I Found a Study That Changed Everything I Thought I Knew About Men's Underwear.

Lauryn

Founder, PRIMALS

Your Underwear Is Lowering Your Testosterone. Here Is What the Research Says.

I was deep into researching synthetic chemicals for PRIMALS when I came across a statistic I could not ignore: every time your phthalate exposure doubles, your testosterone drops by 7.72%. Phthalates are one of the most well-documented endocrine disruptors on the planet. They are in plastic food containers, vinyl flooring, and personal care products. But the source most men never think about is the one pressed against their skin 16 hours a day: polyester underwear.

That one study sent me down a rabbit hole. I pulled paper after paper: phthalates, BPA, electrostatic fields, scrotal heat accumulation. I watched the men in my life spend hundreds on testosterone supplements and biohacking protocols while wearing polyester underwear every single day. Nobody was connecting the dots.

Average testosterone levels have dropped roughly 1% per year since the late 1980s. A 40-year-old man today may have 20 to 30% less testosterone than his father did at the same age. Scientists point to synthetic chemical exposure as a primary driver. And the decline began in the 1970s, the exact decade polyester underwear became mainstream. Here is the full breakdown of what the research says.

4 Ways Polyester Underwear Attacks Your Testosterone

This is not a theory. There are published, peer-reviewed studies documenting exactly how synthetic underwear lowers testosterone. It happens through four distinct mechanisms, and they all operate simultaneously every time you put on a pair of polyester boxers.

1. Chemical Endocrine Disruption

Polyester contains phthalates, BPA, and PFAS, all classified endocrine disruptors. Research found men with the highest phthalate exposure had a 15% lower Free Androgen Index (Environmental Health Perspectives, PMID: 22832070). BPA, found in polyester-spandex blends, slashes testosterone production in Leydig cells by up to 39%. These are the primary testosterone-producing cells in the testes. When BPA interferes with their function, your body physically cannot produce the testosterone it needs.

2. Scrotal Absorption

Scrotal skin is 42 times more absorbent than your forearm (Feldman and Maibach). Chemicals in polyester do not just sit on the surface. Researchers confirmed up to 39% of hazardous chemicals in polyester garments migrate into artificial sweat (Chemosphere, 2024). The warmer and sweatier the skin, the faster this transfer happens. The groin is both.

3. Scrotal Heat Accumulation

Testosterone production requires the testes to be 2 to 4 degrees Celsius cooler than the body. That is why they are located outside the body cavity. Polyester is hydrophobic: it pushes moisture toward the skin instead of absorbing it, creating a hot, stagnant microclimate. Research shows synthetic underwear raises scrotal temperature by 0.5 to 0.8 degrees Celsius (PMID: 9240266), enough to impair both sperm development and testosterone synthesis.

4. Electrostatic Disruption

This is the mechanism most people have never heard of. Polyester generates 338.9 volts per square centimeter of electrostatic potential on the scrotum. Cotton generates zero (European Urology, PMID: 1503251). Even a 50/50 blend still hits 148.3 V per square centimeter. These fields directly disrupt testicular function and are a proposed mechanism behind the reduced sperm counts observed in men wearing synthetics.

This Is Not Aging. This Is Exposure.

Most men assume low testosterone is a natural part of getting older. It is not. Epidemiological data from Europe and the United States reveal that average testosterone levels have dropped 20 to 25% over the past two decades, correlating directly with increased exposure to synthetic chemicals.

Before 1941, all underwear was made from natural fibers. Polyester became widespread by the 1970s. That timeline correlates directly with the beginning of the testosterone decline that researchers have documented across every Western nation. If you are wearing polyester underwear right now, and statistically you probably are since 73% of global textiles are synthetic, these consequences are compounding against you every day: fatigue, brain fog, muscle loss, weight gain, depression, low libido, and erectile dysfunction.

And a chemical called benzothiazole, used in textile dyes and found in 93.5% of clothing tested, makes it worse. Stockholm University proved that 62% passes directly through your skin into your bloodstream (PMC6133113). This chemical is classified as an emerging endocrine disruptor. You are absorbing it every day through your underwear.

Organic Cotton Eliminates Every Mechanism That Suppresses Testosterone

Once you understand the four mechanisms, the solution becomes obvious. You need a fabric that contains zero endocrine-disrupting chemicals, generates zero electrostatic charge, breathes to regulate scrotal temperature, and is made without petroleum-based synthetics. Organic cotton checks every single box.

The "100% Cotton" Lie Most Men Fall For

This is the part that made me angry when I was researching. Brands put "100% cotton" on their label and men assume the problem is solved. It is not. Conventional cotton is grown with synthetic pesticides, herbicides, and chemical fertilizers that remain embedded in the fiber. Then the fabric is processed with toxic dyes, formaldehyde finishes, and chemical softeners that are absorbed directly through your skin.

And even a 50/50 poly-cotton blend still generates 148.3 V per square centimeter of electrostatic charge. If any part of the process involves synthetics or toxic chemicals, you are still exposed. That is exactly why PRIMALS went GOTS-certified organic from seed to stitch.

That Is Why We Built the PRIMALS Organic Cotton Boxers

Once I understood the science, I could not un-see it. Polyester was the obvious problem, but so-called "clean" alternatives were full of shortcuts. Brands claiming organic cotton but dyeing it with toxic chemicals. "Natural" labels processed with formaldehyde finishes. Every brand I investigated compromised somewhere in the supply chain. So I built exactly what I would want the men in my life to wear.

The PRIMALS Organic Cotton Boxers are 100% GOTS-certified organic cotton. No polyester blends. No hidden elastane. No toxic dyes. No synthetic pesticide residue. Even the packaging is biodegradable. Over 100,000 men have already made the switch.

For men who prefer a boxer brief style with merino wool performance, we also offer PRIMALS Merino Wool Boxer Briefs with ozone-treated, chemical-free construction.

Frequently Asked Questions About Underwear and Testosterone

You Cannot Optimize Testosterone While Wearing the Chemicals That Suppress It.

You control your diet. You control your sleep. You control your training. Now control the one thing pressed against your most absorbent skin for 16 hours a day.

The fix is simple. Switch to underwear that was never made with plastic, toxic dyes, or endocrine disruptors in the first place.

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