A medium for purifying fluids containing toxic compounds can be made by a method which comprises applying a dispersion of a halogenated heterocyclic N-halamine in an inert liquid carrier to the surface of a substrate having a high surface area and allowing the inert liquid carrier to penetrate into the substrate, wherein the inert liquid carried does not react with halogen atoms in the halogenated heterocyclic N-halamine, wherein the halogenated heterocyclic N-halamine is capable of destroying the toxic compound by oxidative action, and wherein the halogenated heterocyclic N-halamine becomes deposited on the surface of the substrate after the inert liquid carrier penetrates into the substrate.