What Your Activewear Is Doing To Your Body (And Why Nobody In The Industry Wants To Talk About It)

By Aneta & Beata Zubek, Co-Founders of ZUBEK

For years, Aneta had back acne she couldn't explain.

She tried everything. Adjusted her diet. Cut out dairy, then gluten, then alcohol. Saw a dermatologist. Changed her skincare routine three times. Nothing worked — not consistently, not for long. The problem kept coming back, always in the same place, always worse in the weeks after heavy training.

Then one afternoon, for reasons she still can't fully explain, something clicked.

She was standing in the gym changing room in her polyester training leggings, sweating. Tight fabric. Elevated skin temperature. Open pores. And she thought: what if it's this?

She switched to natural fabrics. Within days, her skin started clearing. Within weeks, the problem that had plagued her for years was gone.

Not improved. Gone.

That moment sent us down a research rabbit hole we haven't climbed out of since. What we found — in peer-reviewed journals, published by university researchers, sitting quietly in the scientific literature while the activewear industry sold us recycled plastic and called it progress — was worse than we expected.

This is what we found.

Why Gymwear Is The Most Important Clothing You Own

Before we get to the science, we need to address the question we get asked most: "Why start with activewear? Surely the priority is what you eat, drink, the air you breathe?"

Yes. All of that matters. But here's what most people don't know about skin.

Your skin is not a passive barrier. It is an active, permeable organ — and the rate at which it absorbs substances from its surface is not fixed. It changes dramatically based on two variables that are always present when you exercise: heat and friction.

A study published in the International Journal of Pharmaceutics found that raising skin temperature from 25°C to 39°C — entirely within the range of normal exercise — increased transdermal penetration of test substances by 200–653% within the first 45 minutes of exposure. Not a modest increase. A several-hundred percent increase. The skin you exercise in is not the same barrier as the skin at rest. It is significantly, measurably more permeable.

Add friction — the constant mechanical contact of tight-fitting fabric moving against skin during exercise — and that permeability increases further. Sweat activates the process. Blood flow to the skin roughly doubles during intense exercise, which increases circulation to the surface layers, accelerating the uptake of whatever is sitting against your skin into the bloodstream.

And here's the detail that stopped us cold: according to dermal absorption research, the scrotal region is the most absorptive skin on the body, followed closely by the armpit and scalp. These are exactly the areas your activewear covers most tightly.

So the question isn't just what is in your leggings. It's what is in your leggings, heated to exercise temperature, pressed against the most permeable skin on your body, for one to three hours, several times a week, for years.

That is a very different question.

What Is Actually In Your Activewear

The average legging, sports bra, or pair of training shorts is 80–90% polyester, nylon, or spandex. These are all, at their molecular level, plastic — derived from petroleum, synthesised from crude oil derivatives, and designed for durability, not biological compatibility.

But the raw fabric is only part of the story.

Modern performance fabrics are not sold in their raw state. They are treated. Moisture-wicking finishes, antibacterial coatings, water-repellent treatments, colour-fixing dye processes, anti-odour applications — each one adds a layer of chemical processing to the fabric before it ever touches your skin.

Many of these processes use compounds from chemical families that researchers have classified as endocrine disruptors: substances that interfere with the body's hormonal signalling. These include:

Phthalates — used as plasticisers and dye fixatives, associated with reduced sperm quality, lower testosterone, hormonal disruption in both sexes, and developmental effects in foetuses exposed in utero.

Bisphenol A (BPA) and related bisphenols — present in many synthetic fabric finishes, classified as endocrine disruptors, linked to reproductive abnormalities, thyroid disruption, and increased cancer risk.

PFAS ("forever chemicals") — used in water-repellent and stain-resistant treatments on performance fabrics. PFAS do not break down in the body or the environment. They have been linked to reduced fertility, lower IVF success rates, endometriosis, miscarriage risk, and immune system disruption.

Antimony — a heavy metal used as a catalyst in polyester production. Present in trace amounts in most polyester fabric; small but measurable quantities are released with heat and sweat contact.

These chemicals do not stay in the fabric. When fabric is heated and compressed against skin — which is what happens every time you train — they migrate. And as we've established, the skin during exercise is primed to receive them.

The Studies The Industry Would Rather You Didn't Read

Now we get to the research. And this is where it gets uncomfortable.

The 1990s: Dogs in Polyester Pants

In the early 1990s, Professor Ahmed Shafik — a surgeon and researcher at Cairo University's Faculty of Medicine, and one of the most prolific researchers in reproductive medicine — began a systematic investigation into the effects of polyester fabric on fertility. His work is meticulous, peer-reviewed, published in reputable scientific journals, and almost completely absent from mainstream conversation.

He started with male dogs.

Twenty-four dogs were divided into two groups. One wore cotton underpants continuously for 24 months. The other wore polyester. Both types were deliberately loose-fitting in the scrotal area, specifically to rule out heat retention as a confounding factor. The dogs were monitored throughout: sperm character, testicular temperature, hormone levels, and — at the end — testicular biopsies.

At the end of 24 months, the cotton group showed no changes. The polyester group told a different story. Sperm count had fallen significantly. Sperm motility decreased. The proportion of abnormal sperm forms increased. And the testicular biopsies showed degenerative changes — structural damage, visible under microscope, in the reproductive tissue of animals wearing polyester.

Critically: testicular temperature showed no significant change throughout the study. The damage was not caused by heat. Shafik proposed a different mechanism: the electrostatic field generated by polyester fabric in contact with skin.

Then he turned to female dogs.

Thirty-five female dogs were divided into groups wearing pure polyester, a polyester-cotton blend, pure cotton, or pure wool for twelve months. The results, published in the Journal of Obstetrics and Gynaecology in 2008, were stark. Dogs wearing polyester-containing fabrics showed significantly reduced progesterone levels during their reproductive cycle. Progesterone, critically, is the hormone required for ovulation and the maintenance of early pregnancy.

They couldn't conceive. Not through natural mating. Not through artificial insemination.

The cotton and wool groups? Normal hormone levels throughout. Normal conception rates.

Five months after the polyester garments were removed, progesterone normalised. The affected dogs conceived successfully.

The Human Study: Zero Sperm

Shafik then moved to human subjects. Fourteen healthy, fertile men volunteered to wear a polyester scrotal sling day and night for twelve months. Their partners took oral contraceptives until the study's fertility effects could be confirmed.

They didn't need to wait long.

Within approximately twenty weeks — five months — every single participant had become azoospermic. Zero sperm in the ejaculate. Their testicles measurably shrank in volume. The fertility effect was total.

The study wasn't measuring a modest decline in sperm quality. It was so effective that Shafik published the findings in the journal Contraception under the title: "Contraceptive efficacy of polyester-induced azoospermia in normal men."

He was proposing polyester as a male contraceptive. Because that's what the data showed.

After the sling was removed, sperm count and testicular volume returned to normal in the following months. The effect was reversible — but only once the polyester was gone.

This research was published in peer-reviewed journals. It was not retracted. It was not disproven. It was largely ignored.

2024: Microplastics in Every Human Testicle Tested

Shafik's work focused on electrostatic fields and chemical finishes. In 2024, a new body of research added another layer to the story — and this one made international headlines.

Researchers at the University of New Mexico, led by Dr John Yu, published a study in Toxicological Sciences in which they tested 23 human testicles and 47 dog testicles for microplastic contamination. The testicle samples came from autopsies and routine veterinary procedures. None had been selected for evidence of reproductive problems.

Every single sample contained microplastics. Every one.

The human testicles contained nearly three times more microplastic material than the dogs'. Among the plastic types found: polyethylene, PVC, and PET — the materials found in plastic bags, water bottles, packaging, synthetic clothing fibres.

Higher concentrations of certain plastics, particularly PVC, correlated with lower sperm count and lighter, smaller testicles in the canine samples.

"At the beginning, I doubted whether microplastics could penetrate the reproductive system. When I first received the results for dogs I was surprised. I was even more surprised when I received the results for humans." — Dr John Yu

He noted the tight blood-tissue barrier that surrounds the testicles — an evolutionary protection mechanism specifically designed to shield reproductive organs from systemic contaminants. Microplastics had crossed it anyway.

This study followed earlier research finding microplastics in human blood, lung tissue, placental tissue, and breast milk. It also followed a Chinese study published in 2023 in which microplastics were detected in all 30 semen samples and all 6 testicular tissue samples tested — with plastic-contaminated sperm samples showing higher rates of abnormality.

"It's probably one more piece of the puzzle of things that are contributing to declines in male fertility over the years as these environmental exposures keep accumulating." — Dr Sarah Krzastek, Virginia Commonwealth University

Global sperm counts have declined by approximately 50% since the 1970s. Some researchers now project that, on current trajectories, average sperm counts could trend toward zero by 2045. The causes are multifactorial. But microplastics and endocrine-disrupting chemicals — present in synthetic fabrics, released through friction and heat, absorbed through skin — are named in the literature as probable contributors.

"But Isn't This Everywhere? What Difference Does Clothing Make?"

It's a fair question. Microplastics are in the food we eat, the water we drink, the air we breathe. The exposure is ubiquitous. So why focus on clothing?

Because clothing is the one source of exposure where your body's own chemistry is actively working against you.

When you eat, your digestive system filters, metabolises, and processes what enters it. When you breathe, the respiratory system provides some defence. But when chemicals sit against heated, sweating skin during exercise — the permeability window is wide open. The 200–653% increase in transdermal absorption during exercise isn't a small footnote. It is the crucial variable that makes what you wear during training categorically different from ambient environmental exposure.

You cannot eliminate microplastics from your environment. You can eliminate them from the fabric pressed against your skin for two hours every day.

That is not a small thing. That is a meaningful, controllable reduction in your total toxic load — executed through one simple change in your daily routine.

What About "Sustainable" Activewear?

We have to address this directly, because we know it's the objection running through many readers' minds right now.

Most sustainable activewear is recycled polyester or recycled nylon. It is made from reclaimed plastic — bottles, fishing nets, other synthetic waste — broken down and respun into fibre.

We appreciate the environmental intent. We genuinely do. Keeping plastic out of landfill and oceans matters.

But recycled polyester still sheds microplastics with every wash. Recycled nylon still carries the same chemical families as virgin nylon. The fabric may have a cleaner backstory. It does not have a cleaner interaction with your biology.

Recycled plastic pressed against exercising skin at 39°C is still plastic pressed against exercising skin at 39°C.

The industry found a better story. It did not fix the underlying problem.

One more thing that almost nobody mentions: uncertified "natural" cotton is not a clean alternative. Conventional cotton is one of the most pesticide-intensive agricultural products in the world. The pesticide residue does not fully wash out during processing — it persists in the fabric. Additionally, most garments marketed as "natural cotton" still contain synthetic elastics, polyester threads, chemical dyes, and synthetic labels. The headline fabric may be natural. The garment as a whole may not be.

If it doesn't carry GOTS (Global Organic Textile Standard) certification for the full garment, "natural" is a marketing word, not a guarantee.

The Good News

We've just spent several thousand words making the case that most activewear is silently working against your health. That's a lot to sit with.

Here is the good news, buried in every single study we've cited above:

The effects were reversible.

In Shafik's female dog study, progesterone normalised and conception resumed five months after the polyester was removed. In the male dog study, sperm quality recovered in ten out of twelve subjects after garment removal. In the human contraceptive study, sperm count and testicular volume returned to pre-study levels within months of removing the sling.

The body knows what to do. When you stop exposing it to the wrong things, it recovers.

This is exactly what happened with Aneta. Years of back acne, gone within weeks of switching fabrics. We're not claiming one garment change will resolve every hormonal or reproductive issue. But the research is clear that removing the source of synthetic exposure allows the body to begin correcting what was disrupted.

You don't need a dramatic intervention. You need to stop wearing the thing that's causing the problem.

What We Built Instead

ZUBEK exists because we looked at this research — and then looked at the activewear market — and couldn't find what we needed.

Everything we make is plastic-free. Not "mostly plastic-free." Not "lower plastic than conventional." Zero polyester. Zero nylon. Zero synthetic base fabrics. Every fibre, thread, elastic, and label in a ZUBEK garment is natural and OEKO-TEX Standard 100 certified — which means every component has been independently tested for harmful substances, not just the headline fabric.

We use three fabrics:

SeaCell — a blend of lyocell (from sustainably harvested wood pulp) and SeaCell fibre (from seaweed), with 3% elastane for stretch. Breathable, moisture-regulating, biodegradable at end of life, and zero microplastic shedding. This is the foundation of our Core Seaweed Collection.

Merino wool — mulesing-free, OEKO-TEX certified, naturally thermoregulating and odour-resistant. No chemical finishes required.

GOTS-certified organic cotton — grown without pesticides, certified through the full supply chain. The only cotton we consider genuinely clean.

It took us 60 prototype iterations to get the performance right. It costs more to produce than synthetic alternatives. And we would make every one of those decisions again.

Because we believe the clothes you sweat in are not a neutral choice. They are a daily health decision. And you deserve to make that decision with accurate information.

The Question We'd Leave You With

You've spent years optimising your diet. Reading ingredient labels. Choosing clean supplements. Filtering your water. Auditing your household products.

When did you last look at the label on your leggings?

ZUBEK is a natural activewear brand founded by sisters Aneta and Beata in Poland. Everything we make is plastic-free, OEKO-TEX certified, and manufactured in Poland. Shop the Core Seaweed Collection at zubek.com

References

• Shafik, A. (1992). Contraceptive efficacy of polyester-induced azoospermia in normal men. Contraception, 45(5), 439–451. PubMed
• Shafik, A. (1993). Effect of different types of textile fabric on spermatogenesis. Urology Research, 21(5), 367–370. PubMed
• Shafik, A. (2008). Effect of different types of textiles on conception. Journal of Obstetrics and Gynaecology, 28(2), 213–216. PubMed
• Hu, C.J., et al. (2024). Microplastic presence in dog and human testis. Toxicological Sciences, 200(2), 235–240. Oxford Academic
• Zhou, Y., et al. (2023). Detection of microplastics in human testis and semen. Reproductive Toxicology. PubMed
• Franken, A., et al. (2020). Impact of temperatures on dermal absorption. International Journal of Pharmaceutics, 586. ScienceDirect
• Maibach, H.I., et al. (1971). Regional variation in percutaneous penetration in man. Archives of Environmental Health.
• NIEHS. Phthalates and Bisphenol A. Updated 2024. NIEHS
• Swan, S.H. (2021). Count Down. Scribner.
• Ragusa, A., et al. (2021). Plasticenta: First evidence of microplastics in human placenta. Environment International, 146. PubMed

All studies cited are published in peer-reviewed scientific journals. ZUBEK does not claim that wearing our products will prevent or treat any medical condition. The research cited here relates to synthetic fabric exposure generally, not to ZUBEK products specifically.