Microplastics are present in human testicular tissue, with recent research detecting them in 100 percent of samples tested. Higher concentrations have been associated with lower testis weights. The primary source is synthetic fabrics like polyester, which shed hundreds of thousands of microplastic fibers per wash, transferring to the body and accumulating in sensitive tissues like the testicles.
How Microplastics Reach the Testicles
Synthetic fabrics like polyester shed microplastic fibers during wear and washing, releasing up to 700,000 fibers per laundry load [1]. These fibers are small enough to absorb through the skin or inhale, eventually circulating through the body. A 2024 study found microplastics in 100 percent of human testis samples tested, with higher concentrations associated with lower testis weights [2].
Why the Testicles Are Vulnerable
The testicles sit outside the body because they function best at several degrees Fahrenheit below core temperature. This cooler environment is critical for spermatogenesis, the process of sperm production, which slows or impairs when scrotal temperature rises [3]. Microplastics may compound this thermal stress: some contain endocrine-disrupting additives like phthalates, which can leach into sweat and absorb through the skin, potentially interfering with reproductive hormones [4].
The Polyester Problem
Polyester underwear is less breathable and can feel clammy. It holds heat against the skin and sheds microplastics against the scrotum. In experimental studies, men wearing polyester showed greater scrotal temperature increases and more pronounced changes in sperm parameters compared to those wearing natural fibers [5]. Unlike cotton or wood-pulp-derived fabrics, polyester doesn't break down naturally, it persists in the environment and the body.
Heat and Microplastics Together
Scrotal temperature rises further in tight, non-breathable fabrics, which restrict airflow and trap body heat. Prolonged sitting, at a desk or in a car, exacerbates this effect, with studies showing measurable increases in scrotal temperature within 15 to 30 minutes [6]. When combined with microplastic exposure, this thermal stress creates a compounded risk for scrotal thermoregulation.
A Natural-Fiber Alternative
Both problems, the microplastic shedding and the heat trapping, trace back to the same material: polyester. Oakman's boxer briefs replace it with TENCEL-branded MicroModal, a wood-pulp-derived regenerated-cellulose fiber, so it doesn't shed microplastics the way synthetics do. Because MicroModal is spun roughly 20 percent finer than standard modal, it sits closer to the skin and moves moisture outward faster than polyester-rich constructions, which trap heat and slow moisture transport [7]. The fibers are Siro-spun for durability and blended with ROICA V550, a Japanese clean stretch fiber from Asahi Kasei, for a breathable, athletic fit that holds its shape through daily wear.
The design includes a gusset, an extra panel that provides room and airflow, to minimize heat buildup throughout the day. For sauna sessions or other prolonged seated heat exposure specifically, a removable cold pack slots into a front pocket, cooling the scrotum through a layer of soft fabric without direct contact.
Swapping polyester for a wood-pulp fiber takes out one everyday source of microplastic exposure and the heat trapping at the same time.
Join the Founding 100
Oakman is sauna cooling for men, launching early fall 2026. Founding members get first access and founding pricing locked in. Reserve your spot.
Frequently asked questions
How do microplastics affect sperm health?
Microplastics may interfere with reproductive hormones and worsen heat-related stress on sperm production. Higher microplastic levels in testicular tissue have been associated with lower testis weights in recent research.
Does underwear material really matter?
Yes. Studies show that synthetic fabrics like polyester raise scrotal temperature more than natural fibers, and they shed microplastics that can absorb into the body. Natural fibers like TENCEL-branded MicroModal minimize both risks.
Can't I just wash polyester less often?
Even with less frequent washing, polyester sheds microplastics during wear due to friction against the skin. Switching to natural fibers is the most direct way to reduce this source of exposure.
Is the cold pack meant to be worn all day?
No. The removable cold pack is designed for specific high-heat, seated sessions like sauna use, not for general daily wear. The gusseted fit and MicroModal fabric handle everyday thermal management on their own.
This is for general educational and informational purposes only and is not medical advice. It makes no claim about fertility, sperm, or testosterone outcomes from any product. Always talk with a qualified healthcare provider before making changes to your diet, exercise, or health routine. These statements have not been evaluated by the Food and Drug Administration. Oakman products are apparel and are not intended to diagnose, treat, cure, or prevent any disease.
References
- Napper IE, Thompson RC. (2016). Release of synthetic microplastic plastic fibres from domestic washing machines: Effects of fabric type and washing conditions. Marine Pollution Bulletin (Elsevier). [Link]
- Hu CJ, Garcia MA, Nihart A, et al. (2024). Microplastic presence in dog and human testis and its potential association with sperm count and weights of testis and epididymis. Toxicological Sciences. [Link]
- Durairajanayagam D, Agarwal A, Ong C. (2015). Causes, effects and molecular mechanisms of testicular heat stress. Reproductive BioMedicine Online. [Link]
- Rodprasert W, Toppari J, Virtanen HE. (2021). Endocrine Disrupting Chemicals and Reproductive Health in Boys and Men. Frontiers in Endocrinology. [Link]
- Shafik A. (1993). Effect of different types of textile fabric on spermatogenesis: an experimental study. Urological Research. [Link]
- Bujan L, Daudin M, Charlet J-P, Thonneau P, Mieusset R. (2000). Increase in scrotal temperature in car drivers. Human Reproduction (Oxford University Press). [Link]
- Jhanji Y, Gupta D, Kothari VK. (2015). Thermo-physiological properties of polyester-cotton plated fabrics in relation to fibre linear density and yarn type. Fashion and Textiles (SpringerOpen). [Link]
