As already mentioned, aromatic polyester polyols and polyisocyanurate (PIR) structures, for example, contribute to fire safety. The use of halogen-containing polyols is also usual. Non-reactive additives are trialkyl, trishalogen alkyl and triaryl phosphates. Triethyl phosphate, tris-chlorisopropyl phosphate and diphenylcresyl phosphate are typical examples. Solid, insoluble flame retardants are scarcely used as they are problematical from the processing point of view. However, they could play a more important role in the future.