Why did airport security swab my hands with wet paper?
Before entering a commercial plane, I had to go through the usual security check at Prague airport (PRG). When I passed the metal detector, it went off. I assume it was because I forgot to take off my wedding ring.
I was approached by the security guy and instead of a full check (which I was expecting), the security guy marked both sides of my hands with some piece of wet paper, told me "wait there", left with that paper (maybe checking something with the paper, I do not know exactly) and after some time he told me I was free to proceed to the Duty Free zone behind the security check.
While relieved, I am wondering: Does anyone have a clue what kind of security check was that?
There are also places which do security at a vehicle checkpoint where instead of swabbing your hands, they swab your steering wheel, on the assumption that most people never clean their steering wheel.
The likelihood of the metal detector being set off by a wedding ring is extremely low. I travel for work (flying every two weeks) and I never take my ring off for security. There is a random sample of passengers who are selected for extra screening, irrespective of metal detection. I don't know about Prague airport, but the machines at Heathrow and Stansted in the UK have a different bleep for that. If there is a single bleep that sustains as long as you are in the machine, it has detected metal. If it bleeps a few times then it has just selected person for screening.
As others have said, it was explosive trace detection. If you've been handling explosives (except for a few types) it will pick that up. Unfortunately it will also pick up some innocent things--common culprits are hand cremes containing glycerin and fertilizer (say, by walking across recently fertilized grass. Unfortunately, ammonium nitrate is both plant food and very stable high explosive.)
Given all the comments about false positives, rather than calling this an _exposives test_, they should call it; _A test for chemicals, some of which are found in explosives but often are found in innocuous everyday materials._
I had a job where I handled all manner of explosives quite frequently. My backpack, keychain, tools, laptop and even some clothes I had never worn to work would frequently swab hot at airports. Not all of these systems are very good, but quite a few of them are really sensitive and actually do work.
Often at security they swab my backpack and some of my possessions with a small piece of damp paper. It is supposed to pick up traces of the materials I've been handling. Then they put it in a machine that analyzes those traces. At customs and immigration it's set to detect various illicit drugs (I've seen this on Border Security) but at security I believe it's set to detect explosives.
Certain people are chosen randomly for a swab. Since you had no residue on your hands that would require them to inspect you further, they sent you on your way. This wasn't caused by you leaving a ring on. Leaving a wedding ring on is fine.
I recently chatted to the Police guy in TXL that was doing this to me. He said it's only for explosives, as the residue of drugs would not be strong enough for the machine to pick up. He swabbed my hands, my belt and the inside of my backpack. He also let me watch how he put it in the machine and laughed at my comment that it's easier than blowing for an alcohol test.
I always find it funny because all you would need to do is _wash your hands_ before the swab.
@Insane, actually, washing your hands is not all that efficient. It will always leave some percent of the traces and if the test is sufficiently sensitive (which it is), it can probably pick the traces after washing your hands a couple of times. The kind of sensitivity was available in chemical labs for years; they just had to put it into sufficiently automated device so they can operate it without a chemistry major.
@Insane further, they typically swab *stuff you've been touching* with the theory that you touched that stuff before you washed your hands. Not foolproof, but since it's quick, they can do it a lot, and perhaps it occasionally turns something up.
@Insane, also water is not that good solvent for the substances they are looking for and soap only helps with fats. I would expect the "damp" towel to be impregnated with solvent (alcohol, toluene…) specifically selected to work well for the target substance.
@Insane, we are saying that the measure is effective and I believe that is generally OK, because part of the effectiveness is that it forces the potential wrongdoers to complicate their plans, which increases the risk of failure or disclosure at other points (the shoe and slips bombing attempts are considered example of this; they managed to make bombs small enough to evade detection, but that just made them too small to actually cause significant damage).
@Insane Besides what everyone else mentioned about washing, it is also worth noting that doing this might actually make it more likely for you to test positive. If you use soap or lotion that has glycerine, you can actually test positive for explosives and get a (very thorough...) searching. This just happened to a good friend of mine last month when we were traveling out of country. She had just applied lotion before going through security and earned herself a full cavity search.
Sometimes it doesn't even have to be something you have applied. I used to work at a business lounge where were served miners. The soap we used was glycerine free and was used because it didn't set the detectors off. One day after an 11hr shift, I tested positive for TNT coming back through security (forgot my keys). They re-did the test and it disappeared. Sometimes trace can be left on you from others, the air or things you have brushed passed. Likewise so can the detector pad.