A 1971 study did something you would never get past an ethics board today. Researchers applied a test compound to different patches of skin on human volunteers-forearm, scalp, forehead, underarm, scrotum-and measured how much made it into the bloodstream. The scrotum absorbed the compound at a rate dramatically higher than the forearm baseline, outpacing every other site they tested (Maibach et al., 1971).
I came across that study while digging into male fertility research, and it shifted how I think about a topic that usually gets boiled down to one piece of advice: keep your testicles cool. Boxers instead of briefs. No hot tubs. Laptop off the lap. That advice is not wrong. But it covers only part of what is happening down there. Heat is a problem for sperm production-that has been settled for a long time-but it might not be the whole problem. It might not even be the most interesting part.
Spermatogenesis works best a few degrees below core body temperature. Raise testicular temperature by even one or two degrees for extended periods, and you start seeing declines in sperm count, motility, and morphology. Sauna users, long-haul drivers, welders-all have documented temporary drops in semen quality after sustained heat exposure. The body bounces back, usually. But that recovery assumes you are removing whatever caused the stress in the first place. The question that keeps surfacing in the research is whether heat is acting alone, or whether it is making something else worse.
The chemical piece most guys never hear about
The scrotum is not just uniquely sensitive to temperature. It is one of the most permeable patches of skin on the male body. Thin outer layer. Lots of blood vessels. Dense with hair follicles. When that skin gets warm and damp-exactly what happens during a long day in synthetic underwear or compression shorts-its ability to absorb whatever is sitting against it goes up significantly.
A 2020 study in Toxicology in Vitro found that raising skin temperature from 25 degrees Celsius to 39 degrees increased absorption of test compounds by more than 200 percent. Earlier research from the British Journal of Dermatology showed that hydrated, occluded skin-the condition created by sweat trapped under a layer of fabric-absorbs more than dry, exposed skin. Stack those findings on top of the 1971 scrotal permeability data, and you get a clear physiological picture: the environment inside your underwear is not passive. It can become a delivery system.
Now look at what that fabric is made of. Polyester is manufactured using an antimony catalyst, and trace antimony can migrate into sweat. A 2021 study using an artificial sweat test confirmed antimony leaching from polyester fabric under conditions that mimic a hot day or a workout. Commercial cotton is often finished with formaldehyde-based resins. Conventional spandex and elastane are polyurethane-a plastic. None of this means wearing synthetic underwear will tank your fertility. It means the chemical load on highly permeable skin is a variable worth considering, especially when motility numbers are already low.
Microplastics, oxidative stress, and the sperm that can't swim right
Research on declining sperm counts has been hard to ignore. A 2017 meta-analysis found a 50 to 60 percent drop in sperm concentration among Western men over roughly 40 years. In 2024, University of New Mexico researchers found microplastics in every single human testis sample they examined. A 2023 study out of China detected microplastic particles in human semen. None of that data proves the particles are causing the motility decline-association is not causation-but the direction of the evidence is consistent.
Endocrine-disrupting compounds like phthalates, bisphenols, and certain pesticides have been linked to reduced sperm motility in multiple human studies. The mechanism tends to involve oxidative stress and disrupted mitochondrial function inside the sperm cell. Sperm mitochondria sit in the midpiece and generate the energy the tail needs to move. If that system gets knocked offline, motility drops. The question is how those compounds reach the testes in meaningful amounts. Food, water, and household dust are one route. Skin absorption through a warm, damp, highly permeable area is another that rarely gets discussed.
There is also the issue of what clothing sheds while you wear it. A 2020 study found that polyester releases microfibers into the air during ordinary movement, not just in the washing machine. The fibers settle into dust, get inhaled, get ingested. That adds a layer of exposure that was not on anyone's radar a decade ago.
Animal data fills in some blanks
Human studies cannot ethically expose men to known endocrine disruptors to measure the effect on fertility. So researchers rely on animal models, and the results have been striking. Controlled rodent studies show that phthalates and bisphenols administered during critical developmental windows reduce sperm motility in offspring. The chemicals disrupt Sertoli cell function and alter epigenetic programming in the germline, with effects that can persist across generations.
The doses used in those studies are often higher than typical human exposure, but not always dramatically so. And real-world human exposure is not a single compound at a controlled dose. It is a low-level mixture arriving through multiple routes over decades. That complexity makes population studies messy. It also makes it difficult to point at one culprit and say “this is the thing.” But the accumulation of consistent findings across species, exposure types, and outcome measures is getting harder to dismiss.
Fertility is a whole-body thing
Most guys I talk to about fertility want to fix one thing. Diet. Exercise. Sleep. A supplement stack. The research says those matter, but it also says motility is sensitive to factors that rarely make the conversation. Excess body fat converts testosterone to estradiol, shifting the hormonal environment that supports sperm production. Sleep restriction lowers testosterone within a week in controlled settings. Chronic stress elevates cortisol, which suppresses gonadotropin-releasing hormone and downstream testosterone output.
The chemical exposure angle has not filtered into the mainstream fertility conversation yet, partly because the evidence is still accumulating, and partly because the solution is not a simple pill. Telling a guy to eat oysters for zinc is concrete advice. Telling him that his underwear might be contributing to a low-level chemical load that stresses sperm mitochondria is harder to wrap into a tidy action plan.
Practical moves that hold up under scrutiny
I am not going to tell anyone to throw out every synthetic garment they own. The evidence does not justify that level of alarm. But for men who are actively trying to conceive or who have been told their motility numbers are low, the physiology does point toward a few specific, practical shifts.
- Manage testicular temperature across the whole day. Not just sauna sessions. Long sitting periods, heavy synthetic layering, and tight fits can trap enough heat to raise scrotal temperature by a degree or more for hours. Standing breaks and looser, breathable fabrics in the groin area cost nothing and make a measurable difference.
- Pay attention to the material that spends all day against permeable skin. The fabric touching your scrotum matters more than the fabric on your shoulders. Natural fibers are not automatically a safe bet-organic cotton can still carry chemical finishes-but fibers that are manufactured without an antimony catalyst and are not polyurethane-based reduce some known variables.
- Shower immediately after heavy sweating, especially post-sauna. Research has detected heavy metals and endocrine disruptors in sweat (Sears et al., 2012). Sauna has solid cardiovascular benefits, but the localized heat spike can temporarily depress sperm production. Using a heat-reflecting barrier over the groin during sauna can preserve systemic benefits while limiting the fertility downside. Then, showering off sweat prevents reabsorption through the same permeable skin you just stressed.
- Get a full workup before assuming the cause. Low motility can come from a varicocele, infection, obstruction, or a genetic factor. A semen analysis and a conversation with a reproductive urologist will tell you more than any amount of lifestyle tinkering ever will.
Where the research is headed
The microplastics and fertility research cycle is spinning fast. Every few months, a new tissue study drops confirming plastic particles in a body compartment that was previously assumed to be clean. The hard part-establishing a dose-response relationship in humans-is still in progress. Once researchers can show whether reducing exposure actually improves semen parameters, the conversation will shift from warning to action.
The textile industry will fund its own studies, and many of those will find no significant migration of chemicals from synthetic fabrics under normal use. That does not make the data invalid, but it is worth knowing who paid for it. On the fertility side, male factor research is finally getting attention proportional to its role in couple infertility. Sperm motility is one of the cleaner endpoints to measure, so expect the data linking environmental exposures to motility decline to firm up sooner than data on morphology or DNA fragmentation.
If you are dealing with low motility right now, the advice landscape is frustrating. Some clinicians still dismiss lifestyle factors entirely. Others push supplement protocols that run hundreds of dollars a month with no guaranteed result. The middle path-reducing known sources of heat and chemical contact, improving metabolic health, managing stress, and getting proper medical testing-is less dramatic. But it is also the path with the strongest research behind it.
That 1971 study on scrotal absorption sat in the dermatology literature for decades without anyone connecting it to the fertility and fabric conversation. It is not the smoking gun. It is a piece of the puzzle that makes the rest of the picture look different once you know it is there.
If you are concerned about sperm motility or fertility, speak with a doctor who specializes in male reproductive health. This article is for informational purposes and does not replace individual medical advice.
Frequently asked questions
does sauna use permanently damage sperm motility
Research shows that sauna use can cause a temporary drop in sperm count and motility due to increased testicular temperature. In most cases, these parameters return to baseline within a few months after reducing heat exposure. Using a heat-reflecting barrier over the groin during sauna sessions and showering immediately afterward are practical steps to limit the fertility impact while keeping the cardiovascular benefits.
can synthetic underwear cause low sperm motility
There is no direct evidence proving that synthetic underwear causes low sperm motility, but the physiology suggests it can contribute to an environment that stresses sperm production. Polyester and other synthetics can trap heat and moisture, and trace chemicals like antimony can migrate into sweat. For men already dealing with low motility, choosing breathable, gentler materials for daily wear is a sensible precaution.
how does the scrotum absorb chemicals differently from other skin
The scrotal skin is thinner, more vascular, and has more hair follicles than most other body sites, all of which increase its permeability. A 1971 study by Maibach and colleagues demonstrated that the scrotum absorbs test compounds at a rate far higher than the forearm or scalp. When combined with warmth and sweat-common conditions inside clothing-that absorption capacity can make the area more vulnerable to whatever it is in contact with.
what does microplastic presence in testicular tissue actually mean for fertility
Several recent studies have found microplastics in human testicular tissue and semen, but this does not yet prove that these particles directly reduce sperm motility. The finding confirms exposure and raises concern because similar endocrine-disrupting compounds have been linked to sperm quality issues through oxidative stress pathways. The link is still associative, and researchers are working to clarify whether reducing microplastic burden improves fertility outcomes.