Anything labelled wrinkle-free, non-iron, permanent-press, or easy-care was cross-linked with a formaldehyde-based resin to lock its shape ¹. The resin slowly breaks down against your skin, releasing free formaldehyde. Cotton takes the cross-linker best, so a pure-cotton non-iron shirt is the highest-residue thing in a wardrobe.

New polyester, nylon, and acetate carry disperse dyes that never bond to the fibre; sweat and friction draw them back out onto skin ². That is why a new black t-shirt rinses dark, and why new denim leaves indigo on your leg.

A single machine wash removes about 60% of the free formaldehyde on the first cycle ³; loose dye comes out on the same schedule. For most people that is enough to notice nothing.

Priority order: non-iron shirts first, then new dark synthetics, then new sheets and pyjamas (eight hours of skin contact a night). A plain undyed cotton t-shirt is the one case where skipping genuinely costs nothing. An Oeko-Tex hangtag means the item was tested for low residual formaldehyde and banned dyes, so its starting load is already much lower.

Skip it and most people get the half-noticed itch that fades within a week. But for the slice whose immune system has learned to react (one in twenty to one in seven of dermatology-clinic patients, fewer in the public), it becomes a rash where fabric presses under heat, on the neck, the inside of the elbows, the waistband ¹. It looks like ordinary eczema, so people blame detergent or stress for years before anyone names the shirt ⁵.