Most people think about what they eat before a workout. What they put in their body, what fuels it, what's clean and what isn't. Almost nobody thinks about what they're wearing.
They should.
Polyester is plastic. Literally.
Polyester is a synthetic fiber derived from petroleum — the same raw material as plastic bottles, packaging, and fuel. When it was first patented in the 1940s and mass-marketed in the 1950s as a “miracle fiber,” nobody was thinking about what happens when it sits against human skin for hours at a time during intense physical activity.
We know now.
Every time you wear polyester — and especially every time you wash it — it sheds microscopic plastic fragments called microfibers. These are a form of microplastics: particles measuring less than 5mm that don't biodegrade and don't leave the environment once they're in it.
According to the United Nations, synthetic textiles are responsible for approximately 35% of the microplastics currently in our oceans. A single laundry load of polyester clothing can release as many as 700,000 microplastic fibers into waterways. One polyester shirt, washed every two weeks, releases approximately 52,000 microplastic fibers per year.
That's the environmental problem. The personal health problem is newer — and worse.
The skin absorption finding
For years, the concern about microplastics focused on what happens when you ingest or inhale them. The skin was considered a reliable barrier.
A 2024 study from the University of Birmingham changed that. Researchers used three-dimensional human skin-equivalent models to examine what happens when microplastics come into contact with sweat. They found that microplastics act as carriers for the chemical additives embedded in synthetic fabrics — flame retardants, plasticizers, dyes — and that several of these compounds passed through the skin barrier and into the equivalent of the bloodstream.
The study's lead researcher, Dr. Ovokeroye Abafe, put it plainly: “Microplastics are everywhere in the environment and they play a role as carriers of harmful chemicals, which can get into our bloodstream through the skin.”
Activewear is the highest-risk category. Here's why: heat, sweat, and friction all increase the rate at which microplastics shed from fabric and make contact with skin. A polyester gym set worn during a lifting session or a run is producing exactly those conditions — for an hour or more, pressed against some of the most absorbent skin on the body.
The cardiovascular data
In March 2024, the New England Journal of Medicine published a study that examined arterial plaque removed from 304 patients. More than half of the samples contained detectable microplastics. Patients whose plaque contained microplastics had a 4.5 times higher risk of heart attack, stroke, or death over the following 34 months compared to those whose plaque did not.
The researchers were careful to note this is an association, not proven causation. But a hazard ratio of 4.5, in a study that controlled for diabetes, hypertension, and cholesterol, is not a small signal.
What actually changes it
The fix is straightforward. Natural fibers — organic cotton, Tencel lyocell, merino wool — don't shed microplastics. They're derived from plants and animals, not petroleum. They biodegrade. They don't carry synthetic chemical additives into contact with your skin.
Research published in 2025 suggests that starting with activewear is the most effective single change a person can make, precisely because it's the category worn closest to skin during the conditions — heat, sweat, friction — that maximize microplastic transfer.
This is why BENCHED builds from GOTS-certified organic cotton, Tencel lyocell, and merino wool. Not as a sustainability talking point. Because the alternative is plastic, and what goes against your skin in your hardest moments should meet the same standard as what goes in your body.