NFPA stands for National Fire Protection Association. NFPA ratings have three numbers, each on a scale of 0 to 4. The first number tells how toxic the material is, the second tells how flammable the material is, and the third tells how unstable the material is. In each case a 0 indicates the least toxic/flammable/unstable materials and a 4 indicates the most toxic/flammable/unstable materials.