What does it mean to be inherently flame resistant?

When it comes to protective clothing, it's important to understand the construction of the garment as to more fully comprehend the limitations of the garment. This is particularly important when it comes to flame resistance. The ability of a garment to resist ignition, resist degradation or drip/melt when exposed to flame is paramount to keeping workers and racers safe.

Giving a garment flame resistance properties is generally accomplished in 2 ways. The first is to take materials that would usually drip/melt or catch fire and add compounds that make the fiber less likely to propagate flame and give it the ability to self extinguish within a set time frame. This strategy is inexpensive and effective for garments with short lifespans or minimal anticipated wash cycles. The compounds that render the material (usually cotton, rayon or polyester) flame resistant will wash out with time and protection may decrease. This is a significant limitation of using fire retardant chemicals and is not permitted for use with SFI 3.3/10 or higher garments due to this.

The second strategy is to use inherently flame resistant materials in the construction of the garment. These are fibers that will not drip or melt and will either not propagate flame or quickly self extinguish. Examples of this are oxidized polyacrylonitrile (CarbonX), meta-aramid (Nomex), para-aramid (Kevlar, Twaron), and modacrylic. Each of these fibers have advantages and disadvantages, so at times they are blended together or with other filler fabrics to achieve the desired properties. they also come in different weights or thicknesses depending on the use case. It is standard in the United States to measure this in oz/yard² and in countries using the metric system, this is measured in g/m². These fibers and blends are sold under a variety of brand names but are generally more expensive than garments that are treated with fire retardant.

Reliable light reading:

Islam, M. S., and van de Ven, T. G. M. (2021). "Cotton-based flame-retardant textiles: A review," BioResources 16(2), 4354-4381.