Yes, and in very large numbers. What is less widely reported is that natural fibres shed at broadly similar rates by mass — the difference is what happens to the fibres afterwards. Synthetic fibres persist; wool and other protein and cellulose fibres break down. That distinction, rather than shedding itself, is the honest case for natural fibres in a base layer.
What the measurements actually show
A study published in PLOS ONE measured fibre release from 37 consumer apparel textiles across five consecutive domestic wash cycles. The results are worth reading carefully, because they do not divide neatly along synthetic and natural lines.
| Textile type | Fibre loss per wash | Notes |
|---|---|---|
| Mechanically-treated polyester | 161 ± 173 mg/kg | Fleeces and brushed jerseys — the heaviest synthetic shedders |
| Cotton and wool | 165 ± 44 mg/kg | Comparable mass to treated polyester |
| Woven nylon, filament yarn | 27 ± 14 mg/kg | Smooth woven construction sheds far less |
| Range across all samples | 9.6 to 1,240 mg/kg | Construction matters more than fibre type alone |
Expressed as fibre counts rather than mass, the same study estimated between roughly 8,800 and over 6.8 million fibres per kilogram of textile per wash. Other work has put the figure for a typical 5–6 kg domestic load in the range of six to eighteen million fibres.
Construction matters more than the fibre name
The single clearest pattern in the data is that how a fabric is made predicts shedding better than what it is made of. Mechanically-treated fabrics — fleece, brushed and napped surfaces — shed several times more than smooth woven fabrics of the same fibre, because the treatment deliberately raises loose fibre ends to create a soft surface.
A woven nylon shell sheds relatively little. A polyester fleece sheds a great deal. Both are plastic. Anyone choosing on environmental grounds should look at the construction as well as the fibre content, and a fleece is the garment most worth thinking twice about — particularly since it is also the synthetic garment most often worn against skin and washed frequently.
Where the fibres end up
The fibres are small enough to be a problem for wastewater treatment. Research measuring fibres from machine washing and tumble drying found the most abundant fraction was retained only by filters with a 60 micrometre pore size, with fibres averaging 360 to 660 micrometres long and 12 to 16 micrometres across.
At that size a substantial share passes through treatment plants into waterways. Some is captured in sewage sludge, which in many jurisdictions is then spread on agricultural land, moving the fibres into soil rather than removing them. Tumble drying adds a second pathway, releasing fibres directly into the air.
The difference that actually matters
If wool sheds as much as polyester by mass, the natural-fibre argument has to rest on something else, and it does. A 2024 study in Water, Air, & Soil Pollution assessed the marine biodegradation of wool alongside other textile fibres, including wool treated by the chlorine-Hercosett process used to make it machine washable.
Both untreated and treated wool biodegraded to a high degree, as did cellulose-based viscose rayon. The synthetic fibres showed little or no biodegradation, with polyester in particular showing no apparent degradation over the test period. Wool fibres entering the environment break down; polyester fibres accumulate.
What this does and does not justify
It justifies preferring durable natural fibres for garments washed frequently, and being particularly cautious about fleece. It does not justify replacing functioning synthetic clothing with new wool, because manufacturing a new garment carries an environmental cost that outweighs the shedding difference for a long time.
- Keep what you own. The lowest-impact garment is one already in your wardrobe.
- Wash less often. Every avoided cycle is avoided shedding, which is where merino’s odour resistance has a genuine environmental benefit.
- Buy for durability. A garment lasting twice as long halves its manufacturing impact per wear.
- Be sceptical of fibre-based marketing. Construction, durability and how often you wash a garment matter more than the label.
The rest of the picture
None of this makes wool an environmentally clean fibre. Wool production carries substantial costs of its own — enteric methane from sheep, land use, and water-intensive scouring — and on carbon footprint per kilogram virgin wool compares poorly with polyester.
Our guide on whether merino wool is sustainable works through that side honestly. For reducing shedding from clothes you already own, reducing microfibre shedding in the wash covers what actually works. Our comparison of merino and synthetic base layers covers the performance trade-offs, and base layer tops and merino socks are where most people start.
Related merino guides
- Whether merino wool is sustainable
- Reducing microfibre shedding in the wash
- Merino compared with synthetic base layers
Sources
- PLOS ONE: fibre shedding from consumer apparel textiles
- Scientific Reports: washing synthetic clothes and microplastic pollution
- Environmental Science and Pollution Research: microplastic fibres from machine washing and tumble drying
- Water, Air, & Soil Pollution: marine biodegradation behaviour of wool and other textile fibres
Image: Merino Mart catalogue. Browse base layers.