Dermatology mapped the male body's absorption hotspots long before anyone worried about microplastics. In 1971, Howard Maibach and colleagues at the University of California applied the same test compound to different skin sites in human volunteers and measured how much penetrated over time. The scrotum ranked near the top, far above the forearm baseline and higher than the scalp, forehead, and underarm. That study had nothing to do with plastic or fertility. It was pure skin physiology. Fifty-plus years later, that physiology is the part of the plastic exposure conversation most people skip.
I am not a doctor, and this is not medical advice. I am a men's health researcher who reads primary studies for a living. If you have a fertility concern, talk to a doctor, because individual situations vary. The finding I keep returning to is this: the male reproductive area is one of the most permeable patches of skin on the body, and the conditions that make skin absorb more are the same conditions men create every day. Sitting at a desk, training, sweating through a commute, sitting in a sauna. Understanding the skin route changes how the microplastics debate looks.
A 1971 map that still matters
The Maibach study did something simple and rare in dermatology. Instead of testing absorption on the forearm, the standard site, the team dosed multiple regions in the same volunteers and compared the results. Regional variation was not subtle. Some body sites barely let the compound through. The scrotum absorbed at a rate far above the forearm baseline. The researchers attributed this to anatomy: a thinner outer skin layer, more hair follicles, and a richer blood supply. Those three features create a gate that swings open wider than most of the surface area on a man's body.
That paper is now more than half a century old. The finding has not been overturned. It gets cited in toxicology textbooks because regional skin permeability is a basic fact of human physiology. What changed is the world around the skin. The materials that sit against that permeable patch all day used to be mostly cotton and wool. Today they are mostly synthetic, and synthetics are plastics with a chemical backstory.
Heat and sweat: the absorption multipliers
A 2020 study in Toxicology in Vitro put human skin samples in diffusion cells and raised the temperature from 25 to 39 degrees Celsius. Absorption of the test compounds increased by more than 200 percent. Fourteen degrees is not a lab fantasy. That is the difference between a cool room and skin warmed by exercise, a commute, or a sauna session. Occlusion makes it worse. When skin is covered, warm, and damp, hydration rises and penetration climbs with it, a pattern documented in the British Journal of Dermatology back in 1988.
Combine those findings and you get a mechanism with no speculation required. The male reproductive zone is highly permeable. Heat raises skin absorption by a large margin. Trapped sweat and fabric cover it, which keeps the skin hydrated and warm. None of this requires a claim about any specific chemical or any specific garment. It is the basic physics of skin.
What fabric chemistry does when it meets sweat
You probably assume fabric just sits there. It does not. Textile chemists have a standardized test for what comes off a fabric into sweat, ISO 105-E04. Researchers using it have measured real chemistry migrating out of textiles: antimony from polyester, residual formaldehyde from finished cotton. The polyester finding is concrete. Polyester is manufactured using an antimony catalyst, and trace amounts of that catalyst can leave the fiber and enter artificial sweat in laboratory conditions (Regulatory Toxicology and Pharmacology, 2021). The cotton finding is just as specific: durable-press finishes can leave residual formaldehyde that releases into synthetic sweat, with more release at higher pH (Textile Research Journal, 2016).
Wear adds another layer. A 2020 study in Environmental Science and Technology showed that polyester garments shed microfibers into the air during ordinary movement, at a scale similar to what they shed in the wash once you account for comparable wear time. That moves microplastic shedding out of the laundry room and into the room with you, onto your skin.
The dermal absorption research on plastic additives runs in the same direction. Work using 3D human skin models has shown that additives carried on microplastic particles can become bioavailable across the skin barrier, and that hydrated skin takes up more (Abafe et al., Environment International, 2024). The flame-retardant migration studies used furniture fabrics, not underwear, so this shows the mechanism rather than a measured dose from a brief. That distinction matters. Mechanism is not dose.
Microplastics in reproductive tissue: what the studies actually say
Sensitive analytical methods have now detected microplastics in human testicular tissue and semen. A 2024 study in Toxicological Sciences found them in human and dog testes and reported an association with sperm count (Hu et al.). A 2023 study in Science of the Total Environment detected microplastics in human testis and semen using complementary methods (Zhao et al.). A 2024 study in the same journal found microplastics in every sample of seminal fluid it examined (Li et al.). A multi-site study in China detected microplastics in all semen and urine samples and linked mixed exposure to poorer sperm parameters in regression models (Zhang et al., eBioMedicine, 2024).
Read those sentences again and you will see what they do and do not say. Presence is established. Association is reported. Cause is not proven. No study attributes the particles found in testes to clothing. The particles could arrive through food, water, air, or skin. Most likely all of the above, but the source question is unanswered.
The wider endocrine context sits behind all of this. Population sperm counts have declined over decades, documented in a 2023 update in Human Reproduction Update that reviewed thousands of published reports. Endocrine disruptors in plastics, phthalates, and bisphenols remain the subject of active male reproductive research (Rodprasert et al., Frontiers in Endocrinology, 2021). A 2024 meta-analysis found associations between BPA exposure and lower sperm parameters (Lu et al., Toxics). These are associations, not proof. The causal question remains open.
The honest gaps
Three gaps matter for this conversation. No study has measured microfiber shedding from worn underwear, synthetic or cellulosic, specifically under combined sauna heat and sweat. No study has attributed the microplastics found in human reproductive tissue to clothing versus other sources. No study has measured chemical migration from modern certified underwear under combined heat, sweat, and occlusion. The mechanism chain is built from separate, real pieces: regional skin permeability, temperature and hydration effects on absorption, sweat-driven chemistry migration, and airborne synthetic fiber shedding. The full chain has not been tested end to end.
Not testing the full chain does not make it true or false. Absence of a study says nothing in either direction. It just means nobody has run the specific experiment. My read is that the individual pieces are solid enough that reducing the physical inputs makes sense on its own, without needing a fertility verdict.
The one piece that is well characterized is heat. Sauna-level heat temporarily lowers sperm count and can worsen morphology (Garolla et al., Human Reproduction, 2013). The effect is reversible once the heat exposure stops, because sperm production needs the testicles a few degrees cooler than core body temperature. Recovery after stopping wet heat is documented (Shefi et al., Fertility and Sterility, 2007). Heat is the anchor. Chemistry is the open question.
What you can actually change
None of this puts you at the mercy of the evidence. Two classes of inputs are under your control: heat and chemistry.
- Pick fibers that move moisture instead of holding it. Cotton soaks up sweat and stays damp, while regenerated cellulosics like modal and lyocell show stronger wicking and air permeability in controlled tests (Ozdemir, 2017; Kucukali-Ozturk et al., 2023). No single fiber wins on every measure, and organic cotton is still cotton. The organic label describes how the crop was grown, not how the fabric behaves when it gets hot and wet.
- Fit matters more than men give it credit for. Tighter underwear and long sitting raise scrotal temperature relative to looser underwear and movement (Jung et al., Human Reproduction, 2005; Mieusset et al., Journal of Andrology, 2007). A relaxed fit that lets air move keeps that area closer to the temperature it prefers. That is a physical claim about airflow, not a promise about sperm.
- Get out of wet gear. Damp, covered skin absorbs more than dry skin, and the occlusion research has shown it for decades. After a workout or a sauna session, sitting in sweaty shorts extends the warm, damp conditions that increase penetration.
- Wash new clothes before wearing them. Standard textile production leaves finish chemistry that can migrate, and residual formaldehyde from durable-press cotton is one documented example. A first wash and airing out is a low-cost step.
- Keep the laptop off your lap. Laptop heat raises scrotal temperature in healthy men (Sheynkin et al., Human Reproduction, 2005). Repositioning the device is a simple physical step that removes one direct heat source from the equation.
- If you sauna, the heat effect on sperm is reversible once you stop. That is the Garolla and Shefi data stated plainly. Regular heat exposure is a choice with tradeoffs, and the reversible nature of the sperm effect is the part the research actually supports.
You cannot un-eat the plastic already in the food chain, and you cannot filter the atmosphere. You can choose what sits against the most permeable patch of skin on your body, for how long, and under what conditions. Cooler, drier, cleaner contact beats hot, damp, chemistry-laden contact. I do not need a fertility verdict to know which side of that equation I would rather be on.
Frequently asked questions
why is scrotal skin more permeable than other skin
The scrotum has a thinner outer skin layer, more hair follicles, and a richer blood supply compared to most other body sites. A 1971 study by Maibach and colleagues found it absorbed test compounds at a rate far above the forearm baseline and higher than the scalp, forehead, and underarm. Those anatomical features make it easier for substances to cross the skin barrier.
do microplastics in testicles cause lower sperm count
No study has proven that microplastics directly cause lower sperm count in humans. Multiple studies have detected microplastics in human testicular tissue and semen, and some report associations between exposure and poorer sperm parameters. The presence is established, but cause and effect has not been demonstrated.
does heat from a sauna permanently damage sperm
No, the effect of sauna-level heat on sperm parameters is temporary and reversible. Research by Garolla et al. (2013) shows heat can lower sperm count and worsen morphology, while Shefi et al. (2007) documented that stopping regular wet heat exposure allows semen parameters to recover. Sperm production requires the testicles to stay a few degrees cooler than core body temperature, so ongoing heat exposure matters more than a single session.
can sweat make skin absorb more chemicals from clothing
Yes, warm, damp skin absorbs more than cool, dry skin. A 2020 study found raising skin temperature from 25 to 39 degrees Celsius increased absorption by over 200 percent, and occlusion research shows that covered, hydrated skin allows more penetration. Fabric chemistry like antimony from polyester and residual formaldehyde from cotton can migrate into sweat, but no study has measured this specific pathway from underwear under combined heat and sweat.