Why old sweat clings on

Short answer: The fatty compounds in sweat bind to the fibre, and over many washes a layer builds up. The layer releases odour every time the fabric gets warm or damp. That is why a garment can smell of nothing in the wardrobe and still smell of sweat as soon as you warm up.

Fat binds best to synthetics

Polyester and nylon are chemically close relatives of fat. That is why skin oil sticks to them and is hard to rinse out at low temperatures. Natural and regenerated fibres bind less often in the same way, but they can also get a layer if washed cool and not dried fully between wears.

The layers build up

If you wash cool every time, only part of the fat is removed. The rest stays, and next time more is added on top. After many rounds the layer is thick enough to be felt as slight stiffness or a heavy, sour smell. See bamboo tights for women.

It shows as odour that returns

The garment smells clean in the wardrobe, but as soon as you warm up, the smell is there. It is the old residue being released. Heat and moisture wake the bacteria and loosen molecules from the layer. That is why it often happens in the middle of the day, when the body is warm, and not the moment you put the garment on.

Why sweat itself does not smell

Fresh sweat hardly smells. Only when bacteria have time to act on fat and protein does the odour appear. So it is not the sweat you need to remove, but the residue the bacteria live on. See bamboo underwear for women and bamboo socks for women.

Low temperatures and too little detergent

Cool water and a small dose of detergent save energy but do not always loosen the fat. Follow the dosing on the pack and choose a temperature that suits the fabric but is not lower than it needs to be. If the laundry is very dirty, a higher temperature may be needed.

A tightly packed machine

If the machine is overfilled, the water does not get between the garments, and the fat stays. Fill the drum to about three quarters so the laundry can move freely. It makes a bigger difference than many think and costs nothing.

Drying after washing

A garment that is not completely dry when put away gives bacteria good conditions. Hang it in open air and let it dry in all seams. A tumble dryer can help if the fabric tolerates it, but always follow the care label.

How to remove an old layer

Soak the garment and wash at the highest temperature the fabric tolerates. A detergent with enzymes can break down fat and protein. Repeat a couple of times if needed. If the smell is still there, the residue sits deep, and it may take more washes.

How to avoid it happening again

Wash clothes after use when they have been close to the skin, and do not leave them damp in the basket. Now and then switch to a higher temperature, even if it is not necessary, so the layer never gets thick.