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

• Contact dermatitis from fabric is real and common — usually triggered by trapped sweat, synthetic fibers, or chemical dyes/finishes rather than the fiber alone.
• Rashes typically appear where fabric rubs most: underarms, inner thighs, waistband line, and neck — all high-friction, low-airflow zones.
• New garments often carry residual dye and finishing chemicals; washing once before wearing removes most of what can trigger a reaction.
• If a rash consistently appears with one specific fabric type but not others, that's a strong signal to avoid that fiber going forward, not just that shirt.

The fit-and-fabric-health takeaway: This is the clearest fit-fabric link there is: friction plus trapped moisture plus the wrong fiber is the exact recipe for a fabric-triggered rash.

Next up: check out our guide on the best fabrics for humid weather.