Let's get straight to it — here's what actually matters on this one.

• Cotton is a natural fiber that's generally hypoallergenic, breathable, and gentle against skin — the default recommendation for sensitive or reactive skin.
• Synthetics (polyester, nylon) don't absorb moisture, so sweat sits on the skin surface, which can trigger irritation, especially in humid climates.
• Synthetic fabrics also build up static and trap odor-causing bacteria more than cotton, both of which correlate with skin discomfort over a full day.
• If you need wrinkle resistance, a cotton-poly blend under 35% synthetic keeps most of cotton's skin-friendliness while resisting creasing.

The fit-and-fabric-health takeaway: Skin reactions people blame on 'my skin being sensitive' are frequently just synthetic fabric doing what synthetic fabric does — trap moisture against skin.

Next up: check out our guide on can wrong fabric actually cause skin rashes.