Polyester is the New Birth Control: What Research Says About Synthetic Fabrics and Your Hormones

 

We recently talked about the chemical burden — the total load of synthetic chemicals your body processes every day through food, skincare, fragrance, and household products. But there's one source of daily chemical exposure that almost nobody thinks about: the clothes on your body.

 Specifically, the polyester ones.

Polyester is plastic. It's made from polyethylene terephthalate (PET) — the same material used in plastic water bottles. It makes up about 70% of all synthetic fibers in the world and is found in almost every pair of gym shorts, leggings, hoodies, and sports bras on the market. It's cheap to produce, it lasts a long time, and it's everywhere.

But a growing body of research suggests that this plastic fabric may be doing more to your endocrine system — your body's hormone network — than most people realize. And the effects don't just apply to men. Women are equally affected.

Let's look at what the studies actually found. 

The Study That Started the Conversation

In 1992, Dr. Ahmed Shafik — a well-known surgeon and researcher at Cairo University who published over 500 scientific papers in his career — ran a study that still turns heads today. He had 14 healthy men wear a polyester sling around their groin area nonstop for about five months. The result? Every single man's sperm count dropped to zero. None of them could get anyone pregnant.

After the men stopped wearing the polyester, their sperm counts came back to normal within about five months. The five couples who wanted to have a baby were able to conceive. The polyester had worked like a reversible form of birth control — no pills, no surgery, just fabric.

Dr. Shafik believed the polyester created electrostatic charges through friction with the skin. These charges disrupted how the body makes sperm and regulates temperature. His animal studies supported this: dogs wearing polyester for 24 months had significantly fewer sperm and more damaged sperm. Dogs wearing cotton showed no changes at all.

He didn't stop with males. In a later study on female dogs, those wearing polyester-containing textiles showed lower progesterone levels — a hormone essential for pregnancy — and could not conceive. Once the polyester was removed, their hormones returned to normal and they were able to get pregnant again.

These studies are older and small in scale, and more human research is needed. But they opened an important door — one that newer chemical research has continued to walk through.

What's Actually Hiding in Your Gym Clothes

The concern with polyester goes beyond heat and static. Researchers have found that polyester clothing carries a number of chemicals classified as endocrine disruptors — substances that can mimic, block, or alter the way your hormones work. These chemicals can transfer onto your skin, especially when combined with sweat, heat, and friction.

Here's what studies have identified in synthetic activewear:

BPA (Bisphenol A): The Center for Environmental Health tested popular activewear brands and found BPA levels up to 22 times the safe limit under California law. BPA mimics estrogen in the body. In men, research has linked it to lower sperm quality and reduced testosterone. In women, it's been tied to irregular periods, ovulation problems, and a higher prevalence of PCOS. These chemicals show up most in items labeled "moisture-wicking" or "anti-static" — sports bras, leggings, socks, and athletic tops.

Phthalates: Used to soften plastic and fix prints on synthetic garments. Strongly linked to lower testosterone and poor sperm health in men, and to menstrual irregularities and fertility challenges in women. A 2024 review of over 120 studies found widespread phthalate contamination in textiles, especially in synthetic garments and printed areas.

Antimony: Used as a catalyst in 80–85% of all polyester manufacturing. Present in the majority of polyester garments. When these clothes interact with sweat, research shows up to 2% of the antimony can leach onto the skin. It's classified as a suspected carcinogen and adds to the chemical burden your endocrine system has to process daily.

PFAS ("Forever Chemicals"): Used to make activewear water-resistant and stain-proof. Found at up to 19 times California's safety threshold in garments tested by the Center for Environmental Health. These chemicals build up in the body over time and never break down. Research links them to weakened immune function, liver damage, and reduced fertility in both men and women.

Why Your Workout Makes It Worse

Here's what makes this especially important for anyone who trains: your skin becomes more absorbent when you sweat. That means during your workout — the exact time you're wearing tight, synthetic activewear — your body takes in more of whatever chemicals are in that fabric. Heat, friction, and moisture all speed up the transfer of these substances through the skin and into the bloodstream.

This isn't just theory. Lab studies have demonstrated these chemicals leaching out of polyester into simulated sweat. Your gym clothes are in constant contact with your body, often pressed tightly over the most sensitive areas.

For men, that means chemicals sitting directly over reproductive organs. For women, it means synthetic leggings, sports bras, and underwear pressing against breast tissue and the pelvic area — exactly where endocrine disruptors can do the most damage.

And when you add the synthetic deodorant on your skin, the chemical sunscreen on your face, and the artificial fragrance in your pre-workout — you start to see how fast the total daily exposure stacks up. Your workout gear isn't just one product. It's part of a system of choices that either supports your body or works against it.

The Bigger Picture: A Global Shift in Reproductive Health

A major meta-analysis published in Human Reproduction Update (Levine et al., 2023) — covering data from 53 countries — found that global sperm counts have declined by more than 50% since the 1970s. The rate of decline has also sped up since 2000.

On the women's side, rates of conditions like PCOS, endometriosis, and unexplained infertility have been climbing. No single factor is responsible. But researchers are increasingly recognizing that the daily, cumulative exposure to endocrine-disrupting chemicals — across clothing, food, personal care, and environment — is a significant and underestimated part of the equation.

Your activewear alone isn't going to determine your fertility. But it's one of the easiest exposures to eliminate — and one of the most constant. You wear clothes every single day, for most of the day. That's a lot of skin contact. And if that contact is with plastic fibers carrying BPA, phthalates, antimony, and PFAS, it's worth asking whether there's a better option.

Look for naturally made clothing. From Linen, wool, cotton, etc.

Why We Made 100% Merino Wool Apparel

At Salt of the Earth, we believe what touches your body matters just as much as what goes into it. That's why we created our **100% Merino Wool Black Hoodie** and **100% Merino Wool Black Shorts** — zero polyester, zero synthetic chemicals, zero compromise.

Here's why merino wool is the real performance fabric:

Controls your temperature naturally: Merino keeps you warm when it's cold and cool when it's hot — no chemical coatings needed. It's the original "moisture-wicking" fabric, and it does it without BPA.

Fights odor on its own: Merino has natural antimicrobial properties that keep you fresh without synthetic anti-odor treatments loaded with phthalates. Wear it to the gym, on a plane, or all day long.

Actually breathes: Unlike polyester, which traps heat against your skin and increases the conditions for chemical absorption, merino allows airflow and helps your body regulate temperature the way it's supposed to.

No microplastic shedding: Every time you wash polyester, it releases hundreds of thousands of tiny plastic fibers into the water. Those fibers carry endocrine-disrupting chemicals into our water supply, our food chain, and eventually our bodies. Merino wool is a natural, biodegradable fiber. It sheds nothing toxic.

No endocrine disruptors: No BPA. No phthalates. No antimony. No PFAS. Just natural wool from Australian sheep — the way clothing was always meant to be.

It's one swap. But it removes one of the most constant and overlooked sources of synthetic chemical contact from your daily life — the fabric sitting on your skin all day long.

CORE INSIGHT: Your Clothes Are Your Closest Environment

We spend hours choosing the right foods, the right supplements, and the right training programs. We pick natural electrolytes over ones loaded with sugar and artificial ingredients. But the fabric on our body? Most of us never give it a second thought.

Your clothes sit on your skin for 16+ hours a day. They're closer to your body than almost anything else in your life. Making an informed choice about what that fabric is made of — and what chemicals it carries — is one of the simplest, most overlooked ways to lower the load on your endocrine system.

The closer something is to its natural form, the less likely it is to carry chemicals that work against your body. That's true for your food. It's true for your skincare. And it's true for the hoodie and shorts you throw on every morning.

It's not about fear. It's about awareness. And once you know, you can't unknow.

Shop the 100% Merino Wool Hoodie →

Shop the 100% Merino Wool Shorts →

 

 

References

1. Shafik, A. (1992). "Contraceptive efficacy of polyester-induced azoospermia in normal men." Contraception, 45(5): 439-451.

2. Shafik, A. (1993). "Effect of different types of textile fabric on spermatogenesis: an experimental study." Urology Research.

3. Shafik, A. (2008). "An experimental study on the effect of different types of textiles on conception." J Obstet Gynaecol, 28(2): 213-216.

4. Levine, H. et al. (2023). "Temporal trends in sperm count: a systematic review and meta-regression analysis." Human Reproduction Update, 29(2): 157-176.

5. Center for Environmental Health. "What You Need to Know About BPA in Clothing." (2024).

6. Abraham, A. & Chakraborty, P. (2024). "Bisphenols in daily clothes from conventional and recycled material." PMC/Environmental Science.

7. Martinez-Ibarra, A. et al. (2023). "Endocrine disruptors in plastics alter β-cell physiology." American Journal of Physiology.

8. Kwiatkowski, C.F. et al. (2020). PFAS bioaccumulation and dermal exposure from clothing. Environmental Science & Technology Letters.