Mobile infrared measuring system (air quality signal) A traffic light on the housing displays the carbon dioxide concentration and with that air quality: green LED: CO2 concentration below 1,500 ppm; air quality o.k. yellow LED: CO2 concentration between 1,500 and 2,500 ppm; air of minor quality red LED: CO2 concentration above 2.500 ppm; bad air quality Measuring range: 0-3,000 ppm +/- 2% MBE. Measurable gas: Carbon dioxide (CO2).