I just uploaded a graph of my CO2 sensor in the hallway between bedrooms and the kitchen/living room. It moves around a bit, and it is heavily influenced by people in the house and on the same floor. This got me to start running my HVAC fan at night as we sleep with doors closed with humidifiers on. My house was built in 1980 and is by no means "sealed up tight". We have no mechanical ventilation other than a single bathroom fan that rarely runs. I think the CO2 data is better for identifying trends and getting an idea of how much ventilation can help, but it is by no means cause for alarm. Collect more data...