[blkhawk]

I have had a self-build CO2 monitor for several years now and I find the airplane example Surprising.

AFAIK the air in a Plane is cycled out too fast for that amount to develop. Maybe the Lower air pressure was the cause? Since it was portable it was probably the NIR type? If its not measuring all the time it might also be the heater type - I am really not sure how that type would deal with low pressure. Or was is the "eCO2" type - in that case well I doubt you get anything out of that thing in a plane except a high number.

One thing I noticed is that CO2 seems to "flow and pool" in certain places as it seemingly "rains" down and the room is "filled" from the bottom up. A Table for instance might develop a layer that is thick enough for my meter to hoover it up (it has a fan).

[Retric]

Local passenger density is going to play a role here venting air from low density first class areas isn’t going to do much. Similarly as you mention air flow is important as being in the middle of a large row could have vastly worse airflow than other areas.

So, I could easily see the aircraft venting mostly 700ppm air while some areas hit 3000 ppm internally.

[blkhawk]

3000 ppm is directly breathing at the meter - at least with mine it is.