I have around 50 monitoring devices measuring CO2 in different buildings across England. Occasionally the hourly average readings fall below 400ppm. I know there will be drift/ calibration errors in some of these, but is there a lower limit on what is accepted as a reasonable CO2 level. Similarly, outdoor CO2 has a global average just above 400ppm, but as this is an average, is there an acceptable lower limit below 400ppm?
The World Health Organization (WHO) recommends that indoor CO2 levels should not exceed 1000 parts per million (ppm). The US Environmental Protection Agency (EPA) recommends that indoor levels should not exceed 800 ppm. Therefore, a lower limit of 400 ppm is generally accepted as an acceptable level of CO2 in indoor environments. For outdoor CO2 levels, there is no universally accepted lower limit, however, levels below 400 ppm are considered to be unusual and are typically associated with high-altitude environments.
Ah, sorry. I should clarify my question. I'm not looking for recommended levels with regards to health etc. I'm wondering whether the devices reading something in the range of 350 ppm in an unoccupied room is likely to be a realistic reading or whether the device needs recalibrating. Global average CO2 outdoors is around 410 ppm, but as this is an average, how low can CO2 levels be outdoors?
There could be a powerful sink for CO2 nearby, such as young and dense forest. Within a forest's canopy at low wind speed, you can easily observe CO2 below 300 ppm. Much less probable would be to have such depression inside a living room. Nonetheless, sometimes it happens, e.g., if you have a project involving vast amounts of soda lime (or other CO2 absorbing reactants) in a non-ventilated lab space...
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