There are lots of commercial gadgets like that.

Most of the affordable CO2 meters are relative, not absolute. They set their 400 PPM level based on the lowest value they ever see. That's usually OK, but it's not good enough for places with permanent people occupancy, such as nursing homes. Absolute detectors with NIST calibration are available but around US$500.[1]

[1] https://www.forensicsdetectors.com/products/carbon-dioxide-d...