For the AirGradient device I think the way to calibrate CO2 is to expose it to fresh air at some point, which has a known CO2 level. It’s supposed to auto-calibrate. But if you don’t do that regularly then the readings will drift (without anyway of telling they’re drifting). You also can’t tell when the sensor has calibrated.
IIUC VOC is way more complicated and hard to both calibrate and interpret. I’m not sure I have any faith in that value.
I do think a lot of people are going to be mislead by these monitors, the sensors and devices come with a bunch of caveats that aren’t clearly communicated.
For me what people should most care about is particulates, and at least as far as I know those sensors don’t come with the calibration issues of CO2 and VOC. That’s the sensor my AirGradient is set to alert on.
I was worried our sensors had that autocalibration mode. Our house has new concrete floors. They alternate between absorbing + outgassing CO2 (depending on room temperature), so sometimes our living room reads below 400 ppm.
From what I can tell, the sensors haven't drifted, so they're not using the "below 420 ppm => recalibrate" heuristic.
Reasonable CO2 NDIR sensors use two chambers: one sampling ambient air for the measurement and a sealed chamber with a reference gas for autocal.
edit: It seems AirGradient uses one of the cheap sensors.