A/C refrigerant R-134a, properties: Flammability
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R-134a refrigerant is not flammable. In fact, it tends to inhibit combustion and has a fire extinguishing effect. However, a flame or glowing hot surface, as well as ultraviolet light (from electric welding), will decompose the refrigerant into poisonous gases (hydrogen fluoride, carbonyl fluoride and fluorine). These gases give adequate warning of their presence since they cause irritation of the mucous membrane (throat).
R-134a can become flammable at pressures above ambient pressure in conjunction with air concentrations greater than 60% by volume.