For Dr. Waddington and other science lovers, this was the reading on my CO2 monitor while our full Airbus 321 was being de-iced in Calgary. It's by far the highest CO2 level I've measured - yikes.
CO2 monitoring is used as a proxy for quality of ventilation, i.e. people exhale and if the space they are in is well-ventilated, their exhaled breath doesn't stick around too long and neither do any bacteria or viruses they might breathe out. In the great outdoors, the CO2 ppm is in the low 400s. A measurement below 800 is considered to indicate decent ventilation in a room where people have been hanging out. You could still get sick from someone sitting beside you but a lower CO2 level reflects a lower risk.
When we've flown, the CO2 levels have been 1500-1800 in the air and up to the low 2000s on the ground when the ventilation system isn't running. Airplanes do have HEPA filters so the exhaled air from passengers far away from you will mostly be cleaned before you breathe it in, even though their CO2 may still be there. The HEPA filters don't help with people sitting near you, whose exhaled air reaches you directly. It looks like pilots turn off the plane's air intake completely during de-icing, which lasted about 20 minutes for us.
So the moral of the story is, if you're planning to wear a mask for some or all of your journey, know that the risk is higher when the plane is on the ground and *super* high when it's being de-iced.