Airport security is a system of checkpoints, technology, and procedures designed to prevent dangerous items and potential threats from reaching aircraft. It covers everything from the moment you show your ID to the screening of cargo loaded into the plane’s belly. In the United States, the Transportation Security Administration (TSA) oversees this process, while international standards are set by the International Civil Aviation Organization (ICAO), which first adopted formal aviation security rules in 1974.
How Identity Verification Works
Your airport security experience begins at the document check, where an officer confirms you are who your boarding pass says you are. Traditionally this meant a human comparing your face to your photo ID. Increasingly, airports are replacing that manual check with second-generation Credential Authentication Technology (CAT-2) scanners, which use facial comparison to match the person standing at the podium to the photo on their ID or passport.
This step is voluntary. You can decline the facial scan and have your identity verified by a TSA officer instead. If you’re enrolled in TSA PreCheck Touchless ID, the facial comparison technology lets you move through a dedicated lane without handing over a physical document at all. The system is used only at the podium to confirm your identity. It does not track your movements through the airport or store your image for other purposes.
What Happens at the Body Scanner
Once past the ID check, you enter the screening area. In the United States, body scanners use millimeter-wave technology, which emits low-level radio waves (non-ionizing radiation, the same type produced by cell phones and Wi-Fi routers). Two antennas rotate around your body and construct a fuzzy, generic 3-D outline. The image highlights anything on your person that isn’t skin or clothing fabric, like a phone left in a pocket or a belt buckle.
An earlier generation of scanners called backscatter X-ray machines used low-dose ionizing radiation. Those machines were removed from U.S. airports over a decade ago. The current millimeter-wave units do not use X-rays of any kind.
If the scanner flags an area on your body, a TSA officer will do a targeted pat-down of that specific spot. If you prefer not to go through the scanner, you can request a full pat-down instead.
How Your Bags Are Screened
Your carry-on bag goes through an X-ray machine while you step into the body scanner. Most airports still use traditional 2-D X-ray imaging, where an operator views a flat, color-coded picture of your bag’s contents. But TSA is steadily rolling out computed tomography (CT) scanners at checkpoints. CT machines create a detailed 3-D image of your bag that can be rotated 360 degrees on screen, letting the software automatically detect explosives, including liquid ones. This is the same type of imaging already used to screen checked luggage in the baggage handling area below the terminal.
One practical benefit of CT scanners: at checkpoints equipped with them, you can leave laptops and liquids inside your bag instead of pulling them out and placing them in separate bins. That speeds up the line considerably.
The Liquids Rule, Explained
The 3-1-1 rule limits liquids, aerosols, gels, creams, and pastes in carry-on bags to containers of 3.4 ounces (100 milliliters) or less, all fitting inside a single quart-sized clear bag. Anything larger must go in checked luggage. This rule exists because liquid explosives are difficult to distinguish from harmless liquids on a standard 2-D X-ray.
Two categories are exempt. Medications in any amount can pass through the checkpoint, though they may get additional screening. Infant and child nourishments, including breast milk, formula, and juice, are also exempt regardless of container size. You don’t need to place these items in your quart-sized bag, but you should tell the officer about them before screening begins.
Lithium Batteries and Other Restricted Items
Weapons, explosives, and incendiary devices are outright banned. But some common items fall into a gray area that catches travelers off guard, particularly lithium batteries.
- Under 100 watt-hours: Most consumer electronics (phones, laptops, cameras) have batteries in this range. These are allowed in both carry-on and checked bags with no quantity limit for personal use.
- 101 to 160 watt-hours: Larger batteries, like those in some professional video equipment, require airline approval. You can bring a maximum of two spare batteries in this range, and they must be in your carry-on.
- Over 160 watt-hours: Forbidden on passenger aircraft entirely.
- Spare batteries and power banks: These can never go in checked luggage regardless of size. They must stay in your carry-on, because a lithium battery fire in the cargo hold is far harder for the crew to manage than one in the cabin.
If you check a device with a rechargeable battery, it must be completely powered off, not just in sleep mode, and protected from accidental activation.
TSA PreCheck and Global Entry
Trusted traveler programs let you trade a background check and fee for a faster screening experience. The two most common options work differently.
TSA PreCheck covers domestic departures from U.S. airports. You keep your shoes, belt, and light jacket on, leave laptops and liquids in your bag, and go through a shorter line. It’s best suited to travelers who fly domestically and want to skip the most time-consuming parts of the checkpoint.
Global Entry includes all the benefits of TSA PreCheck, plus expedited customs screening when you re-enter the United States from an international trip. Instead of waiting in a long arrivals line, you use an automated kiosk. TSA’s own guidance: if you travel internationally four or more times a year, Global Entry is worth the upgrade. If most of your flights stay within the U.S., PreCheck alone covers what you need.
Cargo and Behind-the-Scenes Screening
Passenger screening is the most visible part of airport security, but a parallel system operates out of sight. Every piece of checked luggage passes through large-scale CT scanners in the baggage handling area before it’s loaded onto a plane. Air cargo, the freight shipments that share space with passenger bags in an aircraft’s hold, also undergoes mandatory screening. TSA considers the threat of explosives in inbound air cargo significant enough that all carriers must X-ray or otherwise screen cargo before it enters the country.
Since 2020, TSA has been field-testing advanced imaging systems, similar to medical CT scanners, specifically designed to screen small parcels for explosives. These systems produce images that software analyzes automatically, flagging anomalies for human review. Airlines must also follow strict chain-of-custody rules governing how cargo is accepted, handled, and transferred to prevent tampering between the screening point and the aircraft.
The Global Framework
Airport security in any given country is shaped by ICAO’s Annex 17, a set of international standards first adopted in 1974 and updated regularly since. Annex 17 establishes baseline requirements that all 193 ICAO member states are expected to meet, covering everything from checkpoint procedures to airport perimeter security. Individual countries then build their own regulations on top of that baseline, which is why the screening experience can vary noticeably between, say, the U.S. and the European Union. The U.S. tends to be more prescriptive about removing items from bags and about secondary screening procedures, while some European airports have adopted newer CT technology faster and already allow liquids to stay in bags at certain checkpoints.