Your sweat stains can set off TSA body scanners

USA Today

03-07-2025

Travelers enduring scorching temperatures across the U.S. and Europe – or who tend to perspire profusely – may find their sweat stains triggering body scanner alarms at airport security.
In a recent Reddit post, user u/ominous_pan said they couldn't figure out why they set off the millimeter wave scanner at a TSA checkpoint – the machine that creates 3D images of a traveler's body using radio waves. They claimed they had empty pockets and no body piercings.
"I flew for the first time in 15 years this week and both airports flagged my crotch at the arms up scanner," the user wrote.
In the comments, people suggested a potential reason was having sweat stains on her clothing, with several saying this has happened to them.
It turns out that sweat can cause a false alarm at airport security, according to Shawna Malvini Redden, an organizational researcher and author of the book "101 Pat-Downs: An Undercover Look at Airport Security and the TSA."
"The machine will send up an alarm if there's anything out of the ordinary," Redden told USA TODAY. "Conceivably, the machines could bounce off the sweat and cause a false alarm."
Here's what travelers should know about sweat stains setting off false alarms.
How could sweat set off the scanners at airport security?
The machines where you enter and lift your arms use what's called millimeter wave advanced imaging technology, which scans passengers' bodies with non-ionizing radiation frequency, according to the Transportation Security Administration. The waves reflect off the body to detect weapons hidden underneath clothing, but can also catch water or sweat, according to ProPublica. Thicker or more layers of clothing are also more prone to setting off the alarm.
In a statement, a TSA spokesperson confirmed that 'added moisture from a person's body can alter the density of clothing, so it is possible perspiration may cause our advanced imaging technology machines to alarm."
False alarms with these scanners are not uncommon, with wigs, turbans and certain hairstyles also being wrongly detected by the machine. (Travelers can deny this type of screening in favor of a physical pat-down, unless their boarding pass states they've been selected for an enhanced screening.)
Redden added that it's probably not the sweat itself but the "darker shadow" of the stain that's causing the false alarm.
"Due to privacy software that protects passengers from having naked images of them produced by the scanners, (TSA officers) get a notation about an anomaly's general location, but no information about exactly where or what the object might be," she continued.
What should travelers do?
If your sweat triggers the alarm, you'll likely be subject to additional screening, according to TSA.
It may be anxiety-inducing, but there's no reason to panic, said Redden. "Alarms happen all of the time and unless you're smuggling something actually dangerous, the worst that will happen is a manual search in the form of a pat-down," she added. "This is something you can request a private room for and usually only adds a few minutes to your day."