Why do undershirts develop armpit odour that does not wash out?

Short answer: Persistent armpit odour usually comes from body oils, sweat residues and deodorant build-up in the fabric that normal washing does not fully remove, where odour-forming bacteria thrive. Synthetic fibres such as polyester tend to hold oily residues more than cotton, so smells return quickly when the fabric warms up again. Low-temperature washing, short cycles, overloading, too little detergent and fabric softener build-up make it worse. Brands can choose fibres and blends less prone to holding odour, such as cotton, modal or polyester blends with these fibres, consider an odour-control finish with care, and give clear washing advice. Any antimicrobial or odour-control treatment must comply with local rules, such as the EU Biocidal Products Regulation for treated articles, and claims need evidence. Advise customers to wash undershirts after each wear, use enough detergent, avoid fabric softener on technical fabrics and, where the label allows, wash warmer occasionally.

Why odour stays

• Polyester and nylon bind oily residues.
• Deodorant and antiperspirant residues build up in the underarm area.
• Low-temperature short cycles do not remove oils fully.
• Fabric softeners coat fibres and trap residues.

Fabric options

• Cotton, modal or viscose: less prone to holding oily odours.
• Polyester blends with cellulosic fibres.
• Fabrics with odour-control finishes, after checking regulations and durability.
• Merino wool for premium lines.

Care advice for customers

• Wash after each wear, inside out.
• Use the recommended dose of detergent.
• Avoid fabric softener on technical fabrics.
• Wash at the highest temperature the label allows from time to time.
• Dry fully before storing.

Testing

Wear trials with the same testers wearing different fabrics on similar days give the clearest comparison. Wash the garments following ISO 6330 procedures between wears and record odour assessments.

A worked example

As an illustration, if testers report stronger odour from the polyester style than from a cotton-modal blend after a week of wear and washing, the brand could add the blend as an alternative and update care advice for the polyester style.

Responding to complaints

When customers report odour, ask how they wash and dry, and share the care advice rather than blaming them. Repeated complaints on one style or lot point to the fabric or finish rather than individual habits.

Limits

Personal body chemistry and laundry habits vary greatly. No fabric eliminates odour for everyone.

Facts, recommendations and uncertainty

Established: oily residues and deodorant build-up cause persistent odour, especially in synthetics. Recommendation: choose suitable fibres, give clear care advice and use compliant finishes only with evidence. Uncertain: how much each customer's habits contribute.

Sources and further reading

• Regulation (EU) No 528/2012 — Biocidal Products Regulation (treated articles)
• ISO 6330:2021 — Domestic washing and drying procedures for textile testing

Standards and regulations are referenced by their published scope. Test parameters, cycles and pass levels should be agreed with your laboratory for the specific product; legal requirements should be confirmed for each destination market.