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.

The atmospheric windows let in visible light and radio waves.
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.

42 thoughts on “How Much Is That Shirt In The (Atmospheric) Window?

  1. Here’s something to consider: PTFE is teflon, which is a fluoride.

    I’d be concerned about wearing a fluoride compound on my body, with rubbing and sweat and possible transfer. Also, I routinely get scratches, and even gashes, while hiking and am wondering if transfer into the blood is possible.

    This is not the same as cooking on a teflon pan, which is considered safe. In that case the surface comes into contact with food (not your body), is in contact a short time, and is rigidly held so that it does not flex.

    Before wearing this type of fabric I’d like to see a study to determine if doing so is long-term safe.

    1. Maybe one day we will get our fluoride from our clothing rather than from toothpaste? :-)

      Seriously, i wonder if this clothing is really biodegradable if it is coated in PTFE, because afaik PTFE really is not biodegradable?

    2. PTFE is NOT a fluoride. It produces no fluorine ions.
      It is inert, and approved for permanent implants, for pete’s sake. It’s completely non-reactive, biologically.

      Its only problems are that it doesn’t ever really go away, and if you do get it hot enough to decompose some other more active compounds form.

      1. I’m confused.

        Firstly: Websters defines fluoride as 1) a compound containing Fluorine, or 2) the monovalent anion of Fluorine. PTFE is a polymer containing monovalent Fluorines, so… what are you talking about?

        Secondly: Teflon has come under regulatory scrutiny, and is banned or will be banned or in the process of being banned in several areas, including the EU. DuPont paid out a huge settlement to solve the myriad court cases people have brought regarding teflon.

        It’s a little more nuanced due to the difference between PFOA, PFOS, and PTFE, but questioning the safety of a FLUORIDE compound is quite reasonable.

        1. What I gather from a quick googling is that they aren’t trying to bad PTFE (Teflon), but rather trying to limit the emission of PFOAs during various production processes. Including the process of making Teflon.

          So I wouldn’t call it a ban, but rather a forceful way to stop manufacturers from polluting the environment.

          Here’s an article from a manufacturer of PTFE type coatings: https://www.lubrizol.com/Coatings/Blog/2020/07/New-PFOA-Regulations-Will-Impact-PTFE

          Interesting how they forget to mention how bad PFOAs are for humans and animals…

        2. Monovalent fluorine is an ION. An ion is an atom with one or more electron vacancies, and is a charged particle. Which is why salts, compounds of oppositely charged ions held together by IONIC bonds, are soluble in water.

          PTFE is a polymer with a backbone made of covalently bonded carbon atoms, and the fluorine atoms COVALENTLY bonded to the carbons. Insoluble in water or organic solvents.

          The environmental concerns come from the monomers used to prepare the product-fluorinated gases. Speaking as a chemical engineer, it is nearly impossible to prevent trace loss of gases or gaseous vapors from manufacturing systems, especially those that dissolve poorly in water (scrubbers often use water to collect contaminants).

          And I suggest to you that using civil court case settlements as evidence of harm is… questionable… Since lawyers that like to sue “big pocket” companies do their very best to obscure the science, and get juries to decide based on fear. “Water drowns people! Hot water burns hundreds every year [shows pictures of horrific burns caused by steam]! Company X has paid out THOUSANDS for damage from this horrible CHEMICAL! Have you been harmed? Call our skeegie law firm so we can enrich ourselves from your pain!”

          1. I amazes me how people can argue with someone regarding a topic they know nothing about, I literally spit my coffee out laughing when I read PWalsh’s comment. This is a man that does not get chemistry.

            Just to add, it takes a lot of energy to break Teflon down, definitely no less than 250 degrees. And like this perfectly penned comment above, the issue with Teflon has always been the manufacturing process.

  2. Sigh. Heat transfer 101: Conduction, convection, radiation.

    You can wear anything you like and the first two are what matter over the course of a day unless you’re in very dry air and in very bright sun. Come out to a nice cloudy day in the Midwest (or southern Asia) when it’s over 100°F and 80+% relative humidity and see where reflective material gets you. Wicking polyester (CoolMax™ and its relatives) blends help more by allowing evaporative cooling on its surface.

    1. Yep. PTFE is probably one of the least reactive polymer materials known and probably not biodegradable at all. There are essentially no reactive bonds in it. It will survive sitting in concentrated sulfuric acid. Even when extreme heat does break it down, it forms highly toxic and environmentally damaging compounds:
      https://en.wikipedia.org/wiki/Polytetrafluoroethylene#Ecotoxicity.

      Maybe the author can rewrite this part of the article to clarify which of the materials is “supposed to be biodegradable”. The polymer that forms the bulk of the fibers seems to be polylactic acid, which is probably biodegradable, but the coating of PTFE might stop that also.

  3. “PTFE is hydrophobic, so it will repel water from the outside and sweat from the inside.”

    Maybe I’m just not understanding… If it repels sweat, that mean the sweat will get trapped between the fabric and your skin. If this is true, that’s not a good thing. Not allowing sweat out could lead to rashes and blistering to say the least. It may even cause the wearer to overheat.

    I’m sure what’s meant is “moisture wicking”.

    1. The nature of the fibres with that treatment is such the cloth fibres itself won’t absorb the moisture…
      But its still a woven structure with spaces between the fibre, so it should breath and let sweat out (and rain in) all while staying ‘dry’ itself…

      1. Ah, yes. Moisture wicking. Leave it to those wily Chinese to “invent” the material Nike uses in its Dry-Fit athletic apparel. Other companies have products made of similar materials sold under various brand names. Race (5K, 10K, etc.) organizers normally include a shirt branded with the race and sponsors logos made from such material in each participants packet. I have dozens. They’re quite comfortable.

  4. From The Fine Article: “Registering the person’s temperature with an infrared camera, they found that the half that was covered in their structured material was typically about 3º C cooler than the one that wasn’t.” (italics mine)

    Uhhm… An infrared camera measures temperature by measuring the amount of infrared radiation emitted. Lower measured temperature means LESS heat radiated away (or lower emissivity at the thermal wavelengths). Exactly the opposite of what is claimed of the material…

      1. What else would you make clothing in? Wool is both warm and many people have allergies to it, so not suitable for everything, silk is stupendously expensive for good reason, and cotton a ecological disaster to grow commercially – clothing people in natural fibres when there are so many people… Some types of plastics for clothing seem pretty much a certainty for a long time to come…

        1. Wool is awesome, hands down, where/when you can use it. Hot in the summer. But ask me about my toasty toes all winter long.

          I know what you mean about cotton, etc. There’s definitely some long-term / short-term tradeoff being made between cotton and synthetics production and waste, and I’m not sure I understand it well enough to make any decisions more informed than “I like the way cotton feels”. And microplastics in the water (like my fleece sheds with every wash) give me the creeps. But cotton farming, and especially bleaching, isn’t without its downsides either.

          Cue the hemp folks, or something.

          But I don’t think that my clothing is a significant contributor to anything. I’m actually wearing a 15-yr old fleece ATM, so it’s well ecologically amortized at this point. I have pants that are older than my son. I guess if I were more of a clothes horse…

          I’ll use teflon for its intended purpose, though. Making sliding mechanical parts.

          1. Sounds good to me, still have clothes that date back to something around the time I stopped getting taller…

            Good as hemp is for many things I don’t think its really going to make good clothes either, rather rough from what I know of it…

          2. Ecologically amortized, indeed! Almost all of my synthetic clothes are still in good shape many years on (largely around a decade). They never have to be ironed, and presumably lose very little material in washing since they’re still around this far on. I only wear the fleece in bitter cold, though.
            I originally got them for backpacking and camping but they quickly became my regular wardrobe because they block the sun, breathe well, are comfortable, and are presentable enough to be worn wherever I need to go.

            Oh, and don’t knock teflon in clothing until you’ve tried a nice rain jacket!

        2. @Foldi-one, how about linen or bamboo fibre? Both of those are basically “grass” and can grow on marginal land using much less water than cotton. Completely biodegradeable too.

          1. No personal experience with bamboo fibres, but I understand it to be a great deal of work to make them into useful clothing fibres. So while the plant itself is a great source of materials being so fast growing the processing requirements might be too high.

            It is certainly possible to use a great many natural fibres, including no doubt many we are still not mentioning, the question is can we actually sustainably get enough of such things to keep the population of the planet clothed and in the right type of clothing for their climate – which with such high population levels I don’t think we can.

            But with so many options out there including leathers as well as woven natural fibres maybe we can – this is not one I’ve tried to do the napkin maths on, too much I don’t know about the various useable fibres globally, and questions like just how long do bamboo fibre clothes last? Its all well and good it being rapidly growing durable sustainable plants, but if you need new clothes every 6 months where cotton/wool/silk/leather lasts decades it shifts the practicality of using it.

  5. Going the other way, my SO has knit me sweaters out of heat wave yarn, that is designed to maximize how warm it gets when the sun hits it. She says it is heavier than it’s ply would imply, but it is nice and warm.

  6. As to what else you would make shirts out of, all of my t-shirts are made from recycled plastic bottles (Patagonia). Expensive, but I’m on five years and counting with these shirts. And they don’t feel plasticy, either.

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