THE SHORT ANSWER
There is not strong clinical evidence that wearing everyday polyester underwear lowers sperm count. Polyester is a plastic-based fibre, but finding microplastics in semen does not establish that underwear caused the exposure—or that changing fabric improves fertility.
Research checked 12 September 2026. Written by NUMBNUTS. Educational information, not a diagnosis.
Your underwear label deserves a closer look.
Choosing what sits against your skin is a reasonable part of buying underwear. Choosing a fertility treatment is a different decision. This guide separates three questions often rolled into one: what polyester is, what microplastics studies have found, and what underwear research can actually tell us.
1. Is polyester a plastic?
Yes. Polyester used in clothing is commonly polyethylene terephthalate, or PET. That describes the material; it does not by itself tell us how much a person absorbs from a garment or what happens to their sperm. A bottle-to-boxer visual can explain the shared material family. It cannot prove a medical effect.
2. What have microplastics studies found?
A 2024 study of 113 men in China found that exposure to more microplastic types was associated with poorer semen measures. It was observational: it did not test underwear or show that plastic fibres caused the changes. Read the study.
The findings are not uniform. A separate 2025 study of 45 semen samples did not find a significant association with sperm concentration or count. This is an emerging field with important unanswered questions about sources, exposure and health effects. Read the later study.
3. Does switching underwear improve fertility?
Research on fit, temperature and fabric should not be treated as interchangeable. A 2026 review describes limited fabric-specific evidence and no clear evidence of improved pregnancy outcomes from underwear choices. That leaves a gap between “I prefer this material” and “this material will increase my sperm count.” Read the review.
Polyester or merino: choose with your eyes open.
- Material: polyester is synthetic; merino is a wool fibre. Check the full composition of any garment, including blends and trims.
- Wear: consider fit, comfort, how the garment feels during your day, care requirements and your own skin sensitivity.
- Evidence: neither a natural-fibre label nor a cooling feature is proof of better sperm count, pregnancy or IVF outcomes.
What about heat?
Heat exposure is a separate research question. Our sauna and sperm guide explains one small human study. Evidence about repeated sauna exposure cannot be used to calculate the effect of a particular pair of boxers.
If you are trying to conceive
Make underwear a considered purchase, not a substitute for assessment. Fertility concerns deserve a conversation with a clinician and, where appropriate, semen testing. AUA/ASRM guidance recommends evaluating both partners concurrently in an infertility assessment.
Read the label. Know the difference.
NUMBNUTS combines merino boxers with reusable cooling inserts for cooling comfort. It has not been shown to increase sperm count or improve fertility outcomes.
Review your heat habits with the Heat Check →
The Heat Check is educational, not a fertility test.
Keep reading
Boxers, briefs and fit · Spermmaxxing and the evidence · The male-factor checklist
