This is different from most material that start burning slowly and increase their burning rate as time and temperature increases. Flame retardant additives decrease the possibility of foams catching fire from contact with a small fire source, such as a torch or overheated electrical wire. A fire test on cellular plastic foams under controlled conditions and blueprint specifications is quite different from what may occur with foam in place for years, subject to aging, mechanical damage, building modifications, moderate maintenance, and other changes that normally occur during product lifetime. If you are adding a fire barrier to exposed foam insulation, be sure the fire barrier is conducive to the environment ti will be exposed to. For example, plywood barriers would be used in swine confinements rather than gypsum board due to moisture problems and possible physical damage.
Asbestos is a known carcinogen.
If asbestos-cement board is used as a fire barrier and boards must be cut for fitting, wear a respirator approved for asbestos dust. During sawing, fibers are likely to be jarred loose form the bonding substance and become airborne.