Travelers are thrilled about the possibility that restrictions on liquids in carry-on bags at American airports might soon be relaxed. These rules often lead to inconvenience at security checkpoints. However, more than a third of airports across the United States have not yet upgraded their screening systems to effectively detect liquid explosives, which remain a threat to aviation safety.

While it can be frustrating to discard water and other drinks before passing through security, the real challenge lies in distinguishing between harmless items like hair gel and dangerous substances. This threat was nearly realized in 2006, when authorities in the United Kingdom thwarted a plot to destroy airliners using a homemade chemical mixture concealed in sports drink bottles.

Security experts remain concerned about the vulnerabilities that were exposed by that plot.

Recent remarks by Homeland Security Secretary Kristi Noem have sparked enthusiasm among travelers about the future possibility of carrying more than 3 ounces of toiletries, such as shampoos and gels, and not having to dispose of any beverages they purchase.

“The liquids, I’m questioning. So that may be the next big announcement, is what size your liquids need to be,” Noem said. “We have put in place in TSA a multilayered screening process that allows us to change some of how we do security and screening so it’s still as safe.”

A Transportation Security Administration spokesperson said Noem and the agency “are constantly looking for ways to enhance security, and improve the travel experience for the public.”

New liquids rules aren’t ready

Any changes will come through the same official channels that TSA used to announce this month that travelers can keep their shoes on at checkpoints. That change offers relief from a rule adopted after “shoe bomber” Richard Reid’s failed attempt to take down a flight from Paris to Miami in late 2001 with a small explosive device hidden in his footwear.

The limits on liquids were triggered by the 2006 U.K. arrests. Three ringleaders were eventually convicted.

A massive surveillance operation

Prosecutors told the jury in that case that authorities uncovered the plot after secretly searching the luggage of a man who was on their radar for security concerns after he returned from Pakistan and found the unusual combination of the Tang powdered soft drink and a large number of batteries, according to the BBC. That triggered the surveillance operation that eventually grew to include more than 200 officers.

Agents discovered what appeared to be a bomb factory in a London apartment where odd devices were being constructed out of drink bottles. The plot didn’t make sense until authorities discovered that the men were researching flight timetables and realized they were only trying to make an explosive big enough to rip a hole in a plane.

The meaning of coded emails to contacts in Pakistan only became clear after the arrests, when authorities realized that the quantities of Calvin Klein aftershave being discussed in messages matched the amount of hydrogen peroxide the conspirators had purchased.

Longstanding liquid restrictions

Ever since then, the TSA and authorities in other countries have limited carry-on liquids and gels to 3.4 ounces (100 milliliters) because officials believe that amount is too small to create an explosion capable of taking down a plane. The restriction covered all types of liquids, because X-ray machines at checkpoints couldn’t differentiate between explosive and harmless ones.

The United Kingdom was planning to ease its restrictions last year to allow people to carry up to 2 liters (about half a gallon) of liquid, but that was delayed because several major airports still didn’t have the new scanners that use computed tomography, commonly known as a CT scan, to produce clearer images.

Advanced new screening machines

The new machines allow screeners to switch between two-dimensional and three-dimensional images and turn them to see what may be lurking, said Johnny Jones, secretary treasurer of the chapter of the American Federation of Government Employees that represents TSA workers.

“It’s just a totally life-changing situation,” said Jones, who has worked as a screener since the agency was created. “It’s a difference of being able to see something that would have been hidden by something that you can now see clear as day with the new technology. It eliminates the guesswork.”

Many airports still use the old tech

The problem is that only 255 of the 432 airports across the United States have new CT scanners installed, the interim head of the TSA told Congress this spring. The biggest airports got them first, but they are not expected to be everywhere until 2043.

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It’s possible that a change in carry-on liquid policy could be implemented just at the biggest airports, but that could lead to confusion if travelers, are, say, allowed to bring full bottles on departure only to have them confiscated when they return.

The scanners cost more than $2 million apiece, and they are so much bigger than the old X-ray machines that sometimes floors must be reinforced and checkpoints have to be redesigned.

“I think you need to hold off till we get more systems deployed. I think this one’s a little too early,” said Jeff Price, who teaches aviation security at Metropolitan State University of Denver and has co-authored books on the subject. “The keep your shoes on — I can get on board with that provided we continue to do random screenings. But the liquids, I think we’re too early on that. There’s other layers of security. Yeah, I know that. But not too many of them that prevent this type of attack.”

Vulnerabilities in the TSA system

In the past the inspector general of the Department of Homeland Security has highlighted vulnerabilities in TSA screening that can allow things to slip through undetected.

A 2015 report found that TSA officers failed 95% of the time to detect weapons or explosive material carried by undercover inspectors.

But Jones defended the work of the screeners he represents, noting that since the agency’s creation, no plane has failed to reach its destination because of something that was missed.

“Nothing large has been able to make it through our system since we’ve taken over screening. We’ve protected the skies for 22 years,” Jones said. “Even if you have a slight miss, it doesn’t necessarily mean anything is going to happen on the plane.”