FIBRE
Polyester smelled stronger
In a Ghent University study, polyester shirts worn for a spinning class smelled more intense and less pleasant than cotton ones after the same workout.
Check a fabricSweat and bacteria travel from skin to shirt.
What happens next depends on the fibre. Studies from Belgium, Canada, New Zealand and the UK show polyester and nylon hold on to odours differently from cotton and wool, and that how kit dries after washing counts too.
- Fibre changes the smell
- Washing is not the end
- Damp drying feeds odour
- Fresher kit, by the evidence
THE SHORT ANSWER
Bacteria, fibres
and damp.
Three findings from the research.
Together they explain most stubborn smells.
BACTERIA
Different fibres, different microbes
Micrococci were found in almost all the synthetic shirts and almost only on synthetic ones. Staphylococci were common on both cotton and polyester.
See the numbers
DRYING
Slow, humid drying smells
A 2025 study of synthetic T-shirts found humid drying conditions were linked to characteristic malodours and favoured the growth of particular bacteria.
What to do
FOUR HABITS FROM THE RESEARCH
Wash it,
then dry it well.
None of these needs special products. They follow from what the studies found about fibres, washing machines and drying.
Dry quickly and fully
Humid drying conditions were linked to characteristic malodours, so give kit air and space to dry.
Keep the machine clean
Bacteria form biofilms on washing machine surfaces and can pass to clothes, so the machine is part of the story.
Know what your kit is made of
Polyester and nylon held on to odour more in the studies. A cotton or wool layer may smell less for low-key sessions.
Check the care label
Researchers note cooler washes and less bleach remove fewer microbes from clothes, so see what temperature your kit's label allows.
TWO NUMBERS WORTH KNOWING
Small studies,
clear patterns.
One trial with real workouts,
one lab test across six fibres.
26 people
People who wore test shirts to a spinning class
Shirts were kept for 28 hours, then a trained odour panel scored them. Polyester smelled significantly less pleasant and more intense than cotton.
WORKOUT TRIAL · GHENT UNIVERSITY
6 fibres
Fibre types exposed to a synthetic sweat
Cotton, mercerised cotton and viscose gave off lower levels of ketones and aldehydes at 30 minutes than wool, nylon and polyester.
LAB TEST · OTAGO AND ALBERTA
A 2025 study found that washing reduced known odour compounds on synthetic T-shirts, yet the load of intact bacteria on the clothes went up, as skin bacteria were replaced by microbes from the washing machine.
A QUICK FABRIC CHECK
How much does
it hold on to?
Pick a fibre to see what lab studies found about how it takes up and gives off sweat odours. The bar shows how strongly that fibre tended to hold odour in the research.
Based on Callewaert et al. (2014) and McQueen et al. (2024). Lab results on fibres and yarns; real garments, blends and finishes vary.
THE WASHING MACHINE EFFECT
Clean is not
the same as sterile.
Clothes go through cycles of wearing, washing and drying, picking up microbes from skin, the washing machine, water and laundry products along the way. Researchers describe laundry odour as a whole ecosystem rather than a single cause.
A 2025 study notes that cooler washes and less bleach are better for the environment but can make it harder to remove microbes from clothes. A 2024 review from Manchester Metropolitan University adds that biofilms inside the machine can foul both its surfaces and the clothes.
In conversation
Questions people ask
Why does my polyester gym top smell worse than a cotton one?
In a study of 26 people after a spinning class, polyester shirts smelled significantly more intense and less pleasant than cotton. Different bacteria grew on each fabric: micrococci were found almost only on the synthetic shirts.
Is it the sweat or the bacteria?
Both play a part. Sweat and bacteria transfer from the skin, and the Ghent researchers concluded that the fibre type steers which microbes grow and so what odour may develop.
Does washing kill all the bacteria?
Not necessarily. A 2025 study found washing reduced known odour compounds on synthetic T-shirts, but the number of intact bacteria rose, with washing machine microbes replacing skin bacteria.
Why does kit sometimes smell after it has been washed?
One reason found in research is drying: humid, slow drying conditions were associated with characteristic malodours and favoured particular bacteria.
Is wool good for exercise clothes?
In one lab test, wool released more odour compounds than cotton at first, but like cotton and viscose it released lower amounts of ketones after 3 hours than nylon and polyester.
Sources
Each figure on this page comes from the sources below.
- Callewaert et al. (2014), Microbial odor profile of polyester and cotton clothes after a fitness session, Applied and Environmental Microbiology
- McQueen, Eyres and Laing (2024), Textile sorption and release of odorous volatile organic compounds from a synthetic sweat solution, Textile Research Journal
- Díez López et al. (2025), Unravelling the hidden side of laundry: malodour, microbiome and pathogenome, BMC Biology
- Osta-Ustarroz, Theobald and Whitehead (2024), Microbial colonization, biofilm formation and malodour of washing machine surfaces and fabrics, Antibiotics