Why your sportswear makes you itch

If you have ever finished a workout with a red, itchy patch where your bra band sat, or your waistband pressed in, or your seam rubbed, that is not random. It is also not always your skin's fault.

Most fabric-related skin reactions during sport come from a combination of four things, and most of them are coming from the clothes.

What actually happens to your skin during a workout

Three things change in the skin during exercise. They all matter.

Heat. Your core temperature rises. Blood flow to the skin increases to release heat, which means more nutrients and immune cells move toward the surface. The skin barrier softens. It becomes more reactive, more permeable, more sensitive.

Sweat. You secrete water mixed with salts, urea, lactate, and trace proteins. Sweat itself does not irritate skin in any meaningful way, pure sweat on clean skin is fine. But sweat is also a solvent. It draws things out of fabric and into skin contact.

Friction. Wherever fabric meets skin under load, bra band, waistband, seams along the inner thigh, shoulder straps, there is repeated micro-abrasion. This breaks down the outermost layer of the skin barrier. A fabric that would feel fine standing still can feel like sandpaper after thirty minutes of squats.

Now imagine that fabric is also full of synthetic finishes.

What polyester adds to that equation

Three things, all of them avoidable.

Trapped heat and moisture. Polyester does not breathe the way natural fibres breathe. The structure of the fibre, solid, hydrophobic, tightly knit, keeps moisture against the skin instead of releasing it. The skin under polyester is warmer, wetter, and more occluded than the skin under cotton.

Bacterial overgrowth. The same closed environment that traps moisture also hosts bacteria better than natural fibres do. A 2014 study by Callewaert et al. in Applied and Environmental Microbiology found significantly higher concentrations of micrococci, the bacterial group most associated with workout odour and skin irritation, on polyester garments after a single session, and almost none on cotton. We get into the smell side of that in why polyester smells faster.

Chemical residue. This is the part that is least talked about. Synthetic activewear is rarely "just polyester." It is polyester plus dye, plus a softener, plus an anti-microbial finish, plus a moisture-wicking treatment, plus formaldehyde-based wrinkle-resistance in some cases. Many of those chemicals stay in the fabric after manufacturing. Research from Stockholm University identified thousands of textile chemicals detectable in finished garments, several of them known irritants or sensitizers. When you sweat, those chemicals do not stay in the fibre. They migrate. A 2023 University of Birmingham study showed sweat actively increases the leaching of hazardous chemicals from synthetic microplastics into the skin.

When this becomes a real problem

For most people, occasional irritation that fades within a few hours is not medical. It is a signal, the fabric is wrong for your skin in those conditions, and the fix is changing the fabric.

When it becomes more than that:

• Recurrent rashes in the same locations after every workout
• Blistering, weeping, or peeling skin
• Itching that lasts more than a day
• Spreading patches beyond where the fabric actually touched

These are signs of contact dermatitis or an allergic reaction to a specific chemical. See a dermatologist. They can run patch tests for the specific irritant, common ones in textiles include disperse dyes, formaldehyde resins, and rubber accelerators in elastic. Self-diagnosing this online is not a substitute. For people with eczema, psoriasis, or rosacea, fabric choice is not optional. It is part of the management plan.

What natural fibres do differently

Cotton, in plain terms, breathes. Heat releases instead of pooling. Moisture absorbs into the fibre instead of sitting on the skin. The fabric stops being a sealed environment.

Cotton also does not host bacteria the way synthetics do. The fibre absorbs sweat, the sweat dries out from the inside, and the bacterial concentration on the skin stays closer to baseline.

And, this is the part most articles miss, when the cotton is GOTS certified, the chemical processing has been audited. There are caps on what dyes and finishing chemicals can be used. Formaldehyde-based wrinkle treatments are out. Most azo dyes are out. The fibre that touches your skin has been processed under stricter chemical limits than ordinary cotton, let alone ordinary polyester. What GOTS audits, and what it does not, is in what GOTS, OEKO-TEX and Fair Wear actually mean.

That last part is what fixes most fabric-related skin reactions for most people. The fibre type matters. The chemical processing of the fibre matters more.

What to look for if your skin reacts

Fibre composition first. Aim for a fabric that is mostly natural fibre, cotton, linen, hemp, merino, with at most ten percent stretch fibre. Avoid 100% polyester or polyester-dominant blends for next-to-skin pieces. The full fibre rundown is in best natural fabrics for activewear.

Certified processing. GOTS, OEKO-TEX Standard 100, or both. These actually mean something. "Eco" and "conscious" do not.

No anti-microbial coatings. These are sold as a feature for sportswear. For sensitive skin they are usually a problem. Real natural fibres do not need them.

Loose-fit options where possible. Less friction equals less irritation.

Wash before first wear. New garments carry residual processing chemicals. One pre-wash removes a meaningful chunk.

Where LAINE HILL fits

We make sportswear out of 92% GOTS-certified organic cotton and 8% spandex. The cotton is grown without synthetic pesticides. The processing is audited under GOTS. There are no anti-microbial finishes, no formaldehyde resins, no disperse dyes that we have not verified.

We did not design this for people with sensitive skin specifically. We designed it for what we wanted on our own skin during a workout. People with sensitive skin happen to be one of the groups who feel the difference most clearly, because their reaction threshold is lower.

If you have been wearing polyester through a recurring rash, the experiment is small: try a single piece of GOTS organic cotton activewear for two weeks, watch what changes. If the rash fades, you have your answer. If it does not, that is a reason to see a dermatologist, and the cotton is still better for your skin than the polyester.

We are not making medical claims. We are explaining the chemistry, and giving you a fabric option that does not add to it.

LAINE HILL

Continue reading

• Why polyester smells faster, the bacteria behind workout odour, and what natural fabric does differently.
• Best natural fabrics for activewear, cotton, merino, tencel, bamboo, hemp, modal, linen, compared.
• For half a century we got it wrong, what polyester does, and why we built something different.