A 1971 study measured how quickly a compound soaked through skin at different body sites. The scrotum came in near the top, far above the forearm, scalp, forehead, and underarm. That single dermatology paper gave me a different lens on sperm motility than most supplement lists do.
Most motility advice is additive. Take CoQ10, eat oysters, try ashwagandha. The environmental evidence points the other way. The tissue that makes sperm sits a few degrees below core temperature, absorbs more than almost any other skin, and stays pressed against fabric for most of the day. Before you add anything, the first question is what to subtract.
I am not a clinician. This is what I found by reading across reproductive biology, dermatology, and textile chemistry. A doctor can help with personal decisions, especially if you are trying to conceive.
The temperature baseline shows up in older research than most supplement claims
Sperm production needs the testicles a few degrees cooler than core body temperature. Research shows sustained heat can lower sperm count and worsen morphology. The effect reverses once the heat exposure stops. Two studies anchor this point. Garolla et al. in Human Reproduction, 2013 showed that sauna-level heat temporarily reduced sperm parameters. Shefi et al. in Fertility and Sterility, 2007 showed that semen parameters recover after wet heat stops.
Heat does not need to be extreme to register. Clothing and posture change scrotal temperature. Jung et al. in Human Reproduction, 2005 found tighter underwear and sitting raised scrotal temperature compared to looser underwear and movement. Sheynkin et al. in the same journal measured scrotal temperature rising during laptop use. Minguez-Alarcon et al. in Human Reproduction, 2018 found men who reported wearing boxer-style underwear had higher sperm concentration and total count in a fertility-center cohort. That last one is an association. The temperature pathway makes it plausible.
A dermatology finding that reframes underwear
Maibach and colleagues dosed different body sites with the same compound and measured how much penetrated. Scrotal skin was near the top of the range. The reason is anatomical: thinner outer skin layer, more follicles, more blood supply.
Warm, damp, covered skin absorbs more. A 2020 study in Toxicology in Vitro found absorption through human skin rose by more than 200 percent when skin temperature increased from 25 to 39 degrees Celsius. A 1988 British Journal of Dermatology study showed short-term occlusion raised skin hydration and penetration in vivo. Put those two findings next to the scrotal permeability data and you get a measurable mechanism. This region is set up to take in more of what touches it, especially when it is hot and sweaty.
That changes the underwear conversation. The question is not only whether the fabric feels soft. The question is what sits against that skin all day.
Fabric chemistry moves into sweat
Most men treat fabric as inert. It is not. Polyester is manufactured using an antimony catalyst, and a 2021 study in Regulatory Toxicology and Pharmacology found trace antimony could migrate into artificial sweat under standardized test conditions. A 2016 Textile Research Journal study found residual formaldehyde from durable-press cotton released into synthetic sweat, with more release at higher pH. In 2020, De Falco et al. in Environmental Science and Technology showed polyester garments shed microfibers into the air during ordinary wear, at a scale similar to laundering. The microplastic conversation is not only about washing machine filters. It is happening on your body while you move.
Natural-derived cellulose fibers such as lyocell or modal do not solve everything, but they change what is shed. When they shed, it is biodegradable cellulose, not a persistent plastic microfiber. That distinction is the accurate one. Do not trust any claim that a particular natural fiber sheds fewer fibers by mass. The bench data does not support that. The accurate claim is about what the shed fiber is.
Cotton is a separate issue. It absorbs moisture and holds it, staying damp longer in heat. Organic cotton describes how the crop was grown. It does not change how cotton behaves when hot and wet, and wearer chemical safety comes from dye and finish chemistry and finished-product testing, not the farming method.
Microplastics in the body are established. The fertility link is not.
Sensitive analytical methods have now detected microplastics in human testicular tissue and semen. Hu et al. 2024 found them in human and dog testes and reported an association to sperm count. Zhao et al. 2023 detected them in human testis and semen. Li et al. 2024 found microplastics in every seminal fluid sample in a general-population Raman study. Zhang et al. 2024 detected them in all semen and urine samples in a multi-site Chinese study and linked mixed exposure to poorer sperm parameters in regression models.
Those are associations. No study attributes the particles to clothing specifically. No study proves microplastics lower motility in a causal way. The presence is real and citable. The fertility harm is not settled.
Population-level sperm counts have declined in a 2023 meta-analysis by Levine et al. in Human Reproduction Update. That decline is a backdrop, not proof of a single cause. It makes the exposure conversation harder to dismiss.
A subtraction list that follows from the physiology
If you are working on motility, the lowest-risk changes are mostly about removing heat and unnecessary chemical contact.
- Reduce chronic scrotal heat. Laptops belong on a desk, not a lap. Tight polyester underwear traps heat. Looser, breathable underwear lets air move and keeps the area physically cooler. That is a claim about temperature and airflow, not a promise about fertility. Standing and walking break up long sitting, which also appears in the scrotal temperature studies.
- Read fabric labels with chemistry in mind. Favor fabrics that manage moisture and do not carry persistent plastic fibers. Regenerated cellulose such as lyocell or modal is one option. Check the stretch fiber too. Conventional elastane and spandex are plastic.
- Cut easy endocrine-disruptor exposure. The broader male reproductive health literature points to phthalates, bisphenols, and pesticides as areas of concern (Rodprasert et al., 2021). The low-effort moves are reasonable: fewer highly processed packaged foods, less heavily scented personal care product, filtered water if microplastics are a concern.
- If you sauna, understand reversibility. Sauna-level heat temporarily lowers sperm count and can worsen morphology (Garolla et al., 2013). The effect reverses once heat exposure stops (Shefi et al., 2007). Occasional use is a different exposure than chronic daily heat. If you are actively trying to conceive, have the conversation with a doctor.
What the evidence does not say
Research has not measured chemical migration or dermal uptake from modern certified underwear under combined heat, sweat, and occlusion. The mechanism is shown with furniture fabrics and particle models, not with underwear. No study measures microfiber shedding from worn underwear specifically under sauna heat. The long-term effect of regular heat on circulating testosterone in humans is not settled by large trials.
I am not anti-supplement. CoQ10, carnitine, and zinc have real research in male reproductive health, but they address a different layer than the exposure side. Food sources like oysters and pumpkin seeds give you zinc without supplement guessing games. The subtraction moves cost little and get almost no attention.
The most consistent changes are boring: keep the area cool, choose fabrics by chemistry and fit, reduce plastic contact where it is easy, and understand the difference between reversible heat exposure and daily heat load. That is what the studies support. Everything else is a conversation for you and your doctor.
Frequently asked questions
does heat actually lower sperm motility
Heat affects the whole sperm production process. Garolla et al. in Human Reproduction, 2013 showed that sauna-level heat temporarily lowered sperm count and could worsen morphology, and Shefi et al. in Fertility and Sterility, 2007 found semen parameters recovered after the heat stopped. Motility is part of that same temperature-sensitive system.
can tight underwear affect sperm motility
Tight underwear raises scrotal temperature, especially when you sit for long stretches. Jung et al. in Human Reproduction, 2005 found tighter underwear and sitting increased scrotal temperature compared to looser underwear and movement. A 2018 study in the same journal found men who wore boxer-style underwear had higher sperm concentration and total count, though that is an association, not proof of cause.
does polyester underwear affect sperm motility
The direct effect on motility has not been isolated in a controlled trial. Polyester sheds microfibers during wear, and some of its chemistry can migrate into sweat under standardized test conditions. The known mechanism is that scrotal skin absorbs more than most body sites, especially when warm and damp, so fabric choice matters more there than people assume.
should i stop using the sauna while trying to conceive
Sauna-level heat temporarily lowers sperm count and can affect morphology, but the effect appears reversible after heat stops. If you are actively trying to conceive, ask a doctor about sauna frequency and overall heat exposure. This content is for educational purposes only, not medical advice. Oakman products are designed for physical comfort and cooling and make no claims about fertility, sperm quality, or hormone levels. Consult a healthcare professional for personalized advice.