Check the label on almost any gym top or pair of leggings sold in the UK. Odds are it reads something like 88% polyester, 12% elastane. That is not laziness on the brands' part, and it is not purely about cost. Polyester genuinely does things cotton cannot — and it brings one very specific problem, which is why your gym drawer develops a smell and your bed sheets never do.
What polyester actually is
Polyester is a synthetic fibre spun from petroleum-derived polymers — in plain terms, a very fine, very strong plastic thread. What matters is the control that gives a manufacturer: the fibre can be extruded to an exact thickness, given a specific cross-section, and woven into a fabric that behaves precisely as designed. Cotton, grown in a field, offers no such control.
Why activewear brands choose it
It moves sweat rather than soaking it up. Polyester is hydrophobic — it repels water, holding well under 1% of its own weight in moisture where cotton holds roughly 8%. Instead of absorbing sweat, it pushes it along the fibre surface to the outside of the garment, where it evaporates. That is all "moisture wicking" means. A cotton tee drinks your sweat and then hangs off you, cold and heavy, for the rest of the session.
It dries fast. Because so little water sits inside the fibre, polyester dries in a fraction of the time — useful mid-session, and useful when kit needs turning around for tomorrow morning.
It holds its shape. Polyester resists stretching, abrasion and repeated washing far better than cotton. Blend in a little elastane and you get fabric that stretches with a squat and returns to shape rather than bagging at the knees. It also holds dye tightly, does not shrink, and is cheap to produce at scale — which is a large part of why it dominates the market.
The trade-off nobody prints on the label
Polyester repels water, but it attracts oil. Sweat is mostly water, and water is not what smells. The problem is the sebum and body oils that come with it. Those oils are drawn into the fibre and stay there, and ordinary detergent — formulated for cotton bed linen and office shirts — is not built to shift them at a cool wash.
What is left behind feeds odour-causing bacteria. They multiply, and their by-products are what you smell. This is why kit can come out of the machine apparently clean and then turn the moment you warm up: the residue is still there, waiting for heat and moisture. Our guide to permastink and why it takes hold covers that effect in detail.
Fabric softener makes it considerably worse, coating the fibres in a waxy film that blocks the wicking polyester was engineered for and gives bacteria somewhere else to live. More on that in why softener and gym kit do not mix.
So is polyester the wrong choice?
No — it is a trade-off, not a mistake. Merino wool is naturally more odour-resistant, but costs more and wears through faster under heavy use. Bamboo and Tencel sit in between: less prone to holding smell, but slower to dry. For high-intensity training, synthetics still win on performance. The odour issue is a laundry problem, not a fabric flaw — and laundry problems are solvable.
How to wash polyester kit properly
- Turn garments inside out — oils and bacteria concentrate on the inside surface.
- Wash at 30°C. Hotter degrades elastane and DWR coatings without improving odour removal.
- Skip the fabric softener entirely.
- Do not overload the drum; kit needs room to move for water to flush residue out.
- Air dry where you can. Tumble heat shortens the life of synthetics.
- Use a detergent formulated for the fabric — the step most people miss.
That last point is where Titan Wash comes in. It uses a high enzyme complex targeting the sweat proteins standard detergents leave behind, plus zinc ricineolate, which binds odour molecules so they rinse away rather than linger. Fully active from 20°C, with no optical brighteners, chlorine bleach or phosphates, and safe on polyester, nylon, Lycra, elastane, merino, bamboo, Tencel and DWR-coated fabrics. Add 30g straight to the drum, not the drawer.
For cycle, spin speed and temperature, see our guide to the right washing machine setting for gym clothes.
Frequently asked questions
Why does polyester smell worse than cotton?
Because it holds on to body oils rather than water. Cotton absorbs sweat and releases it in the wash fairly readily. Polyester repels water but attracts oil, so the residue that feeds odour-causing bacteria stays in the fibre unless your detergent is built to break it down.
What percentage of elastane should activewear have?
Typically 5–15%. Below 5% gives limited stretch; much above 15% can feel restrictive and loses shape sooner, as elastane degrades faster than polyester.
Can a hot wash kill the smell in polyester?
It is not worth it. Hot water damages elastane and waterproof coatings, and heat alone does not remove the oily residue causing the odour. A 30°C wash with the right detergent works better and keeps your kit intact.
Get the fabric doing its job again
Polyester is engineered to move sweat away from you. Once residue clogs the fibres, it stops. Titan Wash is £14.99 for a 750g pouch — 25 washes, or 60p per wash — dropping to 49p per wash on the 3-pack at £36.99. If you would rather test it on your worst-offending top first, the trial sample is £2.95.