Fire-resistant clothing is a hot choice. Here’s how it works.

PyroMan helps this company test the gear that soldiers wear to protect themselves from intense flames.

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If you’ve ever spent time hiking, you may have heard this dramatic saying: cotton kills. That phrase exists because of the fact that when you get the material wet—through sweat or precipitation—it wants to stay wet, because it’s hydrophilic. And if it’s cold and clammy out, the clingy, damp garment could start its wearer on the road to hypothermia. 

For that reason, outdoor explorers typically prefer to wear synthetic clothing. Shirts made from polyester, for example, wick away your sweat and don’t hold onto moisture well. A stinky tradeoff is that they tend to smell, even right after you’ve put it on and started working out—here’s an excellent explainer as to why that is the case. 

Odor aside, synthetic clothing has another huge drawback that’s more of a concern for people in the military than a typical weekend explorer. Clothing made out of polyester or nylon burns easily. Cotton will burn too, but not the same way that a synthetic will, which sticks to the skin as it melts. This fact means that anyone in the military who wants to wear outdoor-style gear has to find clothing that’s both synthetic and flame-resistant, two qualities that don’t generally coexist. Someone conducting search-and-rescue operations in the mountains and traveling via helicopters faces the same quandary. Fleece is nice, but it’s also flammable. 

“The holy grail of the outdoor world are nylons and polyesters,” says Steve Tacy, the director of textiles and technology at Massif, a company that makes flame-resistant clothing. 

Massif fire-resistant clothing being tested
PyroMan: Before, during, and after a test at North Carolina State University. Courtesy of Massif / TPACC at NC State University

“But if you look at the chemical makeup of nylons and polyesters, they’re basically fuel—everything in there burns, melts, and drips,” he adds. “And certainly in the military arena, that’s something you do not want. Anything that might adhere to your skin and become molten lava, basically, stuck to your body—it’s terrible.” And while cotton isn’t flame-resistant, at least it doesn’t melt and stick. A pilot wearing a flame-resistant flight suit should wear cotton, not synthetics, under that exterior garment.

[Related: The stealth helicopters used in the 2011 raid on Osama bin Laden are still cloaked in mystery]

It’s not just a hypothetical problem, as Senator Tammy Duckworth notes in her new memoir, Every Day is a Gift. “Nylon melts into the wound, as we found out when guys started getting hit with improvised explosive devices (IEDs) while wearing synthetic Under Armour T-shirts instead of the standard-issue brown cotton ones under their battle dress uniforms,” she writes. In other words, a synthetic Under Armour shirt should not actually be used under armor. 

That’s where flame-resistant clothing comes into the equation. This type of gear is different from what a firefighter wears, because it’s designed to endure a flash fire lasting seconds, and not protect a wearer for minutes in a hot burning building. 

Tacy, of Massif, notes that there’s two different types of flame-resistant clothing. The first category is what’s known as “inherent” protection. “Inherent fibers, from a chemical structure, do not lend themselves to supporting combustion,” he explains. Those have brand names like Nomex and Kevlar.

The second class is when a material is treated to make it flame-resistant. Massif can treat cotton or even nylon—that scary melty substance loved by backpackers and the like—to give it flame-resistant qualities. The company will employ both inherent fabrics and treated ones to create its gear. “Sometimes we combine those technologies into the same fabric,” Tacy notes. One of its well-known items is the Army Combat Shirt

The company subjects the gear to tests to evaluate its fire-resistance. The most dramatic involves hitting a sensor-equipped mannequin wearing the gear with flames for about four seconds. Temperatures reach around 2,000 degree Fahrenheit. The mannequin itself is known as PyroMan, and it’s part of the Textile Protection and Comfort Center at North Carolina State University. “You’re basically talking about four flame throwers that produce a wall of flame,” Tacy notes. A video provided by Massif shows PyroMan, clad in flame-resistant clothing, being immersed in flame—and then once the fire is out, the gear is smoking and charred, but not on fire. 

“If you had standard clothing on that,” Tacy says, ‘it would continue burning, melting, dripping—it would just be a mess.” 

Check out the video, below.

Rob Verger

Rob Vergeris the Technology Editor at Popular Science. He covers aviation, the military, transportation, security, and other complex tech topics. A graduate of Columbia Journalism School, he's also written for The Boston Globe, Newsweek, The Daily Beast, CJR, VICE News, and other publications. Contact the author here.