Airport security screens for weapons, explosives, and other prohibited items that could threaten the safety of an aircraft or its passengers. But the screening process is layered, combining technology and human observation to catch everything from obvious threats like firearms to less intuitive risks like spare lithium batteries or dense blocks of fudge that look suspicious on an X-ray. Here’s what’s actually happening at each stage of the checkpoint.
Weapons, Explosives, and Prohibited Items
The core mission is keeping dangerous objects off planes. The TSA maintains a detailed prohibited items list organized into categories including firearms, sharp objects, flammables, and sporting equipment. In your carry-on, you cannot bring axes, box cutters, swords, ice picks, meat cleavers, brass knuckles, or bows and arrows. Many of these items are allowed in checked luggage, but never in the cabin.
Flammable and explosive materials are banned entirely from carry-on bags, and many are banned from checked bags too. This includes gasoline, butane, propane, gun powder, dynamite, flares, lighter fluid, and flammable paints or aerosols. Even sparklers are prohibited. The reasoning is straightforward: anything that can ignite or detonate in a pressurized cabin poses a catastrophic risk.
How Body Scanners Work
The full-body scanners used in U.S. airports rely on millimeter-wave technology, which sends low-level radio waves around your body using two rotating antennas. The scanner builds a 3D image that can reveal both metallic and non-metallic objects hidden under clothing, including ceramic knives, plastic explosives, or concealed packages that a traditional metal detector would miss.
These scanners do not use X-rays or ionizing radiation. The energy they emit is thousands of times less than what a cell phone produces, according to the CDC. The image generated looks like a fuzzy outline of a body, with any anomalies highlighted for the officer to inspect. If something flags, you’ll typically get a targeted pat-down of that specific area rather than a full search.
What the X-Ray Machine Sees in Your Bag
When your carry-on goes through the conveyor belt scanner, the machine generates color-coded images based on material density. Organic materials (food, paper, fabrics, plastics) typically show up in one color range, while metals and dense inorganic materials show up in another. Officers are trained to identify the silhouettes of weapons, spot wires that could indicate an explosive device, and flag anything unusually dense or opaque.
Newer airports are rolling out computed tomography (CT) scanners that create full 3D views of bag contents. These are the same basic technology used in hospital CT machines, adapted for luggage. When CT scanners are in use, you don’t need to remove laptops or travel-size liquids from your bag, because the 3D imaging lets officers digitally rotate and slice through the image to get a clearer look at layered items.
Explosive Trace Detection Swabs
If you’ve ever had a security officer wipe a small cloth pad across your hands, your laptop, or the inside of your bag, that’s explosive trace detection. The swab picks up microscopic particles from surfaces and is then inserted into an analyzer that can identify residues of explosive compounds.
These systems are particularly useful for catching homemade explosives. Some of the inorganic salts used in improvised bombs (chlorates and perchlorates) are difficult to detect because they don’t release much vapor on their own. Newer swab technology, developed at MIT Lincoln Laboratory, uses an acidic coating that reacts with moisture to convert those low-vapor salts into highly volatile acids the machine can easily identify. This is why you might get swabbed even when nothing in your bag looked unusual on the X-ray. The swab is checking for chemical traces that imaging alone can’t detect.
The 3-1-1 Liquids Rule
You can bring liquids, aerosols, gels, creams, and pastes in your carry-on, but each container must be 3.4 ounces (100 milliliters) or smaller, and all containers must fit in a single quart-sized clear bag. This restriction exists because liquid explosives can be disguised in ordinary-looking bottles, and current screening technology works best when liquid volumes are small enough to quickly analyze.
There are exceptions. Formula, breast milk, toddler drinks, and baby food are allowed in quantities larger than 3.4 ounces, even if your child isn’t traveling with you. These are classified as medically necessary liquids. You should let the officer know at the start of screening and remove them from your bag for separate inspection. TSA may test these liquids for explosives, but they won’t open the container or insert anything into the liquid. Using clear, translucent bottles can speed up the process.
Lithium Batteries
Lithium-ion batteries are a specific concern because they can overheat, spark, or catch fire, especially if damaged. The FAA sets limits based on watt-hours: batteries up to 100 Wh are allowed freely on passenger aircraft, batteries between 101 and 160 Wh require airline approval, and anything over 160 Wh is forbidden entirely. For reference, most phone and laptop batteries fall under 100 Wh, while larger power banks or professional camera batteries may exceed that.
Spare lithium batteries must always go in carry-on luggage, never checked bags. The reasoning is practical: if a battery catches fire in the cabin, the crew can respond immediately. In a cargo hold, a lithium battery fire can spread undetected. Damaged or recalled batteries are banned from the aircraft completely.
Behavioral Screening
Security isn’t only about technology. Behavioral detection officers are positioned at checkpoints, particularly at high-interaction points like the document check, where they can observe passengers up close. These officers watch for behaviors that stand out from the normal baseline of how people act in that environment.
Specifically, they look for verbal and nonverbal signs associated with fear of discovery, the stress of concealing something, or anxiety tied to carrying out a planned act. This can include unusual nervousness, evasive responses, excessive fidgeting, or reactions that seem disproportionate to the situation. The challenge, as DHS research acknowledges, is that many of these same indicators show up in people who are simply anxious flyers, running late, or stressed about unrelated life events. Behavioral screening is a referral tool, not a verdict. It may lead to additional screening, not automatic detention.
Everyday Items That Trigger Extra Screening
Some perfectly innocent items look alarming on a scanner. Dense organic materials are the most common culprits because they can resemble plastic explosives on an X-ray. Coffee beans and ground coffee get flagged frequently. So do blocks of cheese, candles, fudge, gummy candy, loaves of bread, bags of cornmeal, and trail mix. A TSA officer described any “large, unidentifiable organic material” as something that will look bad on the screen.
Tightly packed items also cause problems. Stacks of business cards, sealed board game decks, and multiple laptops layered on top of each other can appear opaque to the scanner, making it impossible to see what’s behind them. That’s a guaranteed bag search. Contact lens solution, while explicitly allowed, looks identical to any other liquid container and will typically get tested. Even clothing can trigger the body scanner: metal caps on drawstrings, underwire bras, or a slipped bra strap have all caused pat-downs.
Some items trigger the explosive trace detector rather than the imaging scanner. Deodorants containing aluminum have set off newer machines. Fresh printing inks on brand-new playing cards have tested positive on swab analysis. Knowing this won’t necessarily help you avoid the extra screening, but it explains why security pulled you aside when all you packed was a sourdough loaf and a bag of dark roast.