Why does polyester smell when cotton does not?

The most useful thing to understand about the smell in synthetic clothing is that it is not on the fabric, it is in a layer chemically attached to it, and the layer is made of the same stuff you would use a degreaser on in a kitchen. Every effective fix follows from that, and every ineffective one — more scented detergent, more conditioner, a longer cycle at the same temperature — fails for the same reason.

Step by step

1. Look at the fibre.A polyester filament is a smooth solid rod. There is no lumen, no pore structure and no capillary route into the interior. Everything that happens to it happens on its surface.
2. Note that it repels water and attracts oil.Polyester is hydrophobic, so sweat beads and wicks along the outside rather than soaking in. It is also oleophilic, so the lipid fraction of sweat and sebum spreads out and bonds strongly to that surface.
3. Add the bacteria.Skin bacteria, chiefly Micrococcus species, metabolise those lipids into short-chain fatty acids and thioalcohols. Those compounds are the sour, sharp smell that people describe as the gym-kit smell, and they are chemically different from the smell cotton develops.
4. See why a normal wash does not remove it.A 30 degree wash with a modest detergent dose is designed to lift water-soluble soil from absorbent fabric. It does not emulsify oil that is bound to a hydrophobic surface. The garment comes out smelling of detergent with the oil still on it.
5. See why fabric conditioner makes it worse.Conditioner works by depositing a lubricating cationic layer on the fibre. On polyester that means adding a fresh oily film over the old one, sealing it in and giving the bacteria more to eat.
6. Understand why cotton behaves differently.Cotton is hydrophilic and porous. Sweat soaks into the fibre, carrying oil into the bulk of it where wash water and surfactant can get at it. Cotton develops a musty smell if left damp, but it does not build the persistent bound-oil odour that polyester does.
7. Take the remedy from the mechanism.You need heat or a surfactant strong enough to release bound oil: a full detergent dose, the warmest wash the label allows, no conditioner ever, a pre-soak aimed at oil, and drying fast so nothing sits damp. That is the whole treatment.

Tips

• This is why a technical top can smell clean out of the wardrobe and sour within five minutes of being worn: body heat volatilises the fatty acids that were always there.
• Silver-ion and other antimicrobial treatments on sportswear reduce the bacteria but do not remove the oil, and their effect diminishes with washing.
• Nylon behaves similarly to polyester but is slightly more absorbent, so it usually holds odour a little less. Polypropylene is the worst of the common fibres for this.
• Wool and merino do not do it, for the opposite reasons: they absorb moisture into the fibre and bind odour compounds internally.

Common mistakes

• Adding more scented detergent or a scent booster — It masks rather than removes, and the perfume itself deposits on the fibre and adds to the residue.
• Washing sportswear on a cold quick wash to protect it — The elastane needs a cool wash, but a cool wash needs a full detergent dose and a longer cycle to have any chance of shifting oil. A cold quick wash does neither.
• Leaving kit balled up in a bag until wash day — Warm, damp, oily fabric in a sealed bag is a bacterial incubator, and the odour compounds produced there bond to the fibre before it ever reaches the machine.

Questions people ask