Not sure what people are interested in CO2 sensors for, but I use them as a proxy for ventilation in indoor spaces, which is then a proxy for covid risk.
It won't tell you if the air is being filtered (need MERV13 or better), but a lot of places for efficiency and/or due to old HVAC equipment don't bring in fresh air, which increases the risk in a more confined and/or crowded space.
Also, some places like airplanes, which have HEPA filters and have 10-20 air changes per hour (ACH), don't run those systems at full power or bring in outside air at fast enough rate until airborne, so PPM can't build up to 2000 ppm vs the ~420 ambient outdoor CO2 level and approx 800 "good" indoor air rate.
I use an Aranet4 device, which uses 2 AA batteries that last like a year.
It won't tell you if the air is being filtered (need MERV13 or better), but a lot of places for efficiency and/or due to old HVAC equipment don't bring in fresh air, which increases the risk in a more confined and/or crowded space.
Also, some places like airplanes, which have HEPA filters and have 10-20 air changes per hour (ACH), don't run those systems at full power or bring in outside air at fast enough rate until airborne, so PPM can't build up to 2000 ppm vs the ~420 ambient outdoor CO2 level and approx 800 "good" indoor air rate.
I use an Aranet4 device, which uses 2 AA batteries that last like a year.