I have spent my working life around fabric. Not the kind photographed under studio lights, but the kind that arrives in rolls, gets cut, sewn, pressed, and returned when something is wrong with it. That vantage point teaches you something the marketing never does: a fibre is a decision with consequences that outlast the garment. For thirty years the activewear industry has been making the same decision, polyester with a little elastane and a performance claim on the hang tag, while quietly handing the consequences to the wearer and to the water system.
That is now changing, and not because of activists. It is changing because consumers can smell the difference, researchers can measure it, and legislators have started writing it into law.
The Market Moved Before the Marketing Did
Natural-fibre activewear used to be a niche for hikers and purists. It is now one of the fastest-moving corners of the category, and the clearest evidence comes from China. A joint white paper from Tmall Innovation Center and The Woolmark Company found that in the twelve months to July 2025, wool sportswear sales there grew 58 per cent by value, while Merino sportswear specifically grew 119 per cent. ANTA, the country’s largest sportswear group, has put 100 per cent Merino into competition kit through its ANTA GUANJUN line, a trail running series that debuted on elite runners at the Hong Kong 100 ultra. Trail running. A category built entirely on synthetics.
The credibility is moving too. At the 2025 US Open, Venus Williams played in a custom Merino kit designed by LUAR with Woolmark. Nobody puts wool on a tennis court in New York in late summer as a gesture.
One caveat, stated plainly: a great deal of this data is published by the wool industry itself, and market-research houses contradict each other on how large the natural share of activewear really is. Treat the direction as reliable and the decimal points as marketing.
Because the baseline remains overwhelming. Textile Exchange’s Materials Market Report 2025 puts global fibre production at a record 132 million tonnes in 2024, of which polyester alone accounts for 59 per cent, around 78 million tonnes. Wool, mohair, cashmere and alpaca together come to less than one per cent. The growth in natural activewear is real. It is also growth from almost nothing.
What Polyester Actually Does
Start with the washing machine. When Bethanie Carney Almroth’s team at Gothenburg tested synthetic fabrics for fibre loss, everything shed, but polyester fleece was in a category of its own: roughly 7,360 fibres per square metre per litre of wash water, against 87 for a smooth polyester knit. Construction matters as much as fibre content, and activewear is built from exactly the constructions that shed most. Brushed surfaces. Raised faces. Loose knits. Cut edges.
The European Commission now counts textiles among the largest sources of microplastic pollution in the EU, and the industry’s standard answer does not solve it. Textile Exchange reports that recycled polyester actually lost market share in 2024, slipping to around 12 per cent of polyester production, and that less than one per cent of the global fibre market comes from textile-to-textile recycling. Most recycled polyester is made from drinks bottles. A bottle that could have been recycled again and again is converted into a garment that sheds for years and then cannot be recycled at all. Blend it with elastane, as every stretch garment does, and you have built a material with no end of life.
And What It Does to the Person Wearing It
This is where the conversation has shifted hardest.
In October 2022 the US-based Center for Environmental Health served legal notices on eleven sportswear brands, Nike, Athleta, The North Face, Brooks, ASICS, FILA and PINK among them, after testing sports bras and athletic shirts for bisphenol A. Its testing indicated wearers could be exposed to up to twenty-two times the level California treats as safe. The brands disputed the findings and said they were investigating. The detail that should concentrate the mind is what CEH reported next: it found the problem only in polyester-based garments containing spandex. BPA is an endocrine disruptor. These are garments worn for hours, against damp skin, by design.
Then there are the finishes. PFAS, the fluorinated chemistry behind water and stain resistance, is being legislated out of apparel on both sides of the Atlantic. California’s AB 1817 and New York’s equivalent took effect on 1 January 2025, with thresholds tightening in 2027 and severe-weather outerwear given until 2028. France began restricting PFAS in textiles in January 2026 and extends the ban to all textiles by 2030. The EU is moving toward a broad restriction of its own. This is no longer a sustainability preference. It is a compliance deadline with a date on it.
And then the thing everyone has actually experienced. Chris Callewaert and colleagues at Ghent University collected T-shirts from twenty-six people after an hour of spin class, sealed them for twenty-eight hours, and had a trained odour panel judge them. The polyester shirts were rated significantly less pleasant and more intense than the cotton ones. The culprit turned out to be micrococci, bacteria that flourished on the synthetics and were barely present on the cotton. The smell is not your fault. It is a property of the fabric.
As for microplastics in the body, particles have now been identified in human blood, lungs, placenta and, in research published in Nature Medicine in early 2025, brain tissue. A widely cited study found microplastics in carotid artery plaque, with higher subsequent rates of heart attack, stroke and death among those patients. Honesty demands the qualifier the clinicians themselves apply: these are associations, not proof of causation, and the dose-response work has not been done. But we have not yet proven harm is a thin foundation on which to build a wardrobe worn against the skin for hours a day.
Why Wool and Organic Cotton Work
Merino’s advantage is not mystical. The fibre absorbs a substantial share of its own weight in moisture vapour before it feels wet, and it releases heat as it absorbs, which is why a Merino base layer holds a steadier microclimate against the skin than a synthetic that pushes liquid sweat to the surface and leaves you cold the moment you stop. Odour compounds are held inside the fibre rather than sitting on it, so the garment can be aired instead of washed. Fewer washes means less energy, less water, and fewer fibres down the drain.
The most interesting evidence concerns the oldest objection to wool, that it irritates skin. A randomised crossover trial led by John Su at the Murdoch Children’s Research Institute put thirty-nine children, aged from four weeks to three years, into superfine Merino base layers for six weeks and cotton for six weeks. Eczema severity fell significantly when the children moved from cotton to wool, and worsened when they moved the other way. The researchers concluded that standard clinical advice to avoid wool deserves revisiting. The trial was funded by Australian Wool Innovation, which does not invalidate it but does mean you read the design rather than the press release. Superfine Merino has since been recognised under asthma and allergy certification programmes.
Note the fibre diameter, because it is the whole argument. These were garments under roughly 18.6 microns. Coarse wool prickles. Superfine Merino bends against the skin instead of pressing into it. Wool is itchy is a statement about a micron count, not about a fibre.
Organic cotton does a different job and should be used for it. It belongs in studio work, low-intensity training, athleisure, and everything worn before and after the session rather than during it, and it grows odour-forming bacteria far less readily than polyester. It is weaker when saturated and slower to dry. Yoga and gym wear, not alpine layering.
The Part the Natural-Fibre Lobby Prefers Not to Print
I am not interested in swapping one set of marketing claims for another.
Wool carries a real climate burden. Sheep are ruminants, they emit methane, and grazing is land-hungry. A 2021 report by the Center for Biological Diversity and Collective Fashion Justice argued that wool’s average climate cost runs several times that of conventional cotton and demands vastly more land per unit of fibre. That report leaned substantially on the Higg Materials Sustainability Index, the same index whose consumer-facing use was suspended worldwide in June 2022 after the Norwegian Consumer Authority found that claims built on it were likely to breach Norwegian marketing law. A genuine concern, carried on contested data. Both things are true at once, and anyone who tells you otherwise is selling something.
Animal welfare is the second unresolved issue. Mulesing is still practised, and certification is thinner than the shelf-talkers suggest. Nor is supply remotely adequate: global cotton production runs to roughly 24.5 million tonnes a year, of which organic is a small fraction. These are premium inputs with limited availability, not drop-in replacements, and pretending otherwise is how greenwashing starts.
This Is a Production Problem Wearing a Fibre Costume
Here is the part that gets missed in every materials debate I have sat through.
Polyester did not conquer activewear because it performs better. It conquered activewear because it is cheap, available in unlimited quantity at short notice, and perfectly suited to the business model the industry actually runs: enormous forecast-driven production placed a year ahead of demand, discounted when wrong, destroyed when unsold. Wool costs more per kilo, moves at the speed of an animal and a season, and punishes anyone holding dead stock. You cannot staple a natural-fibre strategy onto a volume-and-markdown operation. The fibre loses that argument every time.
Which is why I keep returning to the same structural answer: short collections, small batches, fast reloads of what sells, no minimum order quantities, made to order rather than made to forecast. Take out the unsold-inventory risk and the maths on a superfine Merino base layer changes completely, because you are no longer pricing in the production that was always going to be given away.
The regulatory clock makes this urgent rather than aspirational. The EU’s Ecodesign for Sustainable Products Regulation is expected to publish its delegated act for textiles in early 2027, and standards bodies are already pressing for mandatory limits on microplastic release measured against the ISO 4484 test series. If that lands, shedding becomes a product specification rather than a talking point, and the brands that spent the preceding two years quietly reformulating will hold an advantage their competitors have to buy at speed.
What I Would Tell Anyone Building Activewear Now
- Specify the micron, not the fibre. Merino on a label means nothing on its own. Under 18.5 microns is the difference between a garment worn against skin and a garment returned.
- Treat elastane as a budget, not a default. Every percentage point of stretch you add is a percentage point of recyclability removed. Engineer stretch into the knit structure where you can.
- Audit your finishes before a regulator does. PFAS deadlines are in force in US states and moving through France and the EU. An antimicrobial treatment on a synthetic is an admission that the base fibre has an odour problem.
- Match the fibre to the intensity. Merino for thermoregulation and repeat wear. Organic cotton for studio, low-intensity and off-duty. Do not ask either to do the other’s job.
- Test shedding now. The ISO 4484 methods exist. Knowing your own numbers before anyone demands them is cheap. Discovering them in 2027 is not.
- Fix the order model first. No material strategy survives contact with forecast-driven overproduction.
We spent thirty years engineering fabrics to move sweat away from the body, and never asked what we were moving into the body in exchange.
The honest summary is this. Natural fibres are not automatically clean. Wool has a methane problem, organic cotton has a supply problem, and no fibre on earth is sustainable at the volumes this industry produces. But the specific trade the activewear category made, petroleum against the skin, for hours, in a garment designed to make you sweat into it, shedding plastic into the water with every wash, looks worse with every year of research that comes in. The market has noticed. The regulators have noticed. The only question left is which brands move before they are made to.
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