Clothes may not smell immediately after washing but develop an odor as they dry. Could this involve not only how well dirt is removed but also changes in microbial communities during laundering?
In 2025, Celia Díez López and colleagues examined T-shirt odor, odor compounds, bacteria and microbial genes together at the wearing, washing and drying stages. Their aim was to understand the mechanisms involved so that products and laundering methods could control malodor while retaining environmentally considerate practices such as low-temperature washing.
Clothes worn by 31 people were washed, then divided in half for drying
The study took place in Belgium. Participants were selected because preliminary assessments found unpleasant axillary or clothing odors and identified particular odor-related bacteria. Of the initial 32 participants, 31 completed the study: 12 women and 19 men aged 21–66. This was not a randomly selected general-population sample.
Participants wore identical T-shirts made of 93% polyester and 7% elastane continuously for 48 hours. During that time, they did not wash their bodies or use deodorant or perfume. They then washed the shirts in their home machines on a wool cycle at approximately 30°C, using fragrance-free, enzyme-free detergent and fragrance-free fabric softener. Depending on the machine, washing lasted 30–90 minutes, and no oxidizing bleach was used.
An unworn T-shirt supplied by the researchers and other household garments were included in the same load. This allowed the team to investigate what was introduced not only to worn clothes but also to clothes washed alongside them. Samples were also taken from washing-machine door seals, incoming water and other locations.
After washing, each T-shirt was cut in half. One half was kept at approximately 24.5°C and 50% relative humidity, and the other at approximately 31.8°C and 80% relative humidity, for 72 hours. Because temperature as well as humidity differed, the comparison below concerns the drying environments as a whole.
A sensory panel of 10 people rated odor intensity and pleasantness or unpleasantness. The researchers also combined analysis in which people smelled separated compounds with mass spectrometry to identify compounds actually perceived as malodorous. Bacterial abundance, composition and genes were measured using other methods. The 155 samples used for detailed genetic analysis comprised five sample types for each of the 31 participants: the axilla and T-shirts after wearing, after washing and after each of the two drying conditions. The study did not involve 155 people.
Odor reduced by washing returned under warm, humid conditions
Washing improved the pleasantness ratings of worn T-shirts. Drying at approximately 24.5°C and 50% humidity reduced odor intensity compared with immediately after washing. In contrast, keeping shirts at approximately 31.8°C and 80% humidity made the odor stronger and more unpleasant.
Changes in compounds followed a corresponding pattern. For example, 2-methylbutanoic acid and 3-methylbutanoic acid were evaluated together because the analysis could not distinguish them. Their detection disappeared from many T-shirts after washing, then increased again under warm, humid conditions. The authors considered the possibility that bacteria remaining on the shirts or newly introduced during washing regenerated these compounds during drying.
Not every odor compound followed the same pattern, however. Some compounds present before washing rarely reappeared during drying. Moreover, “not perceived” in this analysis does not mean a compound had fallen completely to zero; it can also mean its concentration was below the level at which people could detect its odor.
Washing also changed the bacteria on the clothes
Counts of bacteria with intact cell membranes were higher after washing than immediately after wearing. They increased further under warm, humid conditions and decreased under the comparatively dry conditions. An intact membrane is an indicator of possible viability, but this measurement did not establish that every cell could reproduce or determine its metabolic activity.
Microbial genetic information showed that worn clothes contained many skin-associated bacteria, whereas after washing the relative proportions of bacteria associated with environments such as water and soil increased. Source-estimation analysis also indicated contributions from washing-machine microorganisms and incoming water to bacteria on freshly washed clothes. Bacteria were introduced during washing even onto the unworn T-shirts in the same load.
Under these conditions, then, washing reduced odor compounds but was not simply a process that reduced clothing microbes. The authors viewed it as an exchange of bacteria among laundry, the machine and water, forming communities characteristic of the post-wash stage. Changes in those communities under subsequent environmental conditions offer clues to odor recurrence.
Persistent smells and the mechanisms that let bacteria remain attached
Genetic analysis found that genes associated with antimicrobial resistance and virulence accounted for a greater proportion of microbial reads on washed shirts and shirts dried under warm, humid conditions than immediately after wearing. This compared relative amounts within the analyzed genetic information, not total bacterial counts. These included genes involved in bacterial attachment to surfaces and the formation of film-like communities called biofilms. Such mechanisms could make bacteria more likely to remain on fabric.
What was measured was which genetic information was present in the samples and in what quantities. The study did not measure infection in wearers, the spread of antimicrobial resistance to people or the risk that freshly washed clothes would cause disease. Detecting bacteria or related genes is not itself evidence of harm to health. Moreover, overall potential microbial functions changed less than community composition, and the specific metabolic pathways responsible for particular malodors were not identified.
The authors discussed the possibility that attached bacterial communities accumulating through repeated wearing and washing could contribute to odors that are hard to remove. However, the study did not demonstrate long-term biofilm accumulation in these T-shirts or establish a causal link between such accumulation and odor.
Based on this hypothesis, the authors proposed optimizing technologies that disrupt biofilms, such as enzymes, as a future direction. Since some fabrics are sensitive to high temperatures, the idea is not simply to increase washing temperature, but to address how bacteria persist while limiting environmental impact. This experiment did not compare the effectiveness of particular enzyme detergents.
Summary
In this study, low-temperature washing initially improved the odor of synthetic T-shirts, but warm, humid conditions subsequently made them smell strongly unpleasant again. The results suggest that shifts in bacteria during washing and subsequent increases in bacteria and odor compounds may contribute to the return of odor.
The experiment selected people with strong odors and used specific clothing material, detergent, wearing duration and drying environments. The same changes cannot be assumed for all laundry. Nevertheless, the authors proposed that treating washing and drying as one continuous process, and studying both odor-compound removal and microbial persistence, could support laundering technologies that balance odor control with environmental impact.
References
Díez López, C., Van Herreweghen, F., De Pessemier, B., et al. (2025). Unravelling the hidden side of laundry: malodour, microbiome and pathogenome. BMC Biology, 23, 40. DOI: 10.1186/s12915-025-02147-5 · Full original article.
Original article: © The Author(s) 2025. CC BY-NC-ND 4.0. The research was funded by Givaudan UK Ltd, and the authors include company employees. Some authors also received support from VLAIO and FWO.
