3-color Indicator & Alarm Buzzer: Set the low and high alarm value (400-4000PPM limited range); white backlight means normal, green backlight flashes and the buzzer emits intermittent sound when the CO2 value is between low and high alarm values, red backlight flashes and the buzzer emits continuous sound when the CO2 value exceeds high alarm value.

Air Monitor CO2 Carbon Dioxide Detector  Air Quality Temperature Humidity Monitor Fast Measurement Meter