Abstract: An apparatus for indicating the presence of CO and oil in a compressed breathing air supply provided in a tank. An indicator contains a substance which undergoes an observable physical change when exposed to a contaminant. The indicator is activated and placed in a housing and connected to the air supply and the indicator observed. In a preferred embodiment, the indicator is a single use indicator and contains an indicating gel in a glass tube which is opened by breaking at the time of use. Appropriate reference indicia are provided so the user may visually determine the contaminant level.