The Research: What Underwear Fabric, Heat and Plastics Do to Male Fertility

Does underwear fabric affect male fertility? We do not know, because nobody has run a controlled human study comparing underwear materials under normal wearing conditions. We do know that plastic particles are turning up in testes and semen, fit and heat affect semen parameters, and the one human polyester experiment used a scrotal sling, not underwear.

Last reviewed: 2026-09-18

Has anyone actually tested underwear fabric in men?

No. The two newest reviews say the same thing: there is no controlled human study comparing underwear materials under typical wearing conditions.

Zleczewski and colleagues reviewed the evidence on underwear, heat and male fertility in 2026. This was a review, not a new trial, so it enrolled no participants. It found that looser underwear is linked with modest differences in semen parameters, but not with different pregnancy outcomes. Crucially, the authors said controlled human studies comparing materials under typical wearing conditions are lacking (Zleczewski et al., Translational Andrology and Urology, 2026).

Klotz reviewed research on microplastics in male reproductive tissue in 2026. Again, this was a review, not a new study of participants. The review found that microplastics are regularly detectable in testes, semen and prostate tissue. It also said causation has not been proven in humans and recommended “precautionary, low-risk counselling” (Klotz, Urologie, 2026).

What this means: the fabric study has not been run. Full stop.

What it does not mean: lack of a perfect study is not proof that exposure is harmless. It means brands should stop pretending the answer is settled in either direction.

Are microplastics actually in human testes?

Yes. Researchers at the University of New Mexico tested 23 human testicular tissue samples for microplastics.

They found microplastics in every testis examined. The average concentration was about 330 micrograms per gram of tissue. That is not a typo. Plastic is in every testicle tested in this study, which is a sentence nobody was hoping to read over breakfast (Hu et al., Toxicological Sciences, 2024).

What this means: plastic particles can reach human testicular tissue.

What it does not mean: the study did not show where those particles came from. It did not identify underwear as the source. It did not measure fertility, testosterone or pregnancy outcomes in the men whose tissue was tested.

Are microplastics in semen too?

Yes. Two recent studies found them in semen samples. Neither proves that microplastics cause infertility.

A 2025 study from Tongji University in Shanghai tested semen from 45 men. Researchers found microplastics in 34 samples. PET, the plastic polyester is made from, was the most common polymer, making up 36% of all the particles found. PET exposure was associated with lower progressive motility, 20.6% versus 34.9%, but that difference was not statistically significant. The study found no effect on sperm count (Guo et al., Toxics, 2025).

A 2024 study tested semen and urine samples from 113 men in China. Researchers found microplastics in every semen and urine sample. Each additional type of plastic detected was associated with lower sperm count, concentration and motility (Zhang et al., EBioMedicine, 2024).

What this means: PET and other microplastics are showing up in semen, and the Zhang study found associations with worse semen parameters.

What it does not mean: association is not causation. These studies cannot tell us whether plastics caused the differences, whether another exposure drove both, or which route put the particles there. Food, water, air, packaging, household dust and synthetic clothing are all part of the plastic circus.

What was the polyester sling study?

It was a small human experiment from 1992. It was not an underwear study.

Ahmed Shafik had 14 men wear a polyester scrotal sling for 12 months. All 14 reached zero sperm count after a mean of about 140 days. Sperm production returned after the sling was removed. Serum hormones, including testosterone, did not change (Shafik, Contraception, 1992).

What this means: continuous contact with a polyester sling coincided with a dramatic and reversible sperm effect in this small study.

What it does not mean: boxer briefs are not a scrotal sling. This was 14 men, not hundreds. It does not show that polyester underwear lowers testosterone. In fact, testosterone did not change in the study. Anyone telling you otherwise skipped the part where the paper says exactly that.

What did the dog study find?

It found a reversible drop in sperm count in dogs wearing polyester underpants. Dogs are useful for asking early biological questions. They are not tiny men in boxer briefs.

Shafik’s 1993 animal study assigned 24 dogs to polyester or cotton underpants for 24 months. The polyester group saw sperm count fall. The effect reversed after the polyester underpants were removed. Hormones did not change. This was an animal study (Shafik, Urological Research, 1993).

What this means: fabric exposure was associated with sperm changes in this animal model.

What it does not mean: it does not prove the same thing happens in men. It also does not prove a man’s everyday polyester underwear changes testosterone, fertility or pregnancy odds.

Do boxers beat tighter underwear?

For semen parameters, looser styles came out ahead in one large observational study. The study was about fit and heat, not fabric.

Harvard T.H. Chan School of Public Health studied 656 men. Men who reported wearing boxers had 25% higher sperm concentration than men who wore tighter underwear styles (Minguez-Alarcon et al., Human Reproduction, 2018).

What this means: fit and heat matter. A tighter environment around the boys is not the same thing as a cooler, looser one.

What it does not mean: this study did not compare cotton, merino, polyester or any other fabric. It cannot be used as proof against polyester. It also cannot prove that switching to boxers changes a specific man’s fertility.

Is BPA found in polyester clothing?

Yes. The Center for Environmental Health found BPA in polyester-based activewear at up to 40 times California’s Prop 65 safe-harbor limit.

The testing covered polyester-based sports bras and athletic shirts between 2022 and 2023. BPA is an endocrine-disrupting additive. Polyester textiles can also carry chemical additives such as BPA and phthalates. That matters because clothing sits on skin, plus sweat, heat and friction. Not exactly a spa day for your family jewels (Center for Environmental Health, 2022-2023).

What this means: plastic-based clothing can carry endocrine-disrupting additives.

What it does not mean: this testing did not measure BPA from a specific boxer brief entering a specific man’s body. It did not test Happy Banana. It did not prove that polyester underwear changes sperm or testosterone.

Do microplastics disrupt testosterone?

In animals, researchers have identified mechanisms by which microplastics can disrupt testosterone production. In humans, that causal evidence is absent.

Khawar and colleagues published a 2026 review of animal research on microplastics and testosterone production. It reviewed possible biological mechanisms. It was not a human trial, and it did not establish that microplastics lower testosterone in men (Khawar et al., Journal of Environmental Sciences, 2026).

What this means: there is a biological reason to take the exposure question seriously.

What it does not mean: Happy Banana does not claim polyester underwear lowers testosterone, and we do not claim our underwear raises or protects it. That claim would outrun the human evidence.

What do we make of all this?

1. Nobody has run the study people actually want. There is no controlled human trial comparing underwear fabrics under normal wearing conditions.

2. Plastic is reaching male reproductive tissue. Microplastics have been found in every human testis tested in one study, and in semen studies too.

3. PET is part of the picture. PET, the polymer polyester is made from, was the most common plastic found in the 2025 semen study.

4. Fit matters separately from fabric. The Harvard study points to looser underwear and less heat as a sensible move for semen parameters. It does not answer the fabric question.

5. The precaution is cheap. We do not promise an outcome. We remove an exposure. Happy Banana boxer briefs use 100% merino wool, with no polyester, nylon or spandex in the fabric. The swap takes 30 seconds. Your banana can handle that.

Sources

• Zleczewski et al. Underwear and male fertility review. Translational Andrology and Urology. 2026. https://pubmed.ncbi.nlm.nih.gov/42293842/
• Klotz. Microplastics and male reproductive tissue review. Urologie. 2026. https://pubmed.ncbi.nlm.nih.gov/42734812/
• Hu CJ et al. Microplastic presence in dog and human testis. Toxicological Sciences. 2024. https://hscnews.unm.edu/news/hsc-newsroom-post-microplastics-testicular
• Guo et al. Microplastics in human semen. Toxics. 2025. https://pubmed.ncbi.nlm.nih.gov/40711011/
• Zhang et al. Microplastics in semen and urine. EBioMedicine. 2024. https://pubmed.ncbi.nlm.nih.gov/39342804/
• Shafik A. Polyester-induced azoospermia in normal men. Contraception. 1992. https://pubmed.ncbi.nlm.nih.gov/1623716/
• Shafik A. Textile fabric and spermatogenesis in dogs. Urological Research. 1993. https://pubmed.ncbi.nlm.nih.gov/8279095/
• Minguez-Alarcon L et al. Type of underwear worn and markers of testicular function. Human Reproduction. 2018. https://doi.org/10.1093/humrep/dey259
• Center for Environmental Health. New testing shows high levels of BPA in sports bras and athletic shirts. 2022. https://ceh.org/latest/press-releases/new-testing-shows-high-levels-of-bpa-in-sports-bras-and-athletic-shirts/
• Khawar et al. Microplastics and testosterone mechanisms review. Journal of Environmental Sciences. 2026. https://pubmed.ncbi.nlm.nih.gov/42070873/

Happy Banana sells underwear. This page is not medical advice. If you are concerned about fertility or hormone levels, see a urologist or endocrinologist.