Polyurethane foam insulation burns quickly if ignited, emitting smoke, heat and toxic gases, according to the Occupational Health & Safety Administration. Toxic gases emitted by burning polyurethane foam include carbon monoxide, nitrogen oxides, hydrogen cyanide, benzene, acetaldehyde and acetone.
Some types of polyurethane foam insulation claim to be "flame-resistant" or "fire-retardant" However, these products still cause large amounts of toxic gases when exposed to fire or flame, which are as lethal as a fire.
Polyurethane foam insulation can be used safely if you install it in a flame-retardant and heat-retardant structure, such as gypsum board, according to the US. Consumer Product Safety Commission.
Never allow welding, cigarette smoking or other types of flame around polyurethane foam insulation.