Aramid fibers from companies like DuPont create inherently flame resistant fibers, with brands such as DuPont™ Kevlar® and DuPont™ Nomex®. The actual structure of the fiber itself is not flammable. For inherently flame resistant fibers, the protection is built into the fiber itself and can never be worn away or washed out. In the case of Nomex®, when exposed to flame, the aramid fiber swells and becomes thicker, forming a protective barrier between the heat source and the skin. This protective barrier stays supple until it cools, giving the wearer vital extra seconds of protection to escape.
The other main category is flame-retardant treated (FRT) fabrics. These materials are made flame-resistant by the application of flame-retardant chemicals. A chemical additive in the fiber or treatment on the fabric is used to provide some level of flame retardancy. During a fire, chemically dependent fabrics rely on a chemical reaction to extinguish the flame. This reaction is triggered by the heat of the fire and the amount of time the fabric is exposed to the fire.