Air quality measuring device for CO2 (carbon dioxide), temperature, and humidity with a traffic light function and 5-hour trend display. It can be used in both battery and mains operation. The indoor air quality measuring device is a CO2 meter designed for desktop use. It measures the CO2 concentration in the air and displays the measurement on a large LCD screen. The CO2 level in the air is an important indicator for checking ventilation behavior in offices, classrooms, lecture halls, etc. The device helps ensure that employees regularly ventilate the office. It monitors air quality based on CO2 levels. If the CO2 concentration is below 1000 ppm, the air quality is optimal. Good indoor air quality promotes concentration and keeps aerosol concentration in the air low, as aerosols are continuously mixed with fresh air. The measuring device displays air quality as a numerical value and as a traffic light indicator. By pressing a button, the CO2 levels from the last five hours can be displayed on the screen. In addition to the CO2 measurement function, the device is equipped with sensors for temperature and humidity. The sensors for monitoring air quality have different requirements than those used in industrial or scientific laboratory applications. Therefore, sensor characteristics such as high long-term stability, low maintenance, and low energy consumption are prioritized. This is fulfilled by the indoor air quality measuring device with its NDIR sensor. The non-dispersive infrared (NDIR) absorption measurement allows for accurate and long-term stable measurements with an absolute measurement range of up to 5000 ppm.