What is a TSA Hand Swab?

Hurry up and wait. Remove your shoes and belt. Put your laptop in a bin. Strike a pose in the body-scanning security portal. Put your palms forward, please. Wait, what? TSA officers swab your hands with a cotton cloth to collect explosives residue for testing in an Ion-Mobility Spectrometer (IMS), the machine they put the cloth in that determines if you go to your gate or to a private security screening for a pat down and metal detector.

‌If you undergo hand swabbing at the security checkpoint, you might have set off the Explosives Trace Detector (ETD) alarm in the security portal. Maybe a TSA behavior-detection officer deemed your behavior suspicious. Or, maybe a TSA agent simply had to fill his daily quota of random checks and chose you.‌ The list is longer, but the TSA insists that its selection of passengers for swabbing for explosive trace detection is random. Either way, you’ll still have to go through the full body screening process including the x-ray machine for your carry-on bag.

The transportation security administration considers alarm resolutions – or figuring out what made the machine go "beep" – Sensitive Security Information (SSI), but you can find the info widely disseminated on the internet. You could also, within reason, thank the infamous and inept would-be bomber of a 2010 Northwest flight over Detroit for prompting all this hand-swabbing business. The following year, TSA implemented the program and spent an additional $60 million on ETD machines.

Security techs can calibrate an ETD, or "sniffer," to detect an array of gaseous organic compounds, including drugs, but the TSA maintains they check only for traces of explosive compounds. These compounds can include:

• NG (nitroglycerin)
• ANFO (ammonium nitrate/fuel oil)
• PETN (pentaerythritol tetranitrate)
• RDX (nitramide)
• TNT (trinitrotoluene)
• Tetryl (trinitrophenylmethylnitramine)
• Semtex (plastic explosives)
• Nitrates
• HMX (octogen)
• GSR (gunshot residue)

Even though the TSA doesn't advertise what sets off their ETDs, you can safely assume it's traces of anything that goes "boom."

You might have noticed that nitrates appeared on the list more than once – in particular nitroglycerine – with an emphasis on the "glycerine" part.

Glycerine, a common ingredient in everyday items as innocuous as soap and hand lotion, can set off an ETD and warrant a private check. Traces of nitroglycerine tablets prescribed for angina, in addition to fertilizer residue on shoes and golf clubs, can set off an ETD.

Calling an ETD sensitive is an understatement. IMS technology can detect explosives residue in parts per million, so picking up on a minuscule amount of glycerine in hand lotion is well within its capabilities. Your best bet for avoiding a false positive from a tsa swab and avoiding additional screening? Clean your hands with a glycerine-free hand sanitizer or alcohol before you reach the security point.