I don't think local outdoor CO2 levels vary by much. Sure, if you are near a significant source (a highway or a factory burning things), then you'll have some locally elevated levels. Even then, however, the locally elevated levels are unlikely to have a significant effect on health, since it is very easily diffused.
Another side of this issue is that CO2 is relatively difficult to measure reliably, because it is a very unreactive molecule. It is not susceptible to oxidation or most other reactions that can be used to determine its presence. Most reliable sensors use an IR light source and measure its attenuation by CO2.
Another side of this issue is that CO2 is relatively difficult to measure reliably, because it is a very unreactive molecule. It is not susceptible to oxidation or most other reactions that can be used to determine its presence. Most reliable sensors use an IR light source and measure its attenuation by CO2.