Environmental monitoring devices for detecting and warning users of unhealthy levels of a given substance are disclosed having more than one sensor for each substance to be detected. A processing unit, wirelessly coupled to the sensors in the devices can be configured to receive each of the output signals from the sensors, determine a detection signal for the substance based on the output signals, determine a gain of a majority of the sensors, and generate a calibration action responsive to the output signals deviating by a threshold amount, wherein the calibration action comprises adjusting a gain of a deviating sensor to correspond with the gain of the majority of sensors.