How new CT scanners ended the 100ml fluid rule at Heathrow

Heathrow has abandoned its 100ml liquid rule. This scanner technology has made it possible

New CT scanners can create a 3D model of your carry-on luggage, helping airport staff spot risks without requiring you to unpack or decant liquids into tiny bottles.

By Eric Sullivan edited by Claire Cameron

If you’ve traveled by plane in the last 20 years, you know the choreography of checkpoints: small bottles of liquid in a clear bag, laptop out, shoes off, pockets empty. This is the most universal travel ritual since pretending that the middle seat offers enough legroom. But things are starting to change: at London’s Heathrow Airport, one of the busiest airports in the world, the dancing is starting to fade.

Last week, Heathrow completed a massive security upgrade that allows travelers to keep electronic devices in their bags and carry liquids in containers with a volume of up to two liters, far more than the long standard limit of 100 milliliters. Thanks to technology: Better imaging and software have pushed checkpoints from two-dimensional X-rays to computed tomography (CT) scanners that build a three-dimensional model of your bag.

Why limit liquids on planes anyway?

On supporting science journalism

If you enjoy this article, please consider supporting our award-winning journalism by subscribe. By purchasing a subscription, you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

The maximum limit of 100 ml (3.4 ounces) for carry-on liquids was set in 2006 in brutal response to a foiled transatlantic liquid explosives plot. At that time, checkpoint scanners were actually digital shadow puppets. They produced 2D images in which a shampoo bottle and a dangerous substance could be difficult to distinguish, especially when such an object was buried under a tangle of charging cables and power supplies. The solution was a workaround: reduce the liquids to 100ml until the machines could handle it.

The new class of material is the CT checkpoint. Heathrow’s deployment includes systems such as Smiths Detection’s HI-SCAN 6040 CTiX, which captures more than one or two static angles. The scanner rotates an X-ray source around the bag, capturing an image approximately every half degree. That’s about 720 frames per rotation.

The system then reconstructs these slices into a high-resolution 3D model of the carry-on bag. Security officers can then scroll through the viewed data set, rotate the bag, zoom in front of a laptop, and inspect the carry-on bag for density and compositional clues that a flat image tends to blur.

The real upgrade is the algorithm

The real innovation of the new scanners, however, lies in the move to automated algorithms. The systems are C3 certified, a European standard that means the system meets a higher bar for detecting potential threats, including liquids, without requiring passengers to unpack everything.

In many settings, this allows controllers to stop searching for every bottle of sunscreen in a bag and instead focus on system indicators. The machine is less likely to be disrupted by all the mess we’re all carrying around, which has convinced regulators to start relaxing the rules in some places.

A word of warning: don’t throw away your Ziploc bags yet. While Heathrow has improved its security checkpoints, other airports are lagging behind. So even if you leave Heathrow with a Costco-sized bottle of sunscreen, your return airport will probably put you through the old routine.

The same is true in the United States. The Transportation Security Administration is aggressively installing CT scanners at airports, but changing policy is another matter — and any rule changes will likely delay until CT is widespread enough to avoid disparate protocols. So for now, on this side of the pond, American travelers are still stuck with a paltry 3.4 ounces.