End of service life indicating systems for layered filter cartridges

A method of filtering contaminants from a gas, comprising:

• providing a filter cartridge, the filter cartridge comprising:
• a sealed cartridge housing comprising:
• a gas inlet;
• filter media;
• and
• a gas outlet, wherein the filter media comprises multi-layer construction comprising:
• a first sorbent layer;
• a second sorbent layer closer in proximity to the gas outlet than the first sorbent layer;
• wherein the first sorbent layer has a higher adsorption capacity and/or higher adsorption rate than the second sorbent layer;
• and
• a sensing element adjacent to the first and second sorbent layers such that an indicating element of the sensing element is located at the interface between the first and second sorbent layers;
• causing gas to flow through the filter cartridge;
• detecting a sensing response in the sensing element;
• and
• replacing the filter cartridge.