How Much Is That Shirt In The (Atmospheric) Window?

Summer is fading into a memory now, but as surely as the earth orbits the sun, those hot and sweaty days will return soon enough. And what can you do about it at the level of a single, suffering human being? After all, a person can only remove so much clothing to help cool off. Until someone figures out a way to make those stillsuits from Dune, we need an interim solution in which to drape ourselves.

We’ve seen the whitest paint possible for cooling buildings, and then we saw a newer, whiter and more award-winning paint a few months later. This paint works by the principle of passive cooling. Because of its color and composition, it reflects most light and absorbs some heat, which gets radiated away into the mid-infrared spectrum. It does this by slipping out Earth’s atmospheric window and into space. Now, a team based in China have applied the passive cooling principle to fabric.

Wait, What’s the Atmospheric Window?

Technically speaking, there are two atmospheric windows — one in the infrared spectrum, and another in the radio spectrum. For the purposes of this discussion, we are only concerned with the infrared window. But let’s back up a bit.

What gets absorbed. Image via University of Rochester’s Department of Physics and Astronomy

The energy radiating from the Sun includes much more than just the light we see and the heat we feel. Earth’s upper atmosphere absorbs gamma rays, x-rays, and some of the ultraviolet part of the spectrum. Think of the upper atmosphere as a blanket that protects Earth from these harmful rays.

There is a hole in the blanket — or a window, if you will — in the low end of the infrared range (roughly 8-14 μm) that lets in visible light and heat because none of the atmospheric gasses absorb that wavelength. That’s a good hole; it’s a hallmark of a habitable planet. So, this is what is meant by the atmospheric window — light and heat can go both ways. The window lets in light and heat, but more importantly, it also lets it escape. This way the planet is nice and warm, but not to a deadly degree.

Fabric of the Universe

So, back to the fabric. To reiterate, passive cooling materials work by reflecting almost all of the incoming light, keeping those energetic photons from heating the surface. These materials also absorb heat from whatever they’re covering and radiate it out.

Passively cooling something like a building is kind of a set-it-and-forget-it type of thing. Clothing, on the other hand, has to be flexible, breathable, and must stand up to repeated washings. So, how does this fabric work already? Much of its reflectivity comes from titanium dioxide powder, like the stuff in some kinds of sunscreen. These titanium dioxide nanoparticles are embedded in fibers made of PLA, which emit radiation (lose heat) in the mid-infrared spectrum.

Then the fabric is further coated with polyetrafluoroethylene (PTFE), which reflects the part of the UV spectrum that the titanium dioxide doesn’t. PTFE is hydrophobic, so it will repel water from the outside and sweat from the inside. The fabric is woven together rather than knit, and has a carefully-calculated pore size. In tests, the fabric reflected more than 92% of sunlight. The team half-covered a vest with their fabric and sat someone in the sunlight while wearing it. They monitored the person’s body with infrared cameras and found that the side covered by the fabric measured an average of 3° C cooler than the side without the fabric.

One Shirt, Please — With a Jolly Wrencher

This stuff seems to tick all the boxes — it’s breathable, washable, and (we assume), comfortable enough to wear all day in the desert. It’s also supposed to be biodegradable, but we have to wonder what effect sweat and laundry detergents and double rubs might have on the fibers’ ability to passively cool someone on a long enough timeline. Since it’s mostly plastic, we do wonder how it feels.

And it comes in any color you want, as long as it’s white. The good news is that it can be embroidered. Dyeing it is a no-no because it will lose its function. But if you can embroider it, you can probably make it visually interesting and still viable. A shirt made of this magic fabric will probably cost even more than that Hypercolor shirt we loved so much in the 90s, but at least it won’t announce our anxiety to the world in living color. But if you need to keep cool at all costs, an atmospheric window-wear shirt might just be the ticket.