A cartridge comprising layers of adsorbent sheet is described. The cartridge includes an indicator that characterizes the consumption state of the adsorbent within the cartridge. The indicator is applied in a way such that discrete areas of indicator are visible. These discontinuous areas of indicator may be applied to the outside surface of the cartridge. Alternatively, the discontinuous areas may be formed by cutting windows in the outermost layer of the cartridge and either coating indicator on the layer beneath the window, placing an indicator layer between the window and the layer beneath it or filling the window with an indicating plug of material so that the indicator is visible from the outside of the cartridge. The indicator layer and indicator plug embodiments allow the use of any indicator with any adsorbent.
