Synthetic workout clothes are designed for performance, but fabrics like polyester and nylon are also especially ‘good’ at holding onto odour. Unlike natural fibres such as cotton, synthetic fibres are hydrophobic, meaning they repel water. That same property causes them to attract and trap oily substances from sweat and skin. Body odour itself isn’t caused by sweat alone, but by bacteria breaking down sweat into smelly compounds. In synthetic fabrics, those oily residues and bacteria can cling tightly to the fibre surface, surviving a typical wash cycle and reactivating as soon as the garment warms up again during your next workout.

Another reason odours persist is the physical structure of many performance textiles. Athletic wear often features tightly woven microfibres engineered to wick moisture away from the skin. This improves comfort, but it also creates tiny spaces where odour-causing bacteria can lodge and form biofilms — bacteria communities that are hard to remove with standard detergents. Over time, detergent buildup, fabric softeners, and body oils can accumulate in these fibres, further trapping smells and reducing the fabric’s breathability. The result is clothing that may smell fresh out of the dryer, but that quickly develops a musty, sour odour once worn.

To combat lingering smells, many people add scented beads or fragrance boosters to their wash. While these products can temporarily mask odours, they don’t eliminate the underlying bacteria and residue causing the smell. In fact, layering fragrance over trapped odour compounds can create a stronger, more unpleasant combination. There are also health and environmental concerns to consider: chemical fragrance additives can irritate sensitive skin, especially in tight-fitting workout gear, and some scent formulations contain chemicals that persist in waterways — in streams, rivers, lakes and beyond.