A comparison of 3 air quality meters with no final conclusion.
None seem to give the same reading, one seems to be entirely stuck or broken at a high reading.

"W17 Indoor Air Quality Monitor Rechargeable CO CO2 HCHO TVOC AQI Detector Tester"
vs
"5 in1 CO2 Temperature Humidity Air Quality Monitor Carbon Dioxide HCHO Detector"
vs
"CEM DT-326" supplied by Rexell and often used in schools in the UK.

I've since also tested the above plus a "CEM DT-182D Air Quality CO2 Monitor" in a different office block; but all within 10cm of eachother.
Photo here: https://imgur.com/a/0WHQJfY
Once again none of the devices matched with results:
W17 (white box): 519ppm
5 in1 (black box): 424ppm
CEM DT-326: 904ppm
CEM DT-182D: 641ppm

I'm still at a loss on which is actually accurate.