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Comment by grahamburger | Hacker News Reader

How fun! Once, after climbing around in a dusty old church attic like this, I had to head straight to the airport for a flight. Barely made it on because something in the dust kept flagging as dangerous. They were using a test strip on my backpack and clothing and analyzing it in some kind of machine. Stressful as I was already pressed for time! Finally got a clean test and they let me through.

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These things are also notoriously prone to false negatives. I know a person who uses the same laptop briefcase to take their handgun to the local indoor shooting range, with magazines and ammo, and also uses that bag as a carry on for travel.

The bag goes right next to the shooting bench at the firing line. It presumably gets a decent amount of powder residue on it, and yet it's been swabbed several times and not set off anything. My buddy is scrupulously careful to remove anything non-airport-approved before taking it on a trip, so the fact that something "bad" was previously contained in the bag has been double/triple checked to be absent before they show up to the airport.

Heard similar stories from many people. I have my own--

I was at a range shooting this old revolver with worn seals. Every shot, unburnt (or sometimes burning) powder was flying backward onto my face and arms. I basically took a gunpowder shower.

Got up like 6 hours later and went to the airport to get on a plane. Got swabbed. Came back negative. Got on the plane.

So... yeah, not really sure what the swabs are doing. I have to assume it's more an issue of "they're not intending to detect that" than "failures to detect" because otherwise the repeated misses here are just a little too big to believe.

Most likely there is no quality control for the security process so no one has noticed that the swabs aren't working properly. Most such processes do not have any QA or QC as far as I can tell and the people actually doing the work have no interest in the effectiveness of the process or influence over it.

The swabs are looking for mercury as well as nitrates and things. A small bit of mercury can do a lot of damage to an airframe - nitrates too of course, but in larger quantities.

If you've been around mercury vapor, you'll set off alarms at the airport. Old church attics are often quite heavy metal.

Also, airports in places with free/open gun culture tend to have their weak spots, I'd imagine, when it comes to false positives ..

Is the solution to a positive swab really to keep swabbing until negative?

Apparently! That or I just have a trustworthy face haha

If this was in the US, it's all security theater. Basically if you get flagged, they swab everything in your bag to find what's getting flagged, and if it's legitimate they pass you through, or they keep swabbing you if its just your hands/arms because they also inspect your bag. It's kind of dumb, but also kind of makes sense.

For extra fun times, baby wipes / wet wipes get flagged as explosives in the US when they use the sniffer devices.

I will be the first to join you on "most of TSA is security theater"; the baby wipes are glycerine getting flagged by ion mobility spectrometry. The machines are fast because they just look for basic chemical building blocks, not doing full spectrometry to figure out the whole compound. So glycerin, nitrates, etc. set it off (and someone else's sticky glycerin e.g. sunscreen can gunk up the machine for multiple subsequent people).

Those guys must have a lot of fun with vapes. The liquid is 50-70% glycerine, and you get the stuff everywhere.

The system is also likely designed to have false positives so they can validate that the security people are doing their job. This is more obvious on the x-ray machines but resembles the "can you spot the phishing email" emails that often get sent out in corporate environments to detect people who need additional security education.

Ion mobility spectrometry is the tech that powers these, and they're VERY sensitive and VERY prone to false positives because it's just looking for ion signatures ("I smell a nitrate/glycerin/etc."). I'm not sure if they're changing modes on the machines between swabs or something, but the machine is a cue for more in-depth inspection, not "zomg I found a bomb". So, some repeated scanning to rule out a false positive or burn off gunk that's accumulated in the machine i.e. we just got a speck of something on the wand or something is gunking up the scanner as opposed to "someone covered in residue that is scary".

TIL! I appreciate the explanation, never considered them to be that sophisticated.

Also, it's not illegal to travel when covered in nitrate dust. Maybe you really were working with explosives or fertilizers earlier in the day. So it really should be just a flag to check that you don't still have the explosives with you.

Do you know if they also scan for drug residue using those devices? A while back I was travelling through a German airport, and got pulled aside to get the skin on my stomach and the inside of my bag swabbed, and I assume tested in the same kind of machine. I don't know what I did to warrant a further inspection, but I wasn't going to make a fuss. I sometimes wonder what would have happened if they'd pulled aside one of the guys in my group who were definitely using drugs

Or to assume it's a false positive.

Shortly after 9/11 I was flying out of Hawaii back to the mainland and I watched the screen when they swabbed my bag. It reported as positive for every single type of explosive (to the best of my knowledge, I was not carrying anything even remotely explosive). The tech looked at the results, said "that can't possibly be right" and waved me on.

In my experience if they test positive the tech will stop and wipe down the machine before trying again. They're highly prone to false positives.

Heathrow I'm guessing ?

In my experience, UK airports seem to not want you to leave the country — contra to the perception the government project regarding not wanting you to stay in the first place. :/

Had a number of experiences where their identity scanners (for some destinations they scan you at the start of security and just before the gate to ensure you get on the right aircraft) failed to be operational resulting in delays that ultimately may have forced me to miss my flight and stay in the UK. On the other hand, arriving from Ireland to the UK results in no checks because of the common travel area — you get waved through a special gate which is reserved for passengers from Ireland. LOL

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