A process for rapidly infusing a synthetic resinous substrate with an alkali metal nitrite, comprises (a) essentially completely dissolving the alkali metal nitrite in supercritical carbon dioxide to form a solids-free solution having from about 1 to 15% by weight of the alkali metal nitrite; (b) contacting the synthetic resinous substrate with the solution for a time sufficient to transfer at least a portion of the alkali metal nitrite into the synthetic resinous substrate while maintaining the carbon dioxide under supercritical conditions; and, (c) decreasing pressure or temperature, or both, on the synthetic resinous material sufficiently to evolve carbon dioxide and leave micronized solid alkali metal nitrite crystals in an amount less than 2% by weight essentially uniformly distributed in the synthetic resinous substrate. If desired, up to about 15% by weight of sodium nitrite or potassium nitrite may be transported and deposited in the substrate if transparency of the substrate is irrelevant, because the substrate tends to become opaque.