Yes, but not for the reasons most men assume.
The conversation usually starts and ends with heat. Boxers vs. briefs. Loose vs. tight. Keep the boys cool, keep the swimmers healthy. That part is real-testicles sit outside the body for a reason, and sustained heat above scrotal temperature does reduce sperm quality. A 2018 study in Human Reproduction found that men who wore boxers had 25% higher sperm concentration than men who wore tight briefs, likely because boxers allow better airflow and temperature regulation.
But heat is only half the story. The other half is about what your underwear is made of, and that's where the science gets more interesting-and more specific.
The Scrotum Is One of the Most Absorbent Areas on Your Body
This is not a detail most men know, and it changes the frame entirely.
A landmark 1971 study by Maibach et al. measured how much of a test compound different body sites absorbed. The forearm was the baseline. The scrotum absorbed nearly 40 times more than the forearm. Higher than the forehead. Higher than the armpit. Higher than the scalp.
The reason is anatomical. The skin on the scrotum is thinner than almost anywhere else. It has more hair follicles and a richer blood supply. That combination makes it exceptionally permeable. What touches that skin doesn't just sit there-it gets in.
This is general dermatology, not a health claim. You can verify it in any textbook on skin physiology. And it means that the question "what is my underwear made of?" is not a luxury concern. It's a legitimate biological question.
Heat and Sweat Increase Absorption Further
Now stack two more mechanisms on top of that.
First, warm skin absorbs more. A 2020 study in Toxicology in Vitro showed that raising skin temperature from 25°C to 39°C (roughly room temperature to sauna temperature) increased absorption of test compounds by over 200%.
Second, damp, covered skin absorbs more. When skin is occluded (covered) and sweating, the hydration level of the outer skin layer rises, and that increases penetration. This was documented in the British Journal of Dermatology back in 1988.
Put these together: you have a highly permeable area, exposed to heat, under clothing that traps sweat. That is the exact environment where what your underwear is made of matters most. The physiology pushes in one direction: clean materials matter most precisely where and when it is hot and damp.
Fabric Chemistry Migrates Into Sweat
Most men assume fabric is inert. It sits there. It does its job. End of story.
It is not that simple.
There is a standardized test called ISO 105-E04 that measures how much chemistry migrates from fabric into artificial sweat. Studies using this test have found real compounds leaving fabric under sweat conditions. Polyester, for example, is manufactured using an antimony catalyst, and trace amounts of antimony can migrate out into sweat. A 2021 study measured this. A 2016 study found residual formaldehyde migrating from finished cotton.
The amounts are small. But "small" means something different when the absorption site is the scrotum, and the conditions are heat and sweat, and the exposure is every day for years.
Synthetics Shed While You Wear Them, Not Just in the Wash
The microplastic conversation has been almost entirely about laundry. Wash a polyester shirt, and thousands of microfibers go down the drain and into the water system.
But a 2020 study showed that polyester sheds microfibers into the air during ordinary wear and movement. The magnitude was in the same ballpark as washing.
This moves the story from "someday in the ocean" to "right now, on your body, in the room with you." If your underwear contains synthetic fibers, you are shedding microplastic particles directly against the most permeable skin on your body, all day long.
What the Research Does and Does Not Say About Fertility
Here is where precision matters.
Multiple studies from 2023 and 2024 have found microplastics in human testes and semen. A 2024 study from the University of New Mexico found microplastics in every single human testicle they examined-23 out of 23 samples. Polyethylene, the plastic used in grocery bags and food packaging, was the most common type.
That presence is real. It is citable. It is concerning.
What is not established is a direct causal link between those particles and worse fertility. The connection is association, not proven cause. And nothing in the research ties the particles specifically to clothing. Microplastics are everywhere-in the air, in the water, in the food. Tracing them back to underwear is not something the current science can do.
So the honest answer is: we know microplastics are reaching the testicles. We do not yet know what that means for fertility, or how much of the load comes from clothing versus other sources. Anyone who tells you different is overinterpreting the data.
What You Can Actually Do
This is where the science meets practical choices, and the practical choices are straightforward.
For temperature: If you are trying to conceive, switching from tight briefs to loose boxers is a low-effort change supported by evidence. The 2018 Human Reproduction study is worth reading. The difference was not small-25% higher sperm concentration in boxer wearers.
For fabric: The physiology makes a logical case for clean materials against scrotal skin. Natural fibers like cotton and cellulose-based fibers like TENCEL Modal are not perfect-they still absorb moisture and can hold heat-but they avoid the antimony issue in polyester and the shedding of plastic microfibers.
For synthetic stretch: Most "natural fiber" underwear still contains elastane or spandex for stretch. Those are plastics-polyurethane, specifically. Some manufacturers are moving to degradable elastane alternatives like ROICA V550, which carries material-health certifications from Hohenstein and Cradle to Cradle. That is a material choice worth knowing about, not a prescription.
For the sauna: If you use a sauna regularly, the heat-and-sweat absorption mechanism is amplified. Consider what your shorts are made of. Cotton or natural fibers are the safer bet against permeable skin under those conditions.
What This All Means for You
There is scientific evidence that underwear choice affects male fertility. The heat mechanism is well-established. The absorption and fabric-chemistry mechanisms are biologically plausible and supported by dermatology and materials science, though not yet tested specifically for fertility outcomes.
The smart move is to treat this the way you treat other low-effort health adjustments: make the choice that aligns with what the evidence suggests, without waiting for perfect proof. Loose fit. Clean materials. Minimal synthetics against permeable skin.
And if you are actively trying to conceive, talk to a urologist or reproductive specialist. They can run actual tests on your sperm quality and give you personalized guidance. The research tells us what general patterns look like. A doctor can tell you what is happening in your specific case.
This article is for educational purposes only and does not constitute medical advice. Consult a healthcare provider for personal medical decisions.