A garment that still smells after washing has a residue problem

Where the smell actually comes from

Fresh sweat is almost odourless. What produces the smell is bacteria living on skin and on clothing, metabolising the components of sweat and sebum and releasing volatile compounds as a by-product. The garment is not the source; it is the substrate.

That distinction has practical consequences. Removing the smell means removing the food supply — the oily residues deposited on the fibre — rather than merely rinsing the fabric. A wash that lifts water-soluble soil and leaves the lipids behind will produce a garment that smells clean when dry and smells again within twenty minutes of being worn.

The reactivation is the giveaway. If a shirt is fine out of the wardrobe and unpleasant as soon as it warms up, the residue is still in it, and washing it again the same way will not change that.

Why synthetics are worse at this than cotton

Polyester and nylon are hydrophobic and oleophilic, which is a slightly technical way of saying they repel water and attract oils. Body oils therefore migrate into and along synthetic fibres readily, and once there they are difficult for a water-based wash to reach.

Cotton behaves the other way. It absorbs water freely, swells, and lets detergent solution get among the fibres, so oily soil is more accessible. A cotton T-shirt worn hard washes properly clean; a polyester one from the same day often does not.

Synthetic fibre surfaces are also smoother and less porous, which sounds like it should help and does not, because the oils sit on the surface as a film rather than being trapped where enzymes and surfactants can work on them. This is the whole reason odour is a recognised problem in synthetic sportswear and largely a non-problem in cotton underwear.

Fabric softener is often the cause rather than the cure

Fabric softeners work by depositing a thin layer of lubricating, usually fatty, material onto the fibre surface. That is what makes cloth feel soft and reduces static, and it is also a coating that traps odour compounds and gives bacteria something to live on.

On towels and sportswear the effect compounds, because the coating also reduces absorbency, so the garment holds less water, rinses less thoroughly and dries more slowly. A towel that has stopped drying you properly has usually been softened into uselessness rather than worn out.

The same logic applies to overdosing detergent. Excess detergent does not rinse out fully, and the residue left behind is itself a deposit that holds soil and smell. More product is not more cleaning, and on synthetics it is frequently the opposite.

What actually shifts an established smell

The first thing to try is a long soak in warm water with the correct dose of a detergent containing enzymes and, where the fabric allows, an oxygen-based bleach. Enzymes break down the protein and fat components of the residue; time lets them work, and a fifteen-minute cycle does not give them enough of it.

A pre-soak with a small quantity of white vinegar or, separately, with a bicarbonate solution both have their advocates, and both do something real: the acid can break up mineral and detergent residues, and the alkali helps neutralise some acidic odour compounds. They should not be combined in the same bath, because they cancel each other out.

Temperature matters more here than in ordinary washing. Where the care label allows it, a warmer wash mobilises fats far better than a cold one, and cold washing is a large part of why odour problems have become more common in wardrobes generally.

Drying is half the problem

Damp cloth is a growth medium. A garment left in the machine for a day after the cycle finishes, or hung to dry in a cold room with no airflow, will develop a musty smell that has nothing to do with body odour and everything to do with what grows in the meantime.

That smell comes from moulds and bacteria colonising damp fibre, and it embeds itself in the same way. Once a load has gone musty, rewashing without addressing the drying conditions gives you a clean garment that will do it again.

The remedy is airflow and speed. Get washing out of the drum promptly, hang it with space between items, and dry it as fast as conditions allow. Sunlight helps for reasons beyond drying, since ultraviolet light kills surface organisms, though it will also fade dyes over repeated exposure, so it is a trade rather than a free gain.