and very dense items appear dark. Both systems flag items for manual inspection when something looks suspicious.
Explosive Trace Detection Swabs
Sometimes a TSA officer will swipe a small cloth pad across your hands, laptop, bag handle, or shoes. That swab is collecting microscopic particles, residue too small to see, and feeding it into an analyzer that can identify the chemical signature of explosives. Even if you washed your hands after handling explosive materials, trace amounts of certain salts and compounds can linger on skin and surfaces.
MIT Lincoln Laboratory developed a chemically treated version of these swabs with an acidic coating that activates when exposed to moisture. It converts hard-to-detect inorganic salt residues (the kind found in certain homemade explosives) into vapors the analyzer can easily identify. This technology is especially useful for catching threats that wouldn’t show up on a body scanner or X-ray, like someone who assembled an explosive device before arriving at the airport.
These swabs can also detect traces of narcotics, though airport screening is focused on security threats rather than drug enforcement.
What Scanners Cannot Detect
No single scanner catches everything, which is why airports layer multiple technologies together. Metal detectors miss anything non-metallic. Millimeter wave scanners see what’s on your body but not inside it. X-ray and CT machines examine your bags but rely on density analysis and operator skill to distinguish a water bottle from a liquid explosive. Trace detection only works if residue is present on the surface being swabbed.
Items hidden inside the body are largely invisible to checkpoint scanners. Liquids in containers that match the density profile of benign substances can also be difficult to flag. The screening system is designed so each technology covers the blind spots of the others, supplemented by behavioral observation, canine units, and random secondary screening.
Medical Devices and Implants
Joint replacements, surgical pins, and other metal implants will trigger walk-through metal detectors. Millimeter wave scanners may also flag the area around a medical device simply because it creates an anomaly under your clothing. Neither situation is a problem, but expect a brief secondary check.
Pacemakers deserve more caution. Dual-chamber pacemakers are susceptible to electromagnetic fields, and earlier models are specifically contraindicated for use near magnetometers. The short duration of a scan makes interference unlikely, but the general guidance is to move through the scanning area without stopping or lingering. If you have a pacemaker or similar implanted cardiac device, you can show your device ID card and request a hand screening instead.
Insulin pumps and continuous glucose monitors are generally safe through millimeter wave scanners and metal detectors. However, some manufacturers recommend unlinking wireless-connected devices (like a pump paired to a glucose meter) during air travel. CGM devices are approved for use through standard U.S. airport screening.
Radiation Exposure
Millimeter wave scanners, the type used in virtually all U.S. airports today, do not use ionizing radiation. They use radio-frequency energy similar to what your cell phone emits.
Backscatter X-ray scanners, which were once common but have been largely phased out, did use low-dose X-rays. Even those delivered only 0.03 to 0.1 microsieverts per scan, equivalent to about 1 to 3 minutes of the radiation you absorb naturally during a flight. You would need roughly 1,000 backscatter scans to equal the radiation dose of a single chest X-ray, and 200,000 scans to match one abdominal CT scan. A six-hour flight exposes you to about 14.3 microsieverts, dwarfing the scanner’s contribution by a factor of more than 100.