Walk into most gym clothing shops and polyester is the default. It's cheap to produce, stretches well, and dries fast — which is exactly why it took over activewear in the first place. But polyester is a plastic, and in the last few years researchers have started finding out what that actually means once you wear it, wash it, and sweat in it for years on end.
This isn't about scaremongering. It's about understanding what the two most common gym fabrics are actually made of, what happens to each of them over time, and how to make an informed choice.
What polyester actually is
Polyester is a synthetic fibre made from polyethylene terephthalate (PET) — the same base plastic used in drink bottles. It's popular in gymwear because it's cheap, durable, quick-drying, and easy to blend with stretch fibres like elastane. None of that is in dispute.
What's newer is the research into what polyester sheds over its lifetime, and where those shed particles end up.
Where polyester microfibres actually go
Every wash cycle, and every hour of wear, polyester clothing sheds microscopic plastic fibres. Washing machines release large volumes of these fibres into wastewater — one estimate from a San Francisco Bay wastewater study put the figure at tens of millions of particles per day from a single treatment plant, with synthetic fibres like polyester identified as the dominant source. From there, fibres move into waterways, the food chain, and drinking water. Fibres are also shed directly into the air through friction during normal wear.
Researchers have since found microplastic particles in some fairly striking places in the human body:
- In blood. A 2022 study found microplastic particles in blood samples from 17 of 22 healthy adults tested.
- In the placenta and breast milk, raising questions researchers are actively investigating around exposure during pregnancy and infancy.
- In reproductive tissue. A 2024 University of New Mexico study detected microplastics — including PET, the plastic used in polyester — in every one of 23 human testis samples examined.
- In lungs, liver, kidneys, and even brain tissue, with more recent tissue samples showing higher concentrations than older ones.
What we don't know yet
It's important to be precise here: finding plastic particles in human tissue is not the same as proving those particles cause specific diseases. The World Health Organization reviewed the evidence on microplastics in 2022 and concluded that the data wasn't yet sufficient to draw firm conclusions about human health risk — and that's still broadly the case today.
What early lab and animal studies do show is that microplastic particles can destabilise cell membranes and trigger inflammatory responses in exposed tissue, and that animal studies link plastic particle exposure to effects on the digestive, respiratory, reproductive, and immune systems. Whether — and how — that translates into measurable harm in humans over a lifetime of exposure is still being worked out. This is a genuinely fast-moving area of research, and the honest answer right now is: we know the particles are getting in, and we don't yet fully know what that means long-term.
How organic cotton compares
Organic cotton is a plant fibre, not a plastic, so the comparison isn't really about which fabric performs better on a spec sheet — it's about what each one is actually made of and what it does over its lifetime.
|
|
Polyester |
Organic cotton |
|
Base material |
Petroleum-based plastic (PET) |
Plant fibre |
|
Sheds microplastics |
Yes, throughout its life |
No |
|
Breathability |
Lower, often needs technical weaves to compensate |
Naturally breathable |
|
Common added chemicals |
PFAS coatings, synthetic dyes, anti-odour finishes |
None in GOTS-certified organic cotton |
|
End of life |
Doesn't biodegrade |
Biodegradable |
|
Stretch |
Excellent on its own |
Needs blending with elastane for stretch |
|
Typical cost |
Lower |
Higher |
Neither fabric wins on every row. Polyester is genuinely better for pure stretch and quick-dry performance in isolation. But if what you care about is what the fabric is made of, what it sheds, and what happens to it after you're done with it, organic cotton has a clear and fairly simple advantage: it isn't plastic, so it doesn't behave like plastic.
Does this mean you should never wear polyester again?
Not necessarily — plenty of technical sportswear will keep using synthetic fibres for the foreseeable future, and the health research is still developing. But for gym clothes that sit directly against your skin for long, sweaty sessions — leggings, sports bras, t-shirts, base layers — it's a reasonable place to start swapping toward a natural fibre, especially given how much of that fabric's life is spent in close, repeated contact with your skin.
Practical takeaways
- Prioritise natural fibres for skin-contact basics — t-shirts, leggings, sports bras, base layers — where fabric spends the most time against your skin.
- Wash synthetic activewear less often and on cooler, shorter cycles if you do wear it, since washing is one of the biggest drivers of microfibre shedding.
- Look for a microfibre-catching laundry bag or filter for any polyester or nylon items already in your wardrobe.
- Check for GOTS certification if organic cotton matters to you specifically — the label isn't regulated everywhere, so certification is the reliable signal.
FAQ
Is polyester gym clothing dangerous to wear? There's no evidence that wearing polyester is acutely dangerous. The concern is around long-term, cumulative microplastic exposure from shedding fibres, which researchers are still actively studying — the WHO says current evidence isn't sufficient to draw firm conclusions either way.
Does organic cotton perform as well as polyester during exercise? It performs differently rather than worse. Organic cotton breathes better and feels cooler over long, steady sessions, but has less natural stretch than polyester-elastane blends unless it's blended with a stretch fibre.
Do all synthetic fabrics shed microplastics, or just polyester? Most synthetic fibres — polyester, nylon, acrylic, polypropylene — shed microplastic fibres. Polyester is simply the most widely used in activewear, which is why it comes up most often in the research.
Is organic cotton completely free of microplastic shedding? Yes — as a natural plant fibre, organic cotton doesn't shed plastic microfibres. It biodegrades naturally rather than breaking down into persistent plastic particles.
This article summarises findings from peer-reviewed research and public health reviews, including studies on blood, placental, and tissue microplastics, and the World Health Organization's 2022 review of microplastics evidence. Associations described are based on early-stage or animal research and are not established causal links to specific diseases in humans.