Lately, the men of the internet who are obsessed with self-optimization have found a new fixation: their underwear. Many standard pairs use polyester or spandex, or sometimes both. What are they doing to the environment and their bodies, wearing clothes that shed all of those microplastics and leach forever chemicals? They want the underwear of their ancestors—underwear made from natural fibers. On Reddit forums such as r/PlasticFreeLiving and r/SustainableFashion, people trade strategies for breaking up with polyester boxer briefs: “Any recs or leads to find linen cloth for making my own underwear?” one exasperated user wondered. Another wanted recommendations for all-natural underwear that would also be formfitting. He knew about organic cotton and hemp, he explained, but the problem was, they chafed.Seemingly everyone is interested in natural fibers. Whitney Crutchfield, who teaches textile development at the Fashion Institute of Technology, told me that she first noticed natural-fiber mania brewing among her sustainability-minded students. Then, a year or two ago, she sensed the discourse shifting to regular consumers. Across the internet, people are purging their closets of synthetics, swearing allegiance to only natural fibers from now on. The enthusiasm seems to cut across gender, culture, and politics. “I need my workout clothes to show my nasty sweat!” the fashion journalist Liana Satenstein wrote last year, praising the emergence of a new generation of natural-fiber workout wear amid posts about French pedicures and vintage Prada thongs. “Make all clothes natural fibers again,” posted Carnivore Aurelius, an anonymous wellness influencer who sells “beef collagen peptides.”Natural fibers are indeed pretty great. Silk, wool, hemp, linen, cotton, and the like—all of which come directly from either plants or animals—don’t carry the baggage of their artificial competitors. They don’t shed microplastics. They’re generally understood to be biodegradable. And although the research is far from settled, synthetics have been linked to adverse hormonal effects. When I visited the website for a natural-underwear brand, a pop-up asked me what I looked for in my underwear: “Fertility health” and “supporting low T” were among the options.Natural fibers have also become an easy way to gauge the quality of a garment. Cotton, wool, and linen generally cost more than polyester or acrylic do, and they can look and feel luxurious in a way that many synthetics don’t. The reason finding a decent sweater these days is so hard is that so many of them are blended with synthetics, and one major reason for all of that blending is reduced production costs. Earlier this summer, when Gap released a reimagined version of a classic sweater from the early 2000s, fans revolted. What had originally been a chunky, cotton-blend knit turtleneck was now an entirely synthetic printed mesh. The anti-synthetic zeitgeist has transcended clothing; it is an attitude. Gen Z apparently now uses polyester as an insult meaning “cheap” and “inauthentic.”But the frenzy has gone too far. Are you really sure it’s always better to buy cotton or wool over a nylon blend? The whole point of man-made fibers is that they can have properties that specifically don’t exist in nature. “There are reasons that these things were produced in the first place,” Crutchfield said. When DuPont unveiled nylon—the world’s first synthetic fiber—to the public at the 1939 World’s Fair, it was a revelation. In the dawning era of synthetics, people would no longer be beholden to the limits of nature; instead, we had chemistry, which was going to free women from the drudgery of endless laundering and usher in an age of universal luxury. This did not, perhaps, play out exactly as one might have hoped, but even today, synthetic fabrics have plenty of benefits.Synthetics can enhance the performance of a garment, or the feel, or the durability, all of which contribute to a garment’s lifespan. The better it serves its intended purpose, the less likely you are to replace it with something new. A beautiful, all-natural raincoat isn’t an especially virtuous purchase if it doesn’t repel rain. On the whole, clothing with some percentage of made-made fiber requires less care than an item made with wool or silk or linen does—no hand-washing, no ironing, no reason to worry that moths might eat your favorite sweater.Sometimes, they really are the best materials for the job. The designer Issey Miyake’s famous pleated pieces are polyester because, as one textile expert explained to NPR, their thermoplastic qualities are what helps them keep their shape. Polyester actively repels moisture and dries quickly, which is why it’s used in so much athletic gear. Wool is stronger and more durable when it’s blended with a bit of nylon—not important for sweaters, crucial for hiking socks. A little spandex can help a pair of cotton leggings keep their shape. If we want the forgiving comforts of stretch denim and athleisure, “we have to make peace with the fact that synthetic fibers are going to be incorporated into our clothes,” Nancy Deihl, a fashion historian at NYU, told me. Yes, synthetic clothing can release microplastics—bad—but it isn’t clear how much swapping out your stretch pants does to meaningfully reduce your exposure.There is another problem with the natural-fiber obsession: Plenty of common fibers are neither synthetic nor natural. Semi-synthetics such as Tencel (the brand name for lyocell), modal, and rayon (otherwise known as bamboo) are made from natural plant material that’s been chemically dissolved and then reconstituted, a process that can be staggeringly toxic or ecologically responsible. Conventional cotton, for example, takes significantly more water to produce than lyocell and modal do.Further complicating matters, although terms such as organic cotton, regenerative cotton, and recycled cotton tell you something about the production of fibers, they do not tell you something about the quality of a garment. Yarn spun from longer cotton fibers is stronger and wears better than yarn spun from shorter ones, but fiber length is a result of the type of cotton plant, or the growing conditions, or how the fibers have been processed. “You can compare the fiber quality of three different cotton fibers, all making the same exact T-shirt,”—same weight of yarn, same knitting machinery, same pattern, same sewing process—“and those three T-shirts can, depending on fiber quality, last wildly different amounts of time,” Crutchfield said. Fiber quality is one reason a 100 percent cashmere sweater can be $50 or upwards of $500. “You must investigate the context and the nuance,” she told me.And fiber, natural or otherwise, is only one of a matrix of concerns that determine whether your clothing is ethical and well made. “Synthetics and natural fibers can both last a long time, but it depends on how they’re used, how they’re worn, and the quality of the garment they were made into,” Deihl said. The look and performance of any given garment are a product of the fibers, but also the fabric weight, the weave, the cut, the construction, and the care instructions—all of which require at least some knowledge to assess, and none of which is easy to evaluate online.In a dizzying consumer landscape of infinite options, it is understandable, even admirable, to want some kind of universal rubric to guide your purchasing. This can’t be cost, which may or may not correspond with quality, and there is no single label guaranteeing sustainability or ethics. You probably don’t know where a fiber was farmed and how it was harvested and processed—whether it’s been bleached and how it’s dyed, and the conditions under which a garment has been constructed—and the people selling it might not, either. Fiber content, though, is listed on the label: therefore, natural fibers! But natural isn’t a reliable shorthand for good—no matter how much you pay for your all-cashmere pants.