What are we wearing when we sweat?

Polyester is a plastic — polyethylene terephthalate, the same polymer family as a drinks bottle — melted and extruded into filaments. Nylon is a polyamide, also extruded, generally softer and more abrasion-resistant. Elastane (also sold as spandex or Lycra) is a polyurethane-based fibre that can stretch several times its length and return, which is what makes modern activewear fit the way it does.

Wool, cotton, linen and hemp are the alternatives, and they behave differently: they absorb moisture into the fibre rather than only moving it along the surface, they are less dimensionally stable, they usually cost more to produce, and they cannot deliver four-way stretch without help.

Blends exist because no fibre does everything. Elastane at 5–20% supplies recovery — the difference between leggings that hold their shape and leggings that bag at the knee by week three. Polyester supplies structure, colourfastness and price. Nylon supplies softness and durability. Small amounts of wool or cotton are sometimes added for feel or odour behaviour.

The cost of blending is disposal. A polyester-elastane knit is difficult to recycle in practice, because separating the fibres at scale is unresolved. Most 'recycled polyester' garments are made from bottles, not from old clothing, and the loop closes once rather than continuously.

Fibre content is declared by law in most markets. Chemical finishes largely are not. Antimicrobial treatments — often silver-based — are applied to slow odour. Water-repellent finishes historically relied on PFAS chemistry, which is now restricted or being phased out in several jurisdictions and voluntarily removed by many brands. Softeners, anti-static agents and print binders are routine.

This is where honest reporting gets hard. Most finishes are used within regulatory limits, many wash out substantially over time, and a treated garment is not a hazard. But a shopper genuinely cannot know what has been applied from the label, and that is a design of the disclosure system rather than an accident.

Plastic-based fibres are hydrophobic. They absorb very little water, so sweat sits on the surface and is moved along by the knit structure — genuinely effective at keeping you feeling dry while moving in cool air, and much less so when the air is warm and still. In a hot room the same shirt stops evaporating and starts feeling like a bag.

Odour is the other well-documented difference. Bacteria that produce the sharpest odour compounds thrive on polyester more than on wool, which is why a merino base layer can go days and a polyester one cannot go two sessions. That is a repeatable finding, not folklore.

Elastane also dislikes heat. Repeated high temperatures — hot rooms, hot washes, tumble dryers — degrade it, which is how stretch garments lose their recovery and finishes lose their evenness. It is one straightforward reason we do not think plastic-based fabric belongs in a sauna.

Textiles shed fibre fragments in use and in the wash. Napper and Thompson (2016) measured hundreds of thousands of fibres released from a single domestic load, with acrylic shedding most and polyester substantially. Later work found that a large proportion of shedding happens in the first few washes of a new garment, and that shorter, cooler, fuller cycles reduce it.

Microplastics have been detected in human tissues — blood, lung, placenta, and in work summarised by Stanford Medicine, the brain. Reviews tracing lifetime exposure describe how particles move from consumption to distribution around the body. That detection is established. What those particles do once they are there is not, and we will not pretend otherwise.

Here is our position plainly. We know plastic-based fibres are persistent, widely used, and capable of shedding, and that clothing worn against warm, damp skin is one of the most intimate exposures we have. We are still learning what that means for human health. For us, uncertainty is a reason to look more closely — and to question where plastic is genuinely necessary rather than simply default.

That is not the same as fear. We would rather you kept the leggings you own for another three years than replaced a working wardrobe out of worry. Reduce what is unnecessary; do not panic about what is already in your drawer.