Abstract: A dosimeter for measuring accumulated dosage of noxious gases, such as carbon monoxide (CO), oxides of nitrogen, (NO.sub.x), etc., incorporates a gas-sensing electrochemical cell which utilizes an electrically biased, hydrated, solid polymer electrolyte (SPE) sensing cell. CO and other noxious gases are oxidized at the sensing electrode and the cell current is a measure of the gas concentration. The cell output current is applied to an electrically resettable current integrating device, which is basically an integrating coulometer in which metal is plated onto a working electrode in response to the cell output current. A fixed, reverse calibration current is driven through the coulometer during dosage readout to deplate the metal from the coulometer working electrode. An accurate timing device measures the time required to deplate the working electrode of the coulometer. This product of the fixed calibrating current and the time required to reset the coulometer, i.e.