A self-contained breathing apparatus for emergency response personnel has face mask with a first port connected by a hose to a tank of compressed air. Back-up smoke inhalation protection is provided for imminent life-threatening situations by configuring a face mask with a canister containing a filter medium, an inlet opening through which air enters and an outlet coupling for attachment to a second port of the face mask. In normal situations the mask is supplied with air from the tank, but should the tank supply become depleted, the canister can be activated. A use-evidencing mechanism is operatively connected to the canister to indicate use of the air filtering system. This mechanism may be a non-reusable cover applied over the inlet opening and/or a valve in the second port of the face mask with a portion that breaks away upon opening of the valve. Such mechanisms provide an indication of previous use of the filtering system and inhibit a fire fighter from using the system in an unauthorized manner, in less than an imminent life-threatening situation.