Is polyester bad for you? What the research actually shows

I trained in polyester for 15 years before I thought about it once. It was just what gear was made of. Rashguards, shorts, the liner in every pair of fight shorts I owned. Nobody in any gym I walked into asked what the fabric was, and neither did I.

Then I started reading, and the honest answer to "is wearing polyester bad for you" turned out to be more specific, and more interesting, than either side of the internet wants it to be.

So is polyester actually bad for you?

The honest answer is yes. Not dramatically: no single study closes the case. But synthetic fabric sheds microplastics, can carry PFAS finishes, and the plasticizer chemistry that comes with it keeps turning up in hormonal research. Three separate problems, all pointing the same direction. Dose and route matter: training gear worn hot and wet against your skin is the worst case, not a shirt on a calm afternoon. But everyday exposure adds to the total. It is not a free pass.

The real picture is harder to wave off than either side of the internet wants it to be. Synthetic fabric sheds tiny plastic fibers. It can carry chemical finishes you can't see. And it does its worst work in exactly the conditions athletes create on purpose: heat, sweat, friction, and hours of skin contact. The question is less "is polyester toxic" and more "how much of this am I choosing to sit in, and for how long."

What is polyester actually made of?

Polyester is plastic. The technical name is polyethylene terephthalate, the same PET that bottles are made from, spun into thread instead of molded into a container. That includes recycled polyester, which starts life as bottles and ends up as the same PET thread.

To make it, manufacturers need a catalyst, and roughly 80 to 85 percent of the world's PET is produced using antimony trioxide. Antimony is the part most people have never heard of. In 2023, the World Health Organization's cancer agency reviewed the evidence and classified trivalent antimony as probably carcinogenic to humans, moving it up from its older, weaker rating.¹ Trace antimony stays in the finished fiber.

I want to be careful here, because this is where it is easy to overstate. That classification is a hazard rating, not a measurement of how much antimony leaches out of a shirt onto your skin. Most of the leaching research looked at PET bottles holding liquid at high temperature, not fabric. So this is a reason to pay attention, and a reason to keep it in proportion. It is one input among several, and all of them point the same direction.

What changed my mind was stacking it next to everything else.

Why combat athletes face more exposure than most

Here is the part that applies to anyone who actually trains.

Most clothing studies imagine a person wearing a dry garment, calmly, for a few hours. That is not grappling. In a hard session you are hot, soaked, and in constant friction with the mat, an opponent, and your own gear. Your skin is open at the pores and your shorts are pressed into you for the better part of two hours.

Skin contact, heat, sweat, and abrasion are the exact conditions that move particles and chemistry out of fabric and onto a body. So a combat athlete is not an average wearer. You are closer to the worst case the studies describe, several times a week, for years. That was the realization I couldn't unsee.

Microplastics: what sheds, and where it ends up

Synthetic clothing sheds microfibers constantly, in the wash and during wear. The numbers are large enough to be worth stating plainly, as long as you state them correctly.

One study of domestic laundry estimated that a single 5 kilogram wash load of polyester fabric releases well over 3 million microfibers.² Another measured roughly 500,000 fibers from a typical polyester wash load and over 700,000 from acrylic.³ Those figures are per wash load, not per garment, and that distinction matters: anyone quoting "700,000 fibers from your shorts" is misreading the paper.

The reason this moved from an environmental story to a personal one is where the particles have turned up. Researchers have now detected plastic particles in human blood,⁴ and a 2024 study found microplastics in 100 percent of the human testis samples it tested.⁵ That study could only confirm the particles were present in the human tissue, not measure what they do. Its one functional signal came from the dogs tested alongside, where certain plastics (PVC and PET) tracked with lower testis weight. This is early science, and particles in tissue do not prove harm. But the finding is consistent: independent studies, multiple tissue types, the same direction. The precautionary case for removing a source of synthetic fiber shedding from what you wear does not require waiting for the last study, especially when a natural fiber alternative removes the question entirely. Shedding in the wash is only one way in, too; the routes those particles take to reach you, including the gym air you breathe, are their own subject.

Does polyester lower testosterone?

This is the question I get asked most, usually phrased as "does polyester lower testosterone," and it deserves a calibrated answer rather than a headline. I gave whether polyester actually lowers testosterone its own dedicated piece.

The fabric itself is not the suspect. The chemistry that rides along with synthetic textiles is. Phthalates, a family of plasticizers common in synthetic materials, have been associated with lower serum testosterone in men at everyday exposure levels in large population studies.⁶ Bisphenol A, another plasticizer, tracked with a reduced free androgen index and higher estradiol in adult men in a separate study.⁷

Read those carefully. They are associational, drawn from population data, and clothing is never isolated as the single source: these chemicals are in packaging, dust, and food contact too. Nobody has run the clean experiment of "wear synthetic shorts, measure your testosterone." But if you are an athlete dialing in sleep, food, and training to protect hormonal health, it is strange to spend two hours a day pressed against the exact chemistry these studies keep flagging. Reducing a known source of phthalate exposure, in your training gear or anywhere else it stays against your skin, is a reasonable call without needing the causal proof. I go deeper on the hormone mechanism, and the other endocrine disruptors in your training gear, in a separate piece.

PFAS: the finish you can't see

The fiber is one thing. What gets sprayed onto it is another.

PFAS, the "forever chemicals," are used across textiles for water and stain resistance, and roughly a third of global PFAS production goes to textiles. The health record is serious: the WHO's cancer agency classified PFOA, one of the best-studied PFAS, as carcinogenic to humans.⁸ Most of the documented human exposure comes through drinking water, food, and the workplace rather than clothing specifically.⁹ Treated apparel is one avoidable contributor to your total load, not the sole cause of anything.

The catch with PFAS is that you cannot see a finish, and a spec sheet rarely lists it. That is its own argument for starting from a fabric that was never treated in the first place. I went deeper on this in PFAS in fight shorts, including what the law is now doing about it.

What actually changes if you switch

I will give you the honest version, including the part that costs me something.

Switch to natural fiber and the microplastic question against your skin mostly goes away, because there is no plastic to shed. There is no PFAS finish, because nothing was sprayed on. Wool brings a real, measured advantage on odor: in controlled wear trials, polyester developed markedly more smell than wool over the same use.¹⁰ Skin-to-skin for two hours, that is not a small thing.

The tradeoff is dry time. Polyester's one genuine engineering win is that it dries fast, and natural fiber dries slower. We close most of that gap by blending organic cotton with Tencel and building a merino liner, but I am not going to pretend a cotton-Tencel short wicks like a sheet of plastic. It doesn't. What it does is let you train without wearing the chemistry. For me that was an easy trade. For you it might be a real one, and you should make it with the actual numbers in front of you, which is the whole reason this article has sources at the bottom. Clothing is one input among several, and a free guide sorts 13 exposure-cutting moves by strength of evidence, not alarm, if you want the rest of the picture.

That is also the entire premise behind what we build. You can see the full material breakdown and the shorts themselves on the shop page. If you're deciding which cut to buy, our buyer's guide to BJJ and MMA shorts sorts it out by sport, inseam, and pockets.

The bottom line

Is polyester bad for you? Yes. Not because any single study closes the case, but because the research keeps saying the same thing: microplastics shed into your body, plasticizer chemistry tracks with hormonal disruption, PFAS finishes are invisible and accumulate. Heat, sweat, friction, and sustained skin contact are what push those things in, which describes training. But everyday wear adds to the total too. The precautionary case for reducing synthetic fabric from what sits against your skin is clear enough without needing the final proof. That is why we build from natural fiber.