Inherently flame resistant fibers are materials that have flame resistance built into their chemical structures. Aramid fibers from companies like DuPont create inherently flame resistant fibers. 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 a brand of inherently flame resistant fiber, 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.
It is very difficult to determine whether protection has been compromised with an FRT fabric. There are ways to test it. Unfortunately, all of these test methods are destructive — there really is no way to test a garment to determine what its current level of flame resistance or arc protection value is, without destroying the garment in a flame or arc flash test.
The flame-resistant (FR) properties of inherently flame resistant fabrics, on the other hand, cannot be washed out or worn away, period. This means the flame-resistant properties of garments made of inherent fibers cannot be compromised. It is crucial for the wearer to know the flame-resistant protection is always there.