Been feeling a bit sluggish in the office recently. Thought about CO2 and so bought a cheapo (but reasonably well reviewed) module. Hooked it up to an ESP32 I had lying about and added it to #homeassistant. Calibrated it outside at 400ppm and then moved it into the office. 1300ppm. Ugh. OK, so I need to open a window every now and again.