fyi, it seems this sensor has to use actual temperature and humidity to compensate the calculation of CO2 and TVOC. I did 48 burn in in a small room (2 square meter) with slightly opened window. I occasionally checked the reading (had lcd display there). It went crazy - sometimes CO2 ~1500ppm, then jumped to 2500, then to 5000ppm and so. Absolutely crazy values.
Then after the 48hrs burn in I put the sensor outside and it went the same crazy way, but this time it did not jump so much, but stabilized around 4000ppm CO2 which is also crazy for outside.
Then I checked actual temperature and humidity outside and set this via mySensor.setEnvironmentalData(humidityVariable, temperatureVariable); (Sparkfun ccs811 library). Interesting is the CO2 reading stood around 400-409 ppm what could be considered quite OK for outside. Now, after 11 minutes since power on, there is 465 ppm - quite much, but the temperature slightly changed and I do not have temp/humidity connected to the arduino, so I can't dynamically apply the compensation. I will try that when I have time.
I still do not know whether this sensor is usable for monitoring indoor air quality, but at least now, with the temp/humi compensation it does not throw so crazy values.