merino wool vs synthetic running shorts

Synthetic running shorts shed plastic. Merino does not.

A synthetic garment can release up to 700,000 microplastic fibres in a single wash — and a similar order of magnitude straight into the air around you as you move. Microplastics have now been detected in human blood, lungs, the placenta, and breast milk. Merino is a protein fibre that biodegrades; there is no plastic to shed, in the wash or on the run. More on that in do your workout clothes shed microplastics?

Sweat, heat, and friction during a run pull residual industrial chemicals out of synthetic fabric and through the skin — antimony from PET production, PFAS “forever chemicals”, phthalate softeners. One 2025 study modelled sweat increasing dermal PFAS absorption from textiles by up to 3,252× versus dry contact, with the highest concentrations in sportswear and water-repellent clothing. Our merino yarn is tested to OEKO-TEX Standard 100 — free of PFAS, formaldehyde, heavy metals, and banned azo dyes.

Most “merino” running shorts are mostly plastic.

Read the label on a typical “merino” running short and you’ll often find something like 86% polyester, 14% elastane — with merino present only as a thin liner, or a few percent in a blend. That garment still sheds microplastics and still carries synthetic chemistry. “100% merino” has to mean the whole short, not a panel. That is the standard we hold ourselves to.

Cotton is the natural fibre most “non-toxic” activewear reaches for — but it’s the wrong tool for running. Cotton soaks up sweat and holds it, going heavy, cold, and chafing on a long run. Merino wicks and releases moisture, regulates temperature, and resists odour between washes. It is the natural fibre that actually performs for running — which is exactly why we chose it. We weighed it against lyocell too; that comparison is in the research.

It’s the usual objection — and the reason most comparison guides hand the win to polyester. The honest answer: a featherweight merino tee will pill faster than plastic, but running shorts aren’t a featherweight tee. At the right weight, and constructed for movement, merino stands up to repeated runs and washes — and unlike synthetics, it resists odour, so it needs washing far less often. Fewer washes mean less abrasion and a longer life. Durability is a design problem, not a fibre verdict, and it’s one we’re engineering for.

RWS (Responsible Wool Standard) — certifies the Australian merino for sheep welfare, responsible land management, and chain-of-custody from grower to yarn. OEKO-TEX Standard 100 — tests the finished yarn for harmful substances, so what sits against your skin is free of PFAS, formaldehyde, heavy metals, and banned dyes. Together they cover both how the fibre is grown and what ends up in the cloth — explained in full in our guide to merino certifications.

Every claim on this page is drawn from peer-reviewed studies, laid out with sources — and the things we deliberately don’t claim — on our research page.