Does Underwear Affect Sperm Count? From Fabric to Fertility

Written By The Mr. Fertyl Research & Education Team

Does your choice of underwear really impact your fertility? It’s a question many men have (perhaps after hearing the classic “boxers vs. briefs” debate). Some experts have shrugged it off, saying guys should just wear whatever’s comfortable [1]. But a growing body of research – and a bit of biological common sense – suggests that your underwear can influence sperm count and quality. In this friendly guide, we’ll normalize the conversation about male fertility and explain how factors like fabric, fit, and breathability could affect your reproductive health. And don’t worry – we’ll also introduce a safe, sustainable option (Mr. Fertyl’s Bamboo Lyocell underwear) that keeps things cool and comfortable down there.

Underwear, Heat, and Healthy Sperm

One of the biggest ways underwear can affect sperm is through heat. The testes are located outside the body for a reason – they need to stay slightly cooler than your core body temperature to produce healthy sperm [11]. In fact, for optimal sperm production, the testicles should be about 2–4°C cooler than the rest of the body (around 94–96°F) [11]. If the scrotum gets too warm (even by a few degrees), the testes can’t produce sperm as effectively, leading to lower sperm counts [11]. Heat literally slows down sperm factories.

Tight-fitting underwear (like briefs or tight boxer-briefs) can push the testicles closer to the body, raising their temperature. Research shows that prolonged heat exposure in the scrotal area can harm sperm production. For example, studies have found that when the scrotum is subjected to higher temperatures for extended periods, sperm count drops and more sperm develop abnormally [12]. This is why fertility specialists often warn men to avoid hot tubs, saunas, or even resting a warm laptop on the lap for too long.

What about everyday underwear? A large Harvard-led study explored this by comparing men who wear boxers versus briefs. The results were eye-opening: men who primarily wore boxer shorts had a 25% higher sperm concentration and 17% higher total sperm count than men who usually wore tight underwear [3]. They also had more motile (“swimming”) sperm on average [3]. In contrast, men who wore briefs or other snug styles had significantly lower counts. Interestingly, the men in tight underwear showed slightly higher levels of follicle-stimulating hormone (FSH), a hormone that stimulates sperm production [3]. Researchers think the body was releasing extra FSH to compensate for heat-damaged sperm – essentially, the brain was saying “uh oh, the sperm are struggling, better boost them with more hormone” [3]. This hormonal clue underscores that the heat from tight underwear was indeed having a negative effect on sperm, and the body was trying to fix it.

The takeaway is pretty simple: keeping things cool is beneficial for sperm. The Mayo Clinic explicitly advises men trying to conceive to “stay cool,” noting that wearing loose-fitting underwear (and avoiding excess heat like hot tubs) may improve sperm quality [2]. In short, briefs aren’t birth control on their own, but if you’re looking to maximize fertility, a roomier, breathable pair of shorts can help your testicles stay at a happier temperature.

Fabric Matters: Breathability vs. Heat-Trapping Materials

It’s not just how tight your underwear is – what it’s made of also matters for fertility. The key concept here is breathability. Breathable fabrics (such as cotton or bamboo-based textiles) allow air circulation and wick away moisture, which helps keep the temperature down in your groin area. Less sweat and heat build-up = a cooler home for your sperm. On the other hand, synthetic fabrics like polyester and nylon can trap heat and humidity. Ever worn polyester and felt a bit swampy? That’s the opposite of what you want for optimal sperm production.

Urologists have pointed out that polyester is a particularly troublesome material when it comes to breathability. It tends to hold in heat and is less breathable than natural fibers, which can raise the testicular temperature and lead to “scrotal heat stress” [8]. Essentially, wrapping the boys in a non-breathable synthetic is like wrapping them in a mini greenhouse – not ideal for keeping cool.

Beyond theoretical concerns, actual experiments have linked polyester underwear to fertility problems. In a remarkable study, scientists tested polyester’s effect by having men wear a polyester scrotal sling (imagine a soft band of polyester supporting the testes). The results? The polyester sling worked a little too well as contraception – the men’s sperm counts plummeted to zero. After about 140 days, all the men became azoospermic (producing no sperm at all) [4]. Luckily, this effect was reversible (once they stopped wearing the sling, their sperm counts gradually returned to normal) [4]. But it’s pretty astounding: simply having polyester in constant contact with the genitals caused a complete, temporary infertility in those men. The researchers theorized that the electrostatic charges generated by the polyester, along with the lack of air flow, disrupted the testes’ normal function [4].

Animal studies have backed this up as well. In one experiment, dogs were made to wear polyester pants for two years. By the end of the study, the polyester-wearing dogs had a significant decrease in sperm count and motility, and an increase in abnormal sperm forms [5]. Their testicular tissue showed signs of degeneration under the microscope. Dogs who wore cotton pants or no pants at all, meanwhile, had no change in sperm health [5]. Once the polyester was removed, the affected dogs’ fertility mostly recovered [5]. This tells us that the fabric itself (not just tightness) had a deleterious effect on spermatogenesis [5]. The authors of the study suggested, again, that electrostatic potential from the synthetic fabric might play a role [5]. It seems bizarre, but think of it like this: our bodies are electrical systems to a degree, and the polyester may create a subtle static field or insulation that interferes with the normal signals or temperature regulation in the testes.

The practical upshot? Choosing breathable, natural materials for your underwear is generally better for sperm. Cotton has long been recommended as a good choice for everyday wear because it’s airy and absorbs moisture. Newer natural fabrics like Bamboo Lyocell (which we’ll discuss soon) are also extremely breathable. If you’re wearing synthetic fabrics that make you sweaty, or if your underwear leaves you feeling hot and damp, it could be creating a less-than-ideal environment for sperm development.

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Hidden Chemical Risks in Your Undies

Heat and airflow are a big part of the story, but there’s another factor to consider: chemicals. We don’t often think of our clothing as a source of chemical exposure, but fabrics (especially synthetic ones) can contain compounds that might affect our health. For men concerned about fertility, two categories of chemicals are worth noting: those that disrupt hormones (endocrine disruptors) and those that might directly affect sperm.

Endocrine-disrupting chemicals (EDCs) are substances that can interfere with the body’s hormonal balance. They can mimic or block hormones like testosterone or estrogen. Over time, exposure to EDCs might impact sperm production or quality. Where would such chemicals be in underwear? Consider some possibilities: the dyes and finishes used on fabric, plastic-based prints or logos on your boxers, or additives used to give elastic waistbands their stretch. For instance, phthalates are chemicals used to make plastics softer and are sometimes found in synthetic fabrics, printing inks, or even certain elastic materials. Studies have shown that men with higher levels of phthalates in their system tend to have lower sperm counts and less motile sperm [6]. These chemicals can leach from products and may be absorbed through the skin or simply accumulate in our bodies over time. While your underwear alone isn’t going to flood you with phthalates, it’s one piece of the puzzle (along with all the other plastic exposures we have daily).

Another example: some clothing is treated with flame-retardant chemicals (common in kids’ pajamas or certain industrial workwear). These flame retardants might keep you safer in a fire, but they have been linked to fertility issues. Research indicates that flame retardant compounds can bioaccumulate in the body and have been associated with infertility and endocrine disruption [7]. You probably wouldn’t suspect your PJs could affect your sperm, but the chemicals in the fabric could quietly be playing a role.

Even pesticide residues in non-organic cotton or heavy metals in fabric dyes could pose risks. The textile industry uses thousands of chemicals, and some of them (like lead or azo-dyes) are known to be harmful if absorbed. The crotch area of your underwear is thin skin with lots of blood vessels – a route by which nasty chemicals could potentially enter. One environmental health report noted that toxic chemicals in textiles (like certain dyes, PFAS “forever chemicals,” and formaldehyde finishes) have a range of health implications, including hormonal effects [7].

And let’s not forget microplastics: When you wash synthetic clothes, they shed tiny plastic fibers. Some of those fibers might remain in the fabric and even get onto your skin. Microplastics have been detected in various human tissues (one study even found them in human placentas) – we’re still learning what that means for health [8]. While we don’t have direct evidence that microfibers from your polyester briefs will lower your sperm count, it’s a rather unsettling thought that minuscule plastic particles could be lurking around your reproductive organs.

Finally, consider volatilized chemicals: ever notice a chemical smell from new clothes or cheap underwear? That could be formaldehyde or other finishing chemicals used to prevent mildew or wrinkles during storage. These are irritants and potential EDCs as well [7]. If your underwear isn’t certified organic or Oeko-Tex (a textile safety standard), there’s no telling what finish or dye chemicals might be present.

In summary, beyond heat, your underwear’s material can expose you to unwanted chemicals. Synthetic fabrics like polyester have even been called “endocrine-disrupting” by some doctors [8], due to chemicals that may disrupt testosterone production when in close contact with the skin [8]. Natural fibers, especially those produced without heavy chemicals, are a safer bet for minimizing these exposures. This is one reason many health-conscious consumers are turning to organic cotton or Bamboo Lyocell underwear – to avoid that daily dose of mystery chemicals right where it counts.

Enter Bamboo Lyocell: A Fertility-Friendly Fabric

By now you might be wondering, “So what should I wear if I want to support sperm health?” The ideal underwear for fertility would do a few things: keep your testicles cool (through good airflow and moisture-wicking), avoid harsh chemicals and synthetic fibers, and still provide comfort and support. Bamboo Lyocell fabric checks all those boxes.

What is Bamboo Lyocell? It’s a modern, eco-friendly fabric made from bamboo pulp. If you’ve heard of Tencel (a popular lyocell made from wood pulp), this is similar – but uses fast-growing bamboo as the raw material. The bamboo is processed in a closed-loop system to create a super soft, smooth fiber that can be woven into fabric. The end result is a natural-based textile that’s amazingly soft, breathable, and sustainable.

Here’s why Bamboo Lyocell is great for fertility (and comfort too):

  • Breathability & Thermoregulation: Bamboo Lyocell is highly breathable. It doesn’t trap heat – in fact, it has natural thermoregulating properties, meaning it helps keep you cool when it’s hot (and can also feel cozy when it’s cooler). Studies comparing fabrics have found that bamboo-based textiles have excellent moisture and temperature management properties – essentially as good as or better than cotton [9]. That means when you sweat, a Bamboo Lyocell fabric will wick away that moisture and allow it to evaporate, cooling you down. Your groin stays drier and more ventilated. This can prevent the overheating and dampness that would otherwise spell trouble for sperm. Men who switch to bamboo or similar moisture-wicking underwear often notice they simply feel “airier” and less swampy downstairs, which is a good sign for temperature control.
  • Natural Antimicrobial Qualities: Bamboo is often touted as naturally antimicrobial, and there’s some truth to that. The bamboo plant contains a compound nicknamed “bamboo kun” which helps it resist pests and germs. Scientific studies have confirmed that bamboo plants and fibers have antibacterial properties [10]. In fabric form, Bamboo Lyocell retains some of this resistance to bacterial growth. What does that mean for you? It means your underwear can stay fresher longer and discourage the growth of odor-causing bacteria or fungi. A cleaner, healthier environment around your privates can indirectly benefit fertility too – for instance, reducing the risk of infections or skin irritations that could impact the sensitive genital area. (Plus, who doesn’t like fresher underwear?)
  • Minimal Chemical Exposure: Bamboo can be grown without intensive pesticides or fertilizers, and the Lyocell production process typically uses non-toxic solvents that are recycled. The result is a fabric that is free from many of the harsh chemicals we discussed earlier. High-quality Bamboo Lyocell underwear is often OEKO-TEX certified or similarly tested to ensure no harmful substances. You won’t find formaldehyde finishes or heavy metals in the dye. And in Mr. Fertyl’s specific case, there’s no direct skin contact with elastane. Why is that important? Elastane (spandex) is a stretchy synthetic fiber used in waistbands or to give fabric some stretch. While elastane helps underwear fit well, it’s still a plastic-based fiber that could cause irritation for some people. Mr. Fertyl’s bamboo underwear is designed so that the elastane is wrapped in the bamboo fabric or otherwise kept beneath a layer of bamboo, meaning only the soft, natural bamboo fabric touches your skin. This design choice ensures you get the stretch and fit benefits of elastane without actually exposing your skin (or your reproductive organs) to it directly. For the wearer, that means less risk of any allergic reaction to elastic and an even lower chance of chemical exposure.
  • Softness and Comfort: From a practical standpoint, you’re more likely to wear comfy underwear consistently – and consistency matters if you want to keep your sperm healthy every day. Bamboo Lyocell feels silky-soft and smooth (many compare it to a blend of cashmere and silk). It’s gentler on the skin than rough fabrics. If you’ve ever had chafing or irritation from cheaper underwear, bamboo fabric could be a game-changer. Happily, taking care of fertility doesn’t mean you have to sacrifice comfort or style. You can have both.
  • Sustainability (Bonus Perk): Choosing Bamboo Lyocell isn’t just good for your body; it’s good for the planet. Bamboo is a highly sustainable crop – it grows extremely fast (some species up to 3 feet a day!), needs little water, and requires no pesticides to thrive. It also regenerates from its own roots, so it doesn’t require replanting and is great for soil health. The Lyocell manufacturing process is designed to recycle water and solvents, making it much cleaner than conventional viscose rayon or polyester production. By wearing bamboo underwear, you’re reducing the environmental impact compared to conventional cotton or synthetic underwear. For eco-conscious dads-to-be, that’s a win-win: you’re protecting your future child’s environment and boosting your chances of conceiving that child.

Mr. Fertyl: Putting It All Together for Fertility and Comfort

Mr. Fertyl’s Bamboo Lyocell underwear is introduced as a smart choice for men focused on fertility, sustainability, and comfort – and now we can see why. It addresses the very issues we’ve discussed:

  • It keeps you cool and dry, thanks to bamboo’s breathable, moisture-wicking nature, thereby helping maintain an optimal temperature for sperm production.
  • It avoids the problematic synthetics and chemicals– no polyester to overheat you, no heavy chemical treatments to worry about. The bamboo fabric is gentle and non-toxic, and even the necessary elastic is kept away from direct contact.
  • It provides a comfortable fit (no one wants to trade fertility for discomfort!). The fabric has a bit of stretch and a lot of softness, making it suitable for daily wear, sports, or lounging. You get support where you need it, but not a vice-like squeeze.
  • It’s sustainable and ethically made, appealing not just to your reproductive health but your values. You can feel good about the environmental footprint (or lack thereof) of your underwear.

Wearing Mr. Fertyl’s underwear is a subtle, proactive step you can take for your reproductive health. It’s part of a holistic approach: of course, you’ll also want to eat healthy, exercise, avoid smoking, etc., for the best fertility outcomes. But optimizing your undergarments is an easy fix that can remove one potential drag on your sperm count. Think of it as creating the ideal home for your sperm – cool, chemical-free, and comfortable.

Perhaps just as importantly, choosing fertility-friendly underwear helps normalize the conversation about male fertility. Women are often reminded to do (or not do) a hundred things for reproductive health; men historically haven’t had as much guidance. By paying attention to something as ordinary as underwear, men can take charge of their fertility in a simple, stigma-free way. Mr. Fertyl aims to make that conversation normal: it’s okay to care about your sperm count, and it’s okay to make lifestyle changes to improve it. It doesn’t make you any less masculine – if anything, it’s a smart and responsible move.

Flat lay of non-toxic bamboo men’s underwear in black and beige, fertility-safe and breathable.

Tight Fit? Toxic Fabric? Time to Switch.

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Bottom Line:

Yes, your underwear can affect your sperm. Factors like heat retention, poor airflow, and exposure to certain synthetic materials can negatively impact sperm production and quality. The good news is that these are factors you have control over. By choosing underwear that is cooling, breathable, and free of harmful chemicals, you’re doing your future fertility a favor. Bamboo Lyocell underwear embodies all those ideal qualities – it keeps your swimmers cool and comfy, without subjecting them to risky synthetics or toxins. And you don’t have to compromise on comfort or style to wear it.

So, if you’re looking to start a family (or even if you just want to maximize your health), it might be time for an underwear drawer upgrade. Your sperm (and your future self) will thank you for it!

  1. Cleveland Clinic – “Are You Unknowingly Lowering Your Sperm Count? When to Worry” (2019). Source.
  2. Mayo Clinic – “Healthy sperm: Improving your fertility.” (Mayo Clinic, 2020). Source.
  3. ScienceDaily – “Largest study yet shows type of underwear is linked to men's semen quality.” (2018). Source.
  4. Shafik, A. “Contraceptive efficacy of polyester-induced azoospermia in normal men.”Contraception, 45(5): 439-451 (1992). Source.
  5. Shafik, A. “Effect of different types of textile fabric on spermatogenesis: an experimental study.”Urol. Res., 21(5): 367-370 (1993). Source.
  6. Wallis, C. – “Common Chemicals May Harm Sperm and Pregnancies, Growing Evidence Shows.”Scientific American (March 1, 2023). Source.
  7. Earth Day – “Toxic Textiles: The Chemicals in Our Clothing.” (Nov 4, 2022). Source.
  8. Indian Express – “Can polyester underwear affect fertility, cause impotence?” (Feb 28, 2023). Source.
  9. Gericke, A. & Van der Pol, J. – “A comparative study of regenerated bamboo, cotton and viscose rayon fabrics: Part 1 – Selected comfort properties.”Journal of Consumer Sciences, Vol. 38 (2010). Source.
  10. Gokarneshan, N. & Khan, A. – “Antimicrobial Properties of Bamboo Fibres.”Biomed. J. Sci & Tech. Res., 33(5) (2021). Source.
  11. WebMD – “Boxers vs. Briefs: Increasing Sperm Count.” (WebMD Health News). Source.
  12. Posterity Health – “Managing Temperature for Male Fertility and Sperm Health.” Source.