To monitor carbon monoxide and moisture content of the air stream, color change indicator wafers have been supported in housing separate from the main purifier housing, with a window for viewing the wafers. Air tight flow connections must be provided between the two housings. In one aspect the gas purification system features improved indicator mounting. An indicator plug is mounted in the purifier housing adjacent a cartridge end cap having a restricted gas exchange opening therethrough; the plug has at least a portion spaced from and communicating with the end cap to define a gas flow zone communicating with the exchange opening; and the plug houses the indicator and the window, and has a gas flow space communicating between the indicator and the flow zone. In preferred embodiments the plug is screwed in the end of a cylindrical purifier housing with its flat end facing the end cap, and the flow space consists of a bore in the plug at that end and relief in a carrier for the indicator. Another aspect the gas purification system features a gland mounted in an end cap opening, and a tube mounted in a gas exchange passage of the purifier housing, the gland having a bore to receive the tube in sealed relation to the cartridge. In preferred embodiments the gland is sealed to the tube by an internal o-ring, and the opening in the second end cap of the cartridge is too small to receive the tube.