A bit more about this...
Carbon dioxide in air is measured in parts per million, "ppm".
The reason it's a useful thing to measure is that both carbon dioxide and covid virus are things that humans breathe out. So if CO2 is building up in a room, then any virus from an infectious person would build up too.
To give an idea of good/bad levels for covid risk:
Outdoor air, which you can think of as the "baseline freshest level", includes around 450ppm CO2, or higher near busy roads.
Indoors, below 600ppm is good, below 700ppm is okay.
700ppm to 800ppm of CO2 is the kind of level where air quality scientists would recommend improving the ventilation.
At 800ppm, if all of that CO2 had come from human breath (not e.g. from using a gas cooker), then about 1% of the air you're breathing in was breathed out by someone else.
2,000ppm would be considered notably poor air quality, and a high risk for transmission of airborne illnesses.