Does Wearing Polyester Affect Male Fertility?
The short answer
Heat is the mechanism, and polyester traps it. That is not a theory, it is the reason your testicles hang outside your body.
The longer answer involves three separate bodies of research, and the numbers in it are worse than most men realise.
Sperm concentration in Western men, 1973 to 2018
Meta-analysis of 185 studies covering 42,935 men across 50 countries, updated in 2022. The rate of decline has more than doubled since 2000. Source: Levine et al, Human Reproduction Update.
Your body already solved this. You are undoing it.
Sperm production needs a temperature a couple of degrees below the rest of you. That is the entire reason the anatomy is arranged the way it is, hanging outside the body where air can get to it.
Polyester is dense and non-porous. It moves sweat to the surface and holds the warmth underneath, against the skin. In a liner, in a waistband, that is the part pressed hardest against you for the whole session.
You are wrapping the one part of your body that evolved to stay cool in a fabric engineered to hold heat in.
Sperm concentration by underwear type, 656 men
Men who most often wore loose-fitting boxers had 25% higher sperm concentration, 17% higher total count and 14% lower FSH than men in tighter underwear. 656 men, 1,186 semen samples. Source: Mínguez-Alarcón et al, Human Reproduction, 2018. This study compared fit, not fabric, but the mechanism is the same one: heat.
The research is about heat. Polyester is a fabric that exists to hold heat against skin. Draw the line yourself.
It is not just heat. The plastic gets in.
In 2024, researchers at the University of New Mexico tested human testicular tissue for microplastics. They found them in every single sample.
Twenty-three out of twenty-three, at a mean concentration of 329 micrograms per gram. Polyethylene was the most prevalent polymer. PET, the plastic your gym shorts are made from, was among the most common found.
The same team found higher concentrations of certain polymers correlated with lower sperm count in the canine samples they tested alongside.
Human testicular samples containing microplastics. Every one tested. Toxicological Sciences, 2024.
Three things the research has established.
Testosterone is falling too
Mean total testosterone in American men aged 15 to 39 fell from 605 to 451 ng/dL between 1999 and 2016. That is roughly 25%, and it held after adjusting for BMI and activity. Lokeshwar et al, 2021, n=4,045.
Phthalate exposure tracks with lower testosterone
A US national health survey found higher urinary phthalate metabolites associated with lower serum testosterone across men, women and children. Phthalates are used in garment prints and coatings. Meeker and Ferguson, JCEM, 2014.
Polyester leaks into sweat
Antimony trioxide is the catalyst used to make PET and is present in 80 to 85% of virgin polyester. Testing found 125 to 470 µg/g in finished fabric, a fraction of which migrated into artificial sweat. Biver et al, 2021.
Nobody in the industry is going to raise this.
Every product developer at every major sportswear brand knows polyester traps heat. They compete on stretch and colourways because those are the arguments plastic can win. What the fabric is doing to the man wearing it is not a competition they enter.
The silence is not an accident. It is a business decision.
What to do about it.
Get the plastic off your skin, starting with the layer closest to it. The liner and the underwear come first, because those are the parts held against you for the entire session.
Nero shorts use a 100% GOTS organic cotton shell and a 100% merino liner at 17.5 micron. Merino absorbs moisture into the fibre and releases it as vapour, and regulates temperature in both directions rather than holding heat in one.
No polyester. No nylon. No elastane. The boxers are 100% organic cotton and nothing else.
We go through the hormone research in more detail on our page about polyester and testosterone.