It is acceptable to wear 100% cotton fabrics under FR clothing which will help provide layering and air gaps that would add to the thermal insulation. Never wear any fabric that may melt when exposed to a flame source such as acetate, nylon, polyester, polypropylene or spandex under or along with your flame resistant protective clothing. Always remember that the FR clothing should always be the outermost layer of garment although he best option would be to wear flame resistant clothing as both the outer layer and undergarments. Flammable apparel should not be used over FR clothing because it may ignite and jeopardize the safety of the user as it transfers heat through the FR fabric.