Most people don't think about what happens to a garment after they stop wearing it. The answer, for the vast majority of clothing produced today, is landfill - where it will remain, largely intact, for decades to centuries.

In the US alone, 85% of all discarded clothing is landfilled or incinerated. Globally, the equivalent of one dump truck full of clothing enters landfill every second. And unlike food waste or paper, most of what's in those trucks will never meaningfully break down.

The reason is fibre composition. Whether a garment biodegrades - and how quickly, and into what - is determined almost entirely by what it's made of.

How Long Does It Take for Clothes to Biodegrade?

Biodegradation timelines vary significantly by fibre type:

The distinction is not merely one of speed. Natural fibres break down into compounds that are either beneficial to soil or cause no net harm. Synthetic fibres do not truly biodegrade - they fragment into progressively smaller plastic particles, ultimately becoming microplastics that persist in soil, water, and living organisms indefinitely.

Why Synthetic Fibres Don't Biodegrade

Polyester, nylon, and spandex are derived from petroleum. Their molecular structure - long chains of synthetic polymers - has no equivalent in nature, which means the microorganisms and enzymes responsible for decomposition cannot process them effectively.

What synthetic fibres do instead is photodegrade and mechanically fragment: breaking into smaller and smaller pieces under UV exposure, heat, moisture, and physical friction. These fragments become microplastics - particles under 5mm that enter soil, waterways, and marine ecosystems, and have been detected in human blood, breast milk, and lung tissue.

The fashion industry produces around 69 million tonnes of synthetic fibre annually. Every one of those garments, at end of life, becomes a microplastic source.

The Laundry Problem: Microplastics Before End of Life

Synthetic fibres don't wait until landfill to release microplastics. The damage begins the first time a garment is washed.

A single wash cycle can release over 700,000 microplastic fibres into wastewater. Most wastewater treatment plants are not designed to filter particles at this scale, so the majority pass through into rivers, lakes, and oceans. Laundering synthetic clothing is the largest single source of microplastic ocean pollution, releasing approximately 2.2 million tonnes of microfibres annually.

Natural fibres also shed during washing - but the shed fibres biodegrade. Wool and cotton particles entering aquatic environments break down into organic compounds within months. Polyester particles do not.

This distinction makes fibre choice one of the most impactful decisions a consumer can make, repeated with every wash cycle, across the entire lifespan of a garment.

Why Wool Biodegrades - and Why That Takes Longer Than You'd Expect

Wool is a protein fibre composed primarily of keratin - the same structural protein as human hair - and contains approximately 50% carbon by weight. When buried in soil or submerged in water, microbial activity breaks keratin down into its constituent elements: nitrogen, sulfur, phosphorus, and potassium - all beneficial soil nutrients.

Wool takes longer to biodegrade than most natural fibres: typically one to five years depending on conditions, compared to weeks or months for cotton and linen. This is not a weakness - it is the same structural property that makes wool durable, resilient, and functional across extreme conditions.

A sheep's fleece must insulate against cold, resist moisture, regulate temperature in heat, and withstand physical abrasion - all simultaneously, for years. The molecular architecture that makes wool capable of all that is also what makes it slow to break down. Relative to polyester, which requires 20–200+ years to fragment (and never truly disappears), wool's five-year decomposition timeline is not a liability. It is one of the most compelling end-of-life arguments for the fibre.

A well-made merino wool garment worn for ten to twenty years, then composted, has a fundamentally different environmental legacy than a polyester garment worn for one to two years and landfilled for two centuries.

Cotton: Fast-Biodegrading, Conditionally

Untreated 100% cotton biodegrades rapidly - within weeks to months under composting conditions. As a plant-based fibre, it is processed by soil microorganisms efficiently, breaking down into carbon, water, and organic matter that builds soil health.

Cotton farming, when done conventionally, carries significant environmental costs: it is one of the most pesticide-intensive crops globally, and conventional processing uses substantial water and chemicals. Organically certified cotton - grown without synthetic pesticides and processed under verified standards - substantially reduces those upstream impacts.

The key qualifier throughout is "untreated." Cotton garments treated with stain-resistant finishes, wrinkle-resistant coatings, or synthetic dyes may not biodegrade cleanly, and blended cotton-synthetic fabrics will leave synthetic residue regardless of how well the cotton breaks down.

What "Biodegradable" Actually Requires

A garment is only genuinely biodegradable if:

1. It is made from 100% natural fibre. Even a small percentage of synthetic content - spandex waistbands, polyester stitching, nylon reinforcement - will not biodegrade, leaving microplastic residue in compost or soil.

2. It is untreated, or treated only with certified-safe finishes. Machine-washable wool is sometimes treated with polyamide resin, which can release microplastics. Many garments across all fibre types are treated with PFAS ("forever chemicals") for water or stain resistance - these persist indefinitely regardless of the underlying fibre.

3. It is certified by a recognised third party. The only reliable way to verify that a garment is made from pure, untreated natural fibres is third-party certification. GOTS (Global Organic Textile Standard) covers both fibre sourcing and chemical processing. OEKO-TEX Standard 100 certifies finished products are free from harmful substances. Without certification, "natural" and "biodegradable" on a label are unverified marketing claims.

At Nui, most of our garments are RWS or GOTS certified. Our merino is sourced from RWS-certified farms. We don't use PFAS treatments. When a Nui merino garment reaches end of life, it can be composted - no qualifications required.

What to Do With Old Natural Fibre Clothing

In order of environmental preference:

Pass it on. Quality natural fibre garments - particularly wool - have genuine secondhand value and long usable lives. Resale, donation, or gifting extends the garment's life and displaces a new purchase.

Repurpose it. Worn natural fibre garments can become cleaning rags, craft material, or insulation. The fibre retains its properties regardless of the garment's condition.

Compost it - but only if it meets the criteria above: 100% natural fibre, untreated or certified-safe finishes, no synthetic components. Cut it into small pieces to accelerate breakdown. A home compost bin with good moisture and microbial activity will biodegrade wool within one to five years, cotton within months.

Landfill as last resort. Even landfilled natural fibres will eventually biodegrade. It is far from ideal, but it is categorically different from landfilling synthetic garments, which will persist for centuries.

Nui's merino garments are made to be worn for years and composted at end of life - certified natural, untreated, and built to last. Explore our merino.