Sure, but it’s not measuring NO2, just things that sometimes correlate with the presence of NO2 in some environment. So, maybe it’s sometimes rightish in a gene

Sure, but it’s not measuring NO2, just things that sometimes correlate with the presence of NO2 in some environment. So, maybe it’s sometimes rightish in a general ball park sense, but often it won’t even be that as these correlations are necessarily based on lots of assumptions that often don’t hold. And this is all ignoring interdevice variability which is often very high on this consumer equipment if the tests are to be believed.