Our CO2 sensor assumes 400ppm to be the lowest background level as that's the outdoor CO2 baseline level. Outside levels of CO2 are generally very low at around 400 ppm (Source: https://www.scientificamerican.com/article/earth-s-co2-passes-the-400-ppm-threshold-maybe-permanently/). Humans inside building tends to be the major source of CO2. When a building is unoccupied for 4 to 8 hours, CO2 levels will tend to drop to outside background levels. If the outdoor CO2 baseline level increases in the future, the change will be reflected on our readings.