Why Can We Only Take Liquids On Planes In Tiny 100ml Bottles?

Believe it or not, four 100ml bottles is safer than one 400ml bottle.


Dr. Katie Spalding

Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory.

Freelance Writer

four small cosmetics bottles cleared for airport security, yellow background
Depending on where you live, these lil guys might soon be a thing of the past. Image credit: Ana_Malee/Shutterstock

Love it or (more likely) loathe it, there’s certainly a routine to boarding a plane. Every day, millions of us line up in snaking queues, separate out our electronics from the rest of our luggage, take off our belts, shoes, and jackets, and desperately try not to make eye contact with the TSA agent who is now busy feeling us up in public. It sucks, but that’s air travel in the 21st century.

One of the many, many rules we abide by to get in the air regards liquids in hand luggage. In countries around the world, there’s a hard limit on how much you can bring, at least in a single container: 100 milliliters (3.4 ounces) or less, submitted through security in a see-through bag ready for inspection.


If you’re one of our younger readers (say, 25 or under) you may not realize it wasn’t always this way – but in fact, the 100ml liquid rule is only less than a couple of decades old. And, to the likely relief of flyers everywhere, it’s already reaching the end of its lifespan.

So, here’s the big question: why did the rule ever exist in the first place? 

Why was the 100ml liquids rule originally brought in?

Liquids in quantities larger than 100ml have been verboten on commercial air travel since 2006 – and like so many flight regulations introduced in the decade following 9/11, it was originally an anti-terrorism measure.

“Intelligence […] suggested that a plot was in existence to blow up transatlantic passenger aircraft, in flight,” said Peter Clarke, then deputy assistant commissioner of the Metropolitan police, in the wake of the foiled transatlantic aircraft plot of 2006. 


“This was to be achieved by means of concealed explosive devices smuggled onto the aircraft in hand baggage,” he explained.

Specifically, the plan was to involve liquid explosives brought onto planes disguised as soft drinks. Plotters carrying acetone peroxide, also known as TATP or (genuinely) “Mother of Satan”, would board the aircraft, before injecting hydrogen peroxide into the bottles. From there, a trace amount of high explosive, contained inside an AA battery combined with a camera flash as detonator, would create an explosion big enough to blow through the fuselage of all of the seven planes targeted in the attack. 

While it wasn’t the first scheme aiming to bring down a plane in such a way – the Bojinka Plot of 1995 also used liquid explosives, and had it gone ahead would have resulted in the deaths of around 4,000 people including the Pope – it was the first since 9/11, in the newly-paranoid world of the War On Terror.

Luckily, the plan was intercepted before it ever went ahead – but had it been successful, experts reckoned that the “smuggled inside beverage containers” method would have left “little or no forensic evidence showing how they had done it.” Immediately, travelers out of the UK were banned from taking anything other than essential travel items as carry-on luggage, while in the US, virtually all liquids were forbidden. 


Before long, however, both countries had instituted rules that allowed liquids in hand luggage so long as they were stored in bottles no larger than 100ml. Soon, the rest of the world followed suit – and within a few months, this new standard for air travel had spread pretty much around the globe.

Why can I take four 100ml bottles on an airplane but not one 400ml bottle?

As justifiable as this limit on fluids originally was, if you’ve ever found yourself impatiently decanting a second or third tiny bottle of shampoo to store in your hand luggage, you’ve probably wondered the same thing: how come I can bring a bunch of small bottles on board, but not one larger bottle? 

After all – couldn’t some potential terrorist just pack, say, four or five smaller amounts of liquid explosives, then detonate all of them? Surely it’s the total amount of liquid brought on board that matters, rather than the size and number of containers it’s in?

It’s a natural assumption, but, surprisingly, it’s wrong. In fact, former director of the TSA Kip Hawley told the New York Times back in 2007, the size of the bottle is actually more important than the liquid inside.


“With certain explosives you need to have a certain critical diameter in order to achieve an explosion that will cause a certain amount of damage,” he explained. “The size of the container itself is part of the security measure.”

Still, couldn’t a potential attacker simply mix multiple bottles together in a larger container once airborne? Well, in theory, yes, but here’s the thing: liquid explosives are, sort of by definition, pretty unstable substances – and trying to mix them on board or in an airport bathroom might just blow up in your face. Literally.

“[TATP] explosions are known to be about 80 percent as strong as TNT, but the substance is much harder to handle,” explained Laura Finney, a chemistry doctoral student at the University of Nottingham, in The Conversation in 2017. “A firm shock or knock is enough to trigger an explosion, which means it’s quite easy to accidentally blow yourself up in the process of making it.”

Even sitting on the shelf, TATP is dangerous – and explosives experts charged with advising on safety measures after 2006 determined that it would be highly impractical for would-be terrorists to try to combine liquid explosives before or during their flight. 


Rather, they concluded, dealing with the kinds of substances necessary to blow up a plane in quantities of 100ml or less would more likely end with a plotter either screwing up the recipe – or harming themselves rather than other passengers. 

How long will the 100ml liquid rule be around?

As inevitable as it may feel today to fly under the strange collection of rules and arguable civil rights abuses of the TSA, it wasn’t all that long ago that no such restriction existed.

Similarly, they won’t be here forever. In fact, in some places, the 100ml rule for liquids has been bent or broken for a while now.

“Medicines, special foods or breast milk… can already be brought in the cabin in quantities over 100ml,” Genoa Airport press officer Nur El Gawohary pointed out back in 2017. In that north-western Italian city alone, another exception had just been added: pesto sauce for pasta.


“Every year hundreds of pesto jars were seized at security controls and thrown away – a waste of food and an annoyance to our passengers,” explained El Gawohary. “We use the same equipment [to check the pesto] that is used to check [other liquids].”

But that raises an intriguing question, doesn’t it? If airports can scan liquid medicines, or pasta sauce, in quantities larger than 100ml, then why do we need the limit at all? 

In fact, we kind of don’t.

In 2019, the UK government announced the rollout of 3D baggage screening equipment that would, officials said, soon put an end to the 100ml liquids rule. Since then, a handful of airports in Western Europe have already ditched the limit thanks to this new tech. In Shannon airport, western Ireland, scrapping the rule in 2022 “halved the time… passengers spend going through security screening,” per The Times.


Since then, all major airports in Ireland have followed suit – and so, too, has England’s London City and Luton airports, with the promise of all major UK airports doing the same by June 2024. Amsterdam’s Schiphol airport, meanwhile, has had the technology since 2021 – though they still recommend obeying the 100ml limit, to make ongoing travel easier through other airports.

So much for Europe, but what about the US? How long before Americans can say goodbye to the 100ml – or, to use the local terminology, 3.4 ounces – liquids rule for air travel?

Well, unfortunately for those traveling out of the land of the free, the answer doesn’t seem to be “soon.” It’s not that the US lacks the technology: nearly 200 of the more than 5,000 airports in the US have already installed the new style of scanners. 

However, according to the TSA, there are currently no plans to remove the existing limits: “While we have [CT scanners] deployed at more checkpoints, we are years away from announcing a change to the current liquids rule,” an agency spokesperson reportedly told Semafor in December 2022.


Nevertheless, as the technology becomes more widespread, and the air travel industry continues to recover after the COVID-19 pandemic slump, experts believe we’ll soon see the 100ml liquids rule relaxed in more and more places.

“From a security point of view, [the new scanners are] able to make very accurate decisions about what the materials are in your bag,” Kevin Riordan, head of checkpoint solutions at Smiths Detection, the company that provides Shannon’s security equipment, told CNN last year. “That’s better security, better decisions.”

But installing these new security measures is “a continuing process,” he added. “It’s different in different global regions. It’ll happen at different speeds.”


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  • aviation,

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  • liquids,

  • explosives,

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