Why doesn't my CO2 reading go below 400ppm? (UK)

The minimum level of CO2 that exists in the Earth’s atmosphere is slightly above 400ppm. This is the observed outside value of CO2. (Source: https://www.scientificamerican.com/article/earth-s-co2-passes-the-400-ppm-threshold-maybe-permanently/).

Our CO2 sensor assumes 400ppm to be the lowest background level as that's the outdoor CO2 baseline level. Humans inside building tends to be the major source of CO2. When a building is unoccupied for 4 to 8 hours, CO2 levels tends to drop to outside background levels.

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