Every wash cycle releases fragments of fibre from clothing into the water. The quantity is large, the mechanism is mechanical rather than chemical, and fabric construction governs most of it.
Shedding is an abrasion process
Inside a drum, garments rub against each other and against the walls. That abrasion breaks fibre ends and pulls loose fragments out of the yarn.
The same process happens in normal wear, but washing concentrates it, adds water to carry the fragments away and does it to a whole load at once.
Which means shedding is a property of how a fabric is built and how it is agitated, and not something a fibre can be entirely engineered out of.
Synthetic fragments persist
Cotton, wool and linen fragments are also released in quantity, but they are cellulose and protein and break down over time in the environment.
Polyester, nylon and acrylic fragments do not degrade on any comparable timescale. They travel through wastewater treatment, and a portion passes through into rivers and the sea.
Treatment plants capture a large share in sludge, but that sludge is frequently spread on land, which relocates the fragments rather than removing them.
Construction matters more than fibre
Loosely spun yarns with many exposed fibre ends shed far more than tightly twisted yarns, and cut edges shed more than finished ones.
Fleece is the clearest case. The pile is created by mechanically raising and cutting fibre ends, which is precisely the structure that releases fragments most readily.
Tightly woven fabrics from long staple or filament yarns shed comparatively little, so two polyester garments can differ enormously in how much they release.
New garments shed most
The first several washes remove loose fibre left over from manufacturing and cutting, and release drops substantially after that.
Release rises again much later as the fabric fatigues and fibres begin to break, so the curve falls and then slowly climbs.
Keeping garments longer therefore reduces total shedding per garment-year, because the high-release opening period is amortised over more use.
What actually reduces release
Fuller loads reduce the free movement that causes abrasion, and cooler, shorter, gentler cycles reduce the mechanical energy applied to the fabric.
Filters fitted to machine outlets capture a meaningful share of fragments, and some jurisdictions have moved toward requiring them on new machines.
None of these eliminate shedding. They reduce the rate, which makes fabric choice and garment lifespan the levers with the largest effect available to a buyer.