Why synthetics smell, and cotton doesn’t

Fresh sweat is almost entirely water and salts with a few trace compounds. On its own it barely smells of anything. The odour everyone recognises is produced afterwards, by the bacteria that live on your skin, breaking down compounds in sweat and sebum and releasing volatile acids and thioalcohols as a by-product.

So the real question is not how much you sweat. It is what the fabric does with it.

Two opposite behaviours

Cotton is hydrophilic. It draws moisture into the fibre itself, holds it, releases it slowly, and lets go of it again in an ordinary cold wash.

Polyester is hydrophobic. It does not absorb. It moves liquid along the surface of the fibre and evaporates it — which is exactly what makes it dry quickly, and exactly what causes the problem. Sebum is oily, polyester is oil-friendly, and the two bind readily. Ordinary washing does not fully shift it.

Odour compounds therefore build up inside the fibre and release again whenever the fabric is warm and damp. This is the familiar experience of a synthetic top smelling within the hour — and smelling again the moment you wear it, despite having just been washed. It is not your imagination and it is not poor hygiene. It is the material.

The industry’s answer, and ours

The usual fix is an antimicrobial finish: silver-based compounds or quaternary ammonium salts applied to the fabric to suppress the bacteria. These are coatings. They sit directly against skin, they wash out gradually, and the garment’s odour performance declines as they go.

Cotton’s answer is that it doesn’t need one.

That is genuinely the entire argument, and it is a fairly boring one. No treatment, no finish, no clever chemistry — a fibre that absorbs and then washes clean.

Two practical notes

Wash cold. It is enough for cotton and it is gentler on the fibre. And skip the fabric softener — it works by depositing a coating on the fibre, which reduces exactly the absorbency you chose cotton for in the first place.