Let’s start at the beginning by defining Modacrylic fibers. Modacrylic fibers are a modified acrylic fiber; thus ‘mod-acrylic.’
Acrylic fiber was originally introduced as a synthetic replacement for wool. Acrylic is made from a combination of polymers of at least 85% acrylonitrile. Modacrylic is modified by manufacturing a fiber of long-chain synthetic polymer combinations composed of less than 85% but at least 35% acrylonitrile and other synthetic polymers.

What is the History of Modacrylic in FR Clothing?
Modacrylic fibers are not new to the FR industry and in fact have been around for years without any known off gassing toxicity issues (Firewear, Glen Guard, Protera, Tecasafe). The amount of Modacrylic fiber in the different FR fabric varies. Firewear, for example, is a blend with 55% Modacrylic compared to Tecasafe at 48% and Protera at 65%. However, there has been some marketplace uncertainty about fabrics using Modacrylic fibers because of off gassing, regardless of its history of no known issues.

So Where Did the Off Gassing Concern Start?
It likely started because of a University of Alberta letter regarding flash fire manikin testing issues. The original letter was written in 2005 regarding the testing of Modacrylic fabrics. The university lab stopped testing Modacrylic blend fabrics because the off gassing affected their sensors causing frequent replacement which was expensive. In 2006, the university published a new letter stating they are “now willing to test FR garments containing modacrylic fibers.” The letter also states that “Reluctance to test clothing or materials containing modacrylic fibers should not be construed as condemnation of modacrylic or modacrylic blends. Most materials used in FR clothing will, under some conditions, produce toxic or harmful compounds. In the case of modacrylic, the problem is that the compounds have an affinity for copper and this adversely affects the copper component of the sensors.”

What Else Can Be Said About Modacrylic Off Gassing?
Most all material (including wool, Nomex, and FR cotton) produce undesirable combustion by-products. It is a matter of whether or not they are at a toxic level. For example: The Modacrylic in DuPont Protera has been tested by DuPont and the trace amount of a potentially toxic combustion by-product antimony trioxide was measured at 20 times below OSHA exposure limits. DuPont also points out that in a fire the toxic gases from the combustion of the fuel and the fire itself are a greater risk than the off gasses from the fabric. In a DuPont paper about toxicity of PPE off gases they state, “Exposure to toxic gasses is of secondary concern to that of escape – especially since the volume of toxic gases generated from most fires far outweigh that which could potentially be generated from a lightweight, thermally protective garment.” It should also be noted that Modacrylic fibers are used in a variety of consumer products requiring flame retardant capability such as children’s sleepwear, mattresses, rugs and draperies which are deemed acceptable.

Also the Kaneka Corporation, the largest manufacturer of Modacrylic fibers, sponsored a toxicity study and concluded: “When the situation is modeled using a flame resistant modacrylic (Protex M), the potential threat to the user becomes insignificant in the face of the hazards associated with thermal exposure,” and “the potential toxicity of pyrolytic gases from flame resistant modacrylics represents almost no risk within the context of occupational use even under the extreme conditions faced by professional firefighters.”