The unit for the concentration of carbon dioxide gas is ppm (part per million). For outdoor air without pollution, it is around 350-450 ppm. For indoor air, its quality is considered to be good if the CO₂ concentration is below 1000 ppm. The acceptable upper limit is 3,500 ppm and 5,000 ppm for the European and USA standards, respectively. If it is above 35,000 ppm (i.e. 3.5% by volume) in a certain place, one should leave it within 15 minutes.
For calibration, put the probe in an environment with 400 ppm of CO₂ and earth the brown wire (left most) of the sensor (i.e. put it at 0V) for at least 7s.