A fire retardant is a substance that slows ignition or reduces the rate at which a polymer burns, without necessarily preventing combustion ^[1]. The action follows four chemical routes: a halogen or phosphorus species interrupts the radical chain of the gas-phase flame, a filler releases water or an inert gas on heating, a char-forming agent promotes a carbonaceous layer at the surface, and a diluent lowers the fuel fraction reaching the flame ^[1]. The char layer separates the condensed phase from the heat flux and from oxygen, so its continuity governs the residual performance ^[2]. Additives are described by the loading needed to reach a limiting oxygen index and a vertical burn rating, and by whether they are reactive or merely blended into the host polymer ^[3]. Bio-based additives act through intumescence or through phosphorus released on decomposition ^[3].