Screening agents will soon begin swabbing airline passengers’ hands to check for explosive residue, the Transportation Security Administration announced on Wednesday. Can’t terrorists just wash their hands before going through checkpoints?
No. The Explosives Trace Detection equipment used to analyze hand and luggage swabs can detect trace elements of explosives down to the nanogram—or billionth of a gram. No matter how many times a bomb maker washes his hands with soap and water, or dry-cleans his clothes, he won’t be able to clean off all the explosive materials he handled the previous day or even that month. Explosive chemicals are quite sticky—partly because of their large size, partly because the chemicals tend to be reactive nitro compounds, which adhere easily to other molecules. Skin, clothes, bags, laptops—everything is a potential surface for the compounds to land and stick. Even if a bomb maker tried to avoid getting traces of explosive materials on himself—by wearing gloves, say, or having someone else package the bomb—it would be hard to evade detection entirely.
What exactly are the machines trying to detect? The official TSA list is classified. But it probably covers any commonplace explosive you can think of, including fertilizer, nitroglycerin (used in dynamite), C4, TNT, RDX, and PETN, the chemical used by Umar Farouk Abdulmutallab on Christmas Day.
There is plenty of room for false positives. Nitroglycerin, for example, is often used in heart medication. Anyone who works on a farm could also set off alarms with trace amounts of fertilizer. But the TSA has procedures in place to quickly identify false positives—like asking a passenger who sets off the alarm if he or she has a heart problem. And explosives manufacturers issue guidelines to employees—who work around chemicals all day—on how to minimize the chance for misunderstanding at the airport. They advise them not to wear their work shoes and clothes when they travel, for example.
Got a question about today’s news? Ask the Explainer.
Explainer thanks Brook Miller of Smiths Detection and Jimmie Oxley of the University of Rhode Island.
Become a fan of Slate and the Explainer on Facebook. Follow us on Twitter.
