An exposure indicating apparatus for monitoring air flowing along a flow-through path extending from the external environment, through an air purifying respirator cartridge and into a face mask. A reversible sensor is releasably attached to the flow-through path so that it can be removed without interrupting the flow of air along the flow-through path. A processing device generates a concentration signal responsive to at least one property of the reversible sensor. The processing device provides and active indication, such as audio, visual, or tactile response to the concentration signal. A flow-through housing may form a portion of the flow-through path. The flow-through housing is preferably interposed between the air purifying cartridge and the face mask. A processor housing containing the processing device and indicator is attached to the flow-through housing or the air purifying respirator cartridge. The reversible sensor is located in the processor housing, the air purifying respirator cartridge or the flow-through housing. The sensor is coupled to the processing device by an optical, electrical, or general electromagnetic coupler covering the frequency range, for example, from DC to RF to microwave.