Fabric guide

The thing that would clean your running shirt is the thing that would ruin it

Polyester holds smell because it is saturated with your own skin oil. Getting oil out of fabric takes heat. And nearly all activewear is blended with elastane, which heat destroys permanently. That is the whole problem, and most advice about it ignores one half.

Last updated 26 July 2026

Polyester repels water and drinks oil

The usual explanation is that synthetics "trap bacteria," which is vague enough to be useless. The actual mechanism has been measured, and it is more interesting than that.

Polyester is hydrophobic — it repels water, which is why it dries so fast. But it is also oleophilic. It readily absorbs oils. And that distinction turns out to be exactly the wrong way round for staying fresh.

Sweat is mostly water and largely odourless on its own. What smells is what bacteria do to sebum — the mix of triglycerides and fatty acids your skin produces, which is their primary food source. A study measuring five underarm bacteria across fabrics found the split starkly:

Polyester is poor at absorbing the harmless part of sweat and excellent at absorbing the part that feeds the smell.

Wool is at the other end again. Its hygroscopic interior absorbs odour compounds into the fibre and holds them there, which is why a merino base layer behaves so differently from a polyester one on the same body.

Why it smells again the moment you start moving

Everyone knows this experience: the shirt comes out of the wash smelling of nothing, and ten minutes into a run it smells like the inside of a gym bag.

Two things are happening. The residual sebum was never fully removed by a 30 °C wash — oil needs heat and surfactant to shift, and a cool cycle gives it neither. And the bacteria are still there, because the act of drying bonds them on irreversibly. The same study found that capillary forces as water evaporates pull bacteria into permanent contact with the fibre, and that no fabric holds enough water on drying to prevent it.

So the garment was never actually clean. It was dry and odourless, which is a different thing. Add warmth and moisture and the reaction picks up exactly where it left off.

This is also why a damp top left in a bag overnight is ruined rather than merely unpleasant. You gave the colony hours of warmth and moisture, and then locked it in place by letting it dry.

Fabric softener is the exact wrong intervention

It is the instinctive fix — the shirt smells, so add something that smells nice — and it makes both problems worse.

Softeners work by depositing cationic surfactants that bond electrostatically to the fibre and stay there as a film. On technical fabric that film does two things, both bad:

• It blocks the capillary channels that move moisture from your skin to the outer surface. That wicking is the entire function you paid for.
• It adds another oil-attracting layer to a fabric whose core problem is already trapped oil.

The same reasoning explains why softener ruins towels. A softer towel is a less absorbent one, because the film sits over the capillary action that made it work.

If softener has been used on a garment for a while, a rinse with white vinegar is the common remedy for stripping the accumulated film. It is chemically plausible and widely recommended, though I have not found a controlled study demonstrating it.

Why you cannot just wash it hotter

Here is where the bind closes.

Elastane — the same material sold as spandex or Lycra — is a segmented polyurethane, and heat degrades the polymer. The damage accumulates with every exposure and none of it reverses. There is no cold wash that undoes it.

Which means the tumble dryer is where activewear actually dies. Dryer temperatures are far past anything elastane tolerates, and each cycle takes a permanent bite out of the recovery that makes leggings hold their shape. The same applies to any pair of jeans with elastane in it, which is most of them.

So the obvious fix — turn the temperature up until the oil comes out — is unavailable for almost everything in the drawer, because almost everything is a polyester–elastane blend.

Unless it is not. Plenty of technical shirts are 100% polyester with no elastane at all, and those genuinely can take a hotter wash when they have gone sour. This is one of the few times reading the composition label rather than the care label tells you something actionable.

Shedding: construction matters more than fibre

Synthetic textiles release microfibres into wastewater when washed. The headline numbers get quoted a lot; the useful finding almost never does.

Mechanically-treated polyester — fleeces and jerseys, anything with a raised or brushed surface — released around 161 mg per kilogram of fabric per wash, roughly six times what woven nylon made from filament yarns released at about 27 mg/kg. Across a set of twelve different polyester fabrics, release ranged from 210 to 72,000 microfibres per gram, per wash.

That range spans more than two orders of magnitude within a single fibre type. "Polyester sheds" is not one behaviour: a fleece sheds enormously, a woven polyester shell sheds very little. If you want to shed less, the fleece is the garment to wash less often — not the windproof.

Beyond that, the levers are full loads, cooler and shorter cycles, and a lower spin — all of which reduce the mechanical stress doing the shedding. Conveniently, that is the same list that keeps the garment alive longer.

What actually works

• Wash it promptly. Never let it dry in a bag — that is what makes smell permanent rather than temporary.
• Turn it inside out. The sebum is on the inside, so that is the surface the water should reach.
• Use enough detergent, and use a liquid. Sports detergents are surfactant-heavy for exactly this reason.
• No fabric softener. Ever. No dryer sheets either.
• Air dry, and enjoy how fast it is. Polyester holds 0.23% of its weight in water. It will be dry before a cotton shirt has started.
• Check the composition. No elastane means a hotter wash is on the table when something has gone properly bad.

The short version

• Polyester absorbs about ten times the sebum cotton does and spreads it where bacteria can reach it.
• Sweat does not smell. What bacteria do to skin oil smells.
• Drying bonds bacteria on permanently. Dry is not the same as clean.
• Fabric softener blocks wicking and traps more oil. It is the worst thing you can add.
• Heat destroys elastane cumulatively. The dryer is the real killer.
• Shedding depends on construction, not fibre. Fleece sheds; woven barely does.

Synthetics are the one part of the wardrobe where the care label genuinely is not enough, because it tells you a temperature and a symbol without telling you the thing that matters — that you are washing oil out of plastic, on a fabric that cannot take the heat which would do it properly. The temperature guide covers why 30 °C is right for almost everything else. This is the exception that proves it.