We seem to get a lot of questions about fire retardant clothing and many misconceptions or understanding revolving around what is fire retardant and good practice. I will deal with the fabrics first.
Generally there are four types of fibre and or blends: 100% cotton, 88% cotton/12% nylon blends and inherently fire retardant fibres such as Nomex (an aramid) or Modacrylic. It is not unusual to find any of these fibres to be blended with other fibres. With the exception of 100% Cotton, which may wash out after about fifty washes, these fabrics fire retardancy will not wash out. In purchasing any garment one should read the laundering instructions or google them to maximize the life of the item. It should be noted fabric softeners should not be used as they can coat the fabric diminishing the effectiveness of the cloth.
In our industry we tend to concern ourselves with hydrocarbon flash. Not all garments labelled as fire retardant meets the criteria for hydrocarbon flash. For example, items that only meet NFPA 701 criteria (A test in which determines the garments ability to deter flames from spreading or in other words self-extinguishes). THE NFPA 701 criterion appears basically to be the standard for curtains and sleepwear.
Having said this one must presuppose that the manufactured when making a claim of fire retardancy has had the product tested in an accredited lab. It also has to meet the standards of the environment one will be working in. If unsure, the provider should be able to get test results.
It should be noted that these tests are conducted under laboratory conditions. In the field if a coverall, for example is soiled with a combustible material, some of that may have to burn off. The point is to keep the item one is wearing as clean as possible. This includes the usage of proper cleaning methods to avoid the garment being contaminated by a lipid (or fat) based substance, such as liquid detergent.
Another point that, to me, has almost reached an urban legend status is the idea that only your outerwear has to be fire retardant. There are two components that should be considered: fire and heat. We know that fire retardant fabrics will not sustain a flame once the source has been removed but heat is another matter. If one is wearing a synthetic material under their clothing it could melt from the heat. If worn against the skin it could be very painful if it melts into the skin and potentially lethal from complications. What should be worn is anything made from a natural fibre or fire retardant cloth against the skin. In the jurisdiction I am in it is the law to wear natural or fire retardant fabrics beneath your outerwear.
Regardless, law or no law you can survive a hydrocarbon flash. Many manufacturers have invested thousands and thousands of dollars to develop and improve products. There is lots of information on the internet about these fibres, textiles and the manufacturing processes. All I care about is that everyone has the information to work safe and be safe.