100% cotton means the fibers are cotton. It does not mean the fibers are not treated with formaldehyde.Sheets on mattress, they are 100% cotton. I'm sure I'm getting it from somewhere though .
Pretty much any textile that says it is wrinkle-free or wrinkle resistant has formaldehyde.
Or odor resistant, stain resistant, cling free, anti shrink, water resistant, mildew resistant, color fast. I'm probably missing some.
Most laundry detergents and most fabric softeners also have formaldehyde which stays in the linens and clothes. The labels rarely say formaldehyde; they say one of many dozens of alternative chemical names for it.