Does Wearing Polyester Lower Testosterone?

Wearing polyester underwear does not appear to lower testosterone based on the available research. The most directly relevant human study, conducted by Egyptian researcher Ahmed Shafik in the early 1990s, found that polyester underwear dramatically reduced sperm counts in men but produced no significant change in reproductive hormones, including testosterone. The concern has persisted for decades anyway, fueled by animal studies, theories about electrostatic fields, and growing awareness of chemicals embedded in synthetic fabrics. Untangling which parts of the worry are legitimate requires looking at polyester the fiber, polyester the chemical cocktail, and the body’s surprisingly complex response to what sits against the skin.

What the Original Polyester Studies Actually Found

Almost everything people believe about polyester and male reproductive health traces back to a single researcher. Ahmed Shafik, a prolific Egyptian urologist, ran a series of experiments from the late 1980s through the 1990s in which he dressed dogs, rats, and eventually human volunteers in underwear made from different fabrics and tracked what happened to their sperm and hormones.

In the human study, 14 men wore a polyester sling (essentially a close-fitting scrotal suspensor) for about a year. Every one of them became azoospermic, meaning their sperm counts dropped to zero, after an average of roughly 140 days. Their testicular volume shrank. But their serum reproductive hormones, including testosterone, follicle-stimulating hormone, and luteinizing hormone, showed no significant change throughout the entire wearing period.¹ A parallel study testing different fabrics on spermatogenesis in dogs confirmed the same hormonal finding: polyester disrupted sperm production, but hormone levels stayed statistically unchanged.²

So the original research that launched this entire conversation actually provides evidence against a direct testosterone-lowering effect. Sperm production and testosterone levels are regulated by overlapping but distinct hormonal pathways, and it is entirely possible to impair one without crashing the other. Shafik himself never claimed polyester reduced testosterone. His interest was in whether polyester could work as a reversible male contraceptive, and the sperm data were striking enough on their own.

The Electrostatic Field Theory

Shafik’s proposed explanation for the sperm damage was not heat, chemicals, or tight fit. He believed the culprit was static electricity. Polyester generates electrostatic charge through friction with the skin, and Shafik measured those charges directly. In human volunteers wearing different fabrics, cotton underwear produced no detectable electrostatic potential on the scrotal surface. Polyester underwear generated an average of about 339 volts per square centimeter, while a polyester-cotton blend produced less than half that.³ The charges were higher during the day than at night, likely because of movement and warmth.

Shafik theorized that these charges created a persistent electrostatic field across the scrotal contents, disrupting the delicate process of sperm maturation in the testes and epididymis.⁴ In a related rat study, animals dressed in polyester pants showed significantly reduced sexual activity compared to those in cotton or wool. The effect was reversible: six months after the pants were removed, sexual behavior returned to normal. Cotton and wool groups showed no decline and actually increased their sexual activity over time.⁵

The electrostatic theory is creative, but it remains largely untested by anyone outside Shafik’s lab. No other research group has independently replicated these findings in humans, and the field of andrology has not broadly adopted the framework. The rat sexual-behavior data are interesting but hard to interpret cleanly: reduced mounting frequency in rats wearing plastic pants could reflect discomfort or irritation rather than a hormonal or electrostatic mechanism. The theory sits in a scientific limbo where it has never been disproven because almost nobody has tried to test it again.

Polyester Itself Does Not Act Like a Hormone

One reason the “polyester lowers testosterone” claim spread so easily is that people conflate the fiber with the chemicals added to it. Polyester, chemically, is polyethylene terephthalate (PET), and the building blocks of that polymer have been specifically tested for hormonal activity. When researchers screened seven polyester monomers using both computer modeling and laboratory binding assays, none showed any meaningful ability to interact with androgen receptors or estrogen receptors. Cell-based activation assays were also negative. Animal toxicity studies using those same monomers over 13 weeks found no androgen- or estrogen-related effects.⁶

This matters because it separates the polymer from its passengers. The polyester molecule sitting against your skin is hormonally inert. Whatever reproductive effects Shafik observed were not caused by polyester acting as an endocrine disruptor in the way that, say, certain pesticides do. The concerns that do have scientific legs involve what gets added to polyester during manufacturing or what happens to it as it degrades, and those are different questions entirely.

The Chemical Passengers in Synthetic Clothing

A polyester shirt fresh off a store rack is not just polyester. The fabric has been dyed, treated with softeners, given wrinkle resistance, and sometimes coated with flame retardants. Many of these finishing chemicals belong to classes that are known or suspected endocrine disruptors, and some can migrate into the body through the skin.

Phthalates

Phthalates are used as plasticizers in textile printing inks, in PVC-based decorative elements on clothing, and in some finishing processes. Modeling research has shown that clothing previously exposed to phthalate-containing environments can increase dermal uptake of certain phthalates beyond what bare skin would absorb at the same air concentrations. For one phthalate in particular, di-n-butyl phthalate, the skin absorption from clothing represented a substantial fraction of total exposure from all sources combined.⁷

The connection to testosterone is indirect but real. Most phthalates interfere with the expression of genes involved in steroid hormone production in Leydig cells, which are the cells in the testes responsible for making testosterone. The concern has been strongest around fetal development, where phthalate exposure can disrupt the programming of male reproductive organs.⁸ Whether the doses absorbed through adult clothing are high enough to meaningfully shift testosterone levels in a grown man remains an open and difficult-to-answer question. The mechanism exists; the dose from a T-shirt probably does not match the doses used in animal experiments.

Bisphenols

Bisphenol A (BPA) and bisphenol S (BPS) turn up consistently in clothing made from both conventional and recycled polyester. A 2024 analysis of everyday garments found these compounds in T-shirts and socks regardless of whether the fabric was virgin or recycled. The highest BPA concentration, 625 nanograms per gram of fabric, appeared in recycled-polyester socks. When the researchers modeled dermal exposure from both dry and sweat-dampened textiles, they found that exposure could exceed the European Food Safety Authority’s updated safety threshold for BPA, especially when the wearer was sweating.⁹

BPA is one of the most studied endocrine disruptors in existence, and its effects on reproductive hormones have been documented across hundreds of animal studies. The wrinkle for clothing specifically is that recycled polyester, which is increasingly marketed as environmentally friendly, may carry higher contamination loads than virgin material. People buying recycled-content athletic wear as a “greener” choice might actually be increasing their chemical exposure during exercise, when sweating is at its peak.

Flame Retardants

Brominated flame retardants, particularly polybrominated diphenyl ethers (PBDEs), have been applied to synthetic textiles for decades, especially in sleepwear, upholstery fabrics, and uniforms. A large population-based analysis using U.S. national health survey data found that higher blood levels of two specific PBDEs were associated with lower total and free testosterone in adult men. The association was strongest in men aged 60 and older.¹⁰ These findings do not prove that flame retardants in your pajamas are dragging down your testosterone, because PBDE exposure comes from many sources including furniture, electronics, and house dust. But clothing is one contributor to the total body burden, and for people in occupations that require treated synthetic uniforms, it can be a meaningful one.

Microplastics and Testosterone

Polyester sheds. Every time you wash a polyester garment, it releases thousands of tiny synthetic fibers into the water. Those fibers enter waterways, food chains, and eventually human bodies. There is also evidence that microfibers shed from clothing during normal wear and are inhaled. The question is whether the microplastic particles that accumulate in human tissues can affect hormone production.

Animal research suggests they can. Mice given drinking water containing polystyrene microplastics over six months showed reduced testosterone, lower levels of the hormones that signal testosterone production (LH and FSH), and damage to testicular tissue. The mechanism appeared to involve downregulation of the enzymes Leydig cells use to synthesize testosterone.¹¹ A separate study using a different type of microplastic, polylactic acid particles, found a dose-dependent drop in serum testosterone in mice. The particles accumulated in testicular tissue and accelerated aging of the Leydig cells themselves.¹²

These are concerning findings, but translating them to a man wearing a polyester workout shirt requires several leaps. The mice were ingesting or being injected with microplastics at controlled doses over long periods. We do not yet know how much microplastic a person absorbs from wearing synthetic clothing versus eating seafood, breathing indoor air, or drinking tap water. Clothing is almost certainly one piece of the exposure puzzle, but how large a piece remains genuinely unknown. A model of microfiber shedding from California apparel washing estimated that installing in-line filters on washing machines could cut synthetic microfiber emissions to the environment by about 79%.¹³ That would reduce waterway contamination, though its effect on personal exposure from wearing the clothing is a separate issue.

The Heat Factor

Polyester is not as breathable as cotton or wool. It traps heat and moisture against the skin, which matters for the testes because sperm production requires a temperature a few degrees below core body temperature. This is why the testes sit outside the body in the first place. Any fabric that insulates the scrotum and prevents heat dissipation could, in theory, compromise the testicular environment.

Research on scrotal insulation in rams provides some context. When ram scrotums were experimentally insulated, Leydig cell function changed over time: at 58 days, the cells’ ability to produce testosterone in response to hormonal stimulation was significantly reduced compared to controls.¹⁴ Interestingly, total testicular testosterone content initially rose during insulation, likely as a stress response to elevated temperature, before the cells’ synthetic capacity began to falter.

This aligns with what doctors have long advised men who are trying to conceive: avoid hot tubs, laptop use on the lap, and tight synthetic underwear. The advice is primarily about sperm quality rather than systemic testosterone levels, but the ram data suggest that prolonged scrotal heating can eventually impair the machinery that makes testosterone at the cellular level. Whether standard polyester underwear creates enough of a temperature increase to trigger that cascade in humans is unclear. Tight-fitting polyester athletic compression shorts worn during intense exercise in hot conditions would be a more plausible scenario than loose-fitting polyester boxers worn around the house.

Why the Simple Answer Keeps Getting Complicated

The honest state of the science is that polyester the polymer does not lower testosterone, but polyester the product, as it actually exists in your drawer, is a delivery vehicle for chemicals that can. The distinction matters because it changes what you should worry about and what you can do about it.

If the problem were the polyester fiber itself, the only solution would be to stop wearing it. But since the fiber is hormonally inert and the concerns center on additives, contaminants, and degradation products, the picture is more nuanced. A well-made polyester garment from a manufacturer that avoids phthalate-containing inks, PBDE flame retardants, and BPA-contaminated recycled feedstock would theoretically pose less endocrine risk than a cheap garment saturated with finishing chemicals. The trouble is that consumers currently have almost no way to know which chemicals are in their clothing. Textile labeling requirements cover fiber content and care instructions but rarely disclose chemical treatments.

For men specifically concerned about testosterone, the evidence-based list of lifestyle factors with much stronger and better-documented effects includes sleep quality, body fat percentage, chronic stress, alcohol consumption, and exercise habits. A man sleeping six hours a night and carrying 40 extra pounds of body fat is dealing with testosterone suppression far beyond anything his underwear fabric could plausibly contribute. That does not make the clothing-chemical question irrelevant, but it puts it in proportion.

Recycled Polyester and the Green Paradox

The push toward recycled polyester in the fashion industry has created an underappreciated wrinkle. Recycled polyester, typically made from post-consumer PET bottles, is marketed as a sustainability win. From a carbon-footprint perspective, it often is. But PET bottles may have accumulated contaminants during their previous life as food packaging, and the recycling process does not always remove them. The bisphenol data cited earlier showed that recycled-fabric socks carried the highest BPA concentrations measured in the study, raising the possibility that recycled polyester garments carry a different chemical profile than virgin ones.⁹

This does not mean recycled polyester is dangerous or that people should avoid it. It means the conversation about sustainable textiles needs to include chemical safety alongside carbon emissions. For athletic wear in particular, where the garment sits directly against sweating skin for extended periods, the combination of recycled polyester and heavy perspiration could maximize dermal absorption of whatever contaminants are present. Brands that invest in cleaning their recycled feedstock more thoroughly would offer a real advantage, but there is currently no standard label or certification that communicates this to shoppers.

What You Can Reasonably Do

Switching entirely to natural fibers is one option, but it is not always practical. Performance fabrics, work uniforms, and affordable clothing all lean heavily on polyester. A more realistic approach involves reducing the exposure that matters most:

Wash new clothes before wearing: A first wash removes a portion of surface-level finishing chemicals, dyes, and residual processing agents. This is especially worthwhile for garments that will sit against the skin.
Choose cotton or wool for underwear: The underwear layer sits closest to the most temperature-sensitive and hormonally active part of the male reproductive system. Switching this single garment to a natural fiber addresses the electrostatic, heat-trapping, and chemical-contact concerns simultaneously.
Be skeptical of “recycled” claims on skin-contact activewear: Recycled polyester is fine for jackets and bags, but for leggings, compression shorts, and base layers worn during heavy sweating, the potential for enhanced dermal absorption of contaminants is worth considering.
Avoid unnecessary flame-retardant treatments: Some synthetic sleepwear and children’s clothing carries flame-retardant coatings that are not legally required. Checking labels and choosing untreated options when available reduces PBDE exposure.

None of these steps will produce a measurable change on a blood test. The chemical exposures from clothing, even in a worst-case scenario, are small relative to other sources. But for someone already optimizing sleep, exercise, and diet for hormonal health, reducing unnecessary synthetic chemical contact is a reasonable next layer of attention rather than a first priority.