An End of Service Life (ESLI) and Residual Life Indicator (RLI), for gas masks and other filters, where the indicator includes a solid conduit with one end closed by a transparent material and the other by a porous material. In one implementation the conduit contains, a powder that can react chromogenically. In another implementation, the porous material includes a reagent that can react with the target gas and release a reactive gas. The transparent end of the conduit is coated internally with a chromogenic reagent that changes color when either the target gas or the gas produced in the reaction reaches it. The ESLI/RLI may be inserted into the filter from different directions to accommodate practical filter or mask designs. The ESLI/RLI may contain more than one chromophore to alert users of the penetration of different gases or classes of gases.