The smell often starts before the wash, while your kit sits in the bag.

A shirt you've trained in carries bacteria from your skin along with your sweat (source). Seal it in a bag, damp and in the dark, and those bacteria keep growing. Odor researchers actually rely on this. In one study, shirts worn for an hour of spinning were sealed in plastic bags and left before a trained panel smelled them (source). That was a small study of 26 people, and the bags were a lab method rather than a test of gym bags, but it shows what a closed, damp space does to worn clothes.

As the consumer group Which? puts it, "Bacteria will collect (and multiply) anywhere that's warm and wet" (source). When they form a film on the fabric that survives the wash, the clothes hold on to the sweat smell (source). Drying matters too. A 2025 laundry study linked long drying in humid air with more bacteria and specific bad smells (source).

So don't leave damp kit zipped in. Which? advises airing it out for a few hours before you wash it, "whether that's outside on the line or indoors on an airer" (source). As the fabric dries, bacterial activity drops (source). After the wash, dry it fully, ideally somewhere that isn't humid.

Get it out of the bag, and let it dry.

Several of these are small studies, and we say so when we cite them.

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