What You're Wearing Against Your Skin Is a Health Conversation Now
The conversation about synthetic activewear and hormone disruption is no longer fringe wellness content. It's showing up in peer-reviewed research, in regulatory toxicology, in university studies. It's not settled science — but it's no longer nothing. And as someone who has specced activewear for decades, I think the industry needs to engage with it honestly rather than wait until it becomes unavoidable.
Here's what we know, what we don't, where the evidence is solid, where it's emerging, and what it means practically for how you design and spec.
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WHAT ARE ENDOCRINE-DISRUPTING CHEMICALS?
Endocrine-disrupting chemicals (EDCs) are substances that mimic, block, or interfere with the body's natural hormones. The endocrine system regulates nearly everything — metabolism, fertility, thyroid function, development, immune response. EDCs don't need to be present in large quantities to cause effects. Hormones operate at parts per trillion, and some EDCs may have non-linear dose-response curves — meaning low doses can sometimes be more disruptive than high ones — an area of active scientific debate.
The World Health Organization classifies EDC exposure as a global health concern. The Endocrine Society has issued multiple scientific statements linking EDC exposure to fertility problems, thyroid dysfunction, developmental delays in children, metabolic disorders, and increased cancer risk.
The question for our industry: how much of this exposure is coming from what we wear?
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WHAT'S IN SYNTHETIC ACTIVEWEAR
Synthetic performance fabrics — polyester, nylon, acrylic, spandex — are petroleum-based plastics. The polymer base is relatively inert. The problem is what gets added during manufacturing.
Plasticizers are added to increase flexibility. Phthalates — a class of plasticizer — are among the most well-documented EDCs. Associated with reduced testosterone production in men, disrupted estrogen signaling in women, and developmental effects in children. Detected regularly in synthetic textiles.
BPA and related bisphenols are used in polyester resins and as dye fixers. BPA is an estrogen-mimic — one of the most studied EDCs in the literature. In 2022, the Center for Environmental Health tested sports bras and athletic shirts from major brands including Nike, The North Face, Athleta, Patagonia, and Adidas, finding BPA levels that could expose wearers to up to 22 times the safe limit under California law — in polyester-spandex garments from brands many consumers consider responsible choices. By 2023, a second round of testing across leggings, shorts, and sports bras found levels up to 40 times that threshold. CEH has found BPA consistently in one category of garment: polyester-based clothing containing spandex.
PFAS — the "forever chemicals" — are used as water repellent and stain resistant finishes on performance fabrics. Linked to thyroid disruption, reduced fertility, and increased cancer risk. They don't break down in the environment or in the body.
Antimony is used as a catalyst in polyester production and remains as a residue in the finished fiber. A suspected carcinogen. Found to leach from polyester fabrics under conditions of heat and sweat.
None of this is inevitable. These are manufacturing choices. But they are widespread choices, and most conventional synthetic activewear carries some combination of them.
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THE SWEAT PROBLEM
A 2023 University of Birmingham study, published in Environmental Science and Technology, found that the oily components of sweat — sebum — act as a solvent for toxic chemical additives in synthetic microplastics, leaching them out and making them available for skin absorption. A 2024 follow-up from the same team, published in Environment International, went further: using 3D human skin models, researchers found that as much as 8% of the chemicals released could be absorbed through skin — with sweatier skin absorbing at higher rates. Sweat doesn't just make you wet. It actively increases the rate at which EDCs migrate out of fabric and into your body.
Your skin is not a barrier to everything. It's highly permeable — especially during exercise when sweat glands are active, body temperature is elevated, and moisture and friction are present. Research shows microplastics can enter the body through sweat glands, hair follicles, and skin wounds.
The implication is direct: a polyester or nylon garment sitting against your skin during a workout is the worst-case exposure scenario. Not a polyester office shirt. Not a nylon jacket. The base layer during exercise — the thing touching your skin while you sweat — is where the exposure risk is highest.
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WHAT MERINO NEXT TO SKIN ACTUALLY MEANS HERE
Merino wool is a protein fiber — keratin, the same protein in human hair and skin. It requires far less chemical treatment to perform. It wicks moisture, regulates temperature, and resists odor through natural fiber architecture and protein chemistry — not through plasticizers or PFAS finishes.
When dyed and processed responsibly — GOTS certified or OEKO-TEX Standard 100 — the chemical load of a merino garment is dramatically lower than conventional synthetic activewear. GOTS controls the chemistry of every stage of production including dyes and auxiliaries. OEKO-TEX tests the finished garment against a list of over 1,000 harmful substances.
The honest claim: there is not yet a peer-reviewed clinical study that directly measures EDC absorption reduction from wearing merino versus synthetic next to skin during exercise. That study doesn't exist yet.
What exists is a well-documented chain of evidence: synthetics carry EDCs → sweat amplifies dermal absorption → merino at its base level does not carry those same chemical loads → merino next to skin eliminates that specific exposure pathway.
That's not a health claim. It's a logic chain I'm confident in as a practitioner, and one that the emerging research consistently supports without yet directly proving.
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THE SUPERWASH COMPLICATION
Superwash merino is not the same as untreated merino from a chemistry standpoint.
The standard chlorine-Hercosett process — used in approximately 75–76% of machine-washable wool globally, with 600 tonnes of Hercosett polymer applied to wool annually — involves chlorination to remove the fiber's natural scale structure, followed by coating with Hercosett 125 resin, a polyamide-epichlorohydrin polymer. (The 600 tonnes / 76% figures come from research cited by Dr. Paul Swan, a wool industry materials scientist, via Northern Playground's supply chain documentation.)
Epichlorohydrin — used in the resin — is a compound of documented human and eco-toxicity. Applied as a microns-thin layer directly to the fiber, researchers have begun flagging potential micro and nanoparticle risks from the coating degrading with wear and washing. The primary documented toxicology concerns are in manufacturing wastewater and environmental persistence — the direct EDC link to the wearer isn't clinically established. But it's real enough that it's being actively studied.
More immediately relevant: superwash treatment seals the wool fiber's cuticle structure — the same structure responsible for merino's natural moisture vapor transmission and thermoregulation. A superwash merino garment is measurably less effective at doing the things that make merino a better skin-contact choice than synthetics. You've compromised the performance argument to gain machine washability.
And: wool treated with the chlorine-Hercosett process cannot be GOTS certified — regardless of how the fiber was grown.
The alternatives exist and are improving. Plasma treatment, enzyme-based processes, and ozone-based approaches can achieve machine washability without chlorine or synthetic resin coatings. They're not yet universally available at scale, but they're where the responsible wool industry is heading.
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THE CONSTRUCTION DISTINCTION MATTERS HERE TOO
Intimate blends — where synthetic and merino fibers are spun together at the fiber level — place synthetic fiber against skin throughout the yarn. If that synthetic carries plasticizers, dye chemicals, or PFAS finishes, they're in direct contact with your skin during every wear.
Core spun and Nuyarn — where nylon is the internal structural filament and merino wraps the outside — eliminate that contact. The synthetic is structural and internal. What touches your skin is wool. This is a meaningful distinction if you accept the premise that skin contact with synthetic fibers during exercise is a relevant exposure pathway — which the University of Birmingham research suggests it is.
The "merino next to skin" claim is not marketing language. It's a construction specification with a health logic behind it. And it depends on the construction actually being core spun — not intimate blend — two things that can look identical on a hang tag.
This distinction is logical but not yet directly studied. The research establishes skin contact with synthetic fibers as a plausible exposure pathway during exercise. It does not yet isolate construction type as a variable. If the contact pathway premise holds, core spun and Nuyarn constructions would theoretically reduce exposure compared to intimate blends — but that's an inference, not a finding. It warrants research. We're naming it here because the construction question is live in the industry and the logic is worth scrutinizing — not because the science is settled.
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WHAT THIS MEANS FOR BRANDS
The EDC conversation in activewear is not going away. It will get louder as more research emerges, as regulatory frameworks tighten around PFAS and bisphenols in textiles, and as consumers become more sophisticated about what's in their gear.
What I recommend now:
Know your construction. Intimate blend or core spun? This is the most actionable question. If you're making a "merino next to skin" claim, it needs to be structurally true.
Know your certifications. OEKO-TEX Standard 100 tests finished garments for harmful substances including phthalates, bisphenols, and formaldehyde. GOTS governs the full supply chain. If you're making performance or health-adjacent claims, these give you something to stand on.
Address the superwash question. If your merino is superwash treated, you can't make the same chemical-free claims as untreated or plasma-treated wool. Know what process your mill uses and communicate it accurately.
Don't overclaim. The clinical study showing direct EDC reduction from merino next to skin doesn't exist yet. Making that specific claim creates liability and erodes credibility when the nuance comes out. What you can say: this is a natural protein fiber that requires dramatically less chemical treatment to perform, and here's our certification to prove it.
Watch the regulatory trajectory. PFAS in textiles is already under regulatory pressure in multiple markets. What's not regulated today may be regulated in three years. Designing with cleaner chemistry now is not just an ethical position — it's risk management.
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THE BIGGER PICTURE
The conversation about what we wear against our bodies is part of a larger shift — connecting environmental health, personal health, and product transparency in ways the industry hasn't had to navigate before.
For decades, performance apparel operated on the assumption that what mattered was function: does it wick, does it dry, does it stretch. Consumers are now asking a different set of questions: what is this made of, what does it do to my body over time, and what happens to it when I'm done with it.
Merino, specced correctly, with honest construction and appropriate certifications, is one of the strongest answers to all three questions. Not because it's perfect — it has real end-of-life challenges, real performance limitations at certain weight ranges, and real complications when superwash is involved. But because a keratin protein fiber that regulates temperature, resists odor, and can be certified clean at every stage of production is genuinely different from a petroleum-based plastic engineered to perform.
That's worth saying clearly. And worth speccing accordingly.
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Bonie Shupe is the founder of Rewildist, a product design and development consultancy specializing in sustainable materials, circular product design, and sourcing strategy for small brands. She has 20+ years of experience on the brand side of outdoor and active apparel, including as VP Product at Paka and Re-founder at Ibex.