Okay, we seem to have go off on the wrong foot here. I am not trying to sell carbon dioxide meters, not do I think they are a reliable indication of covid risk, but they seem to be a neat way of measuring the worst case risk using a cheap, repurposed instrument.
Suppose you are back in my restaurant for some reason: maybe you are a health inspector, and you have to for your job. There are some fans in the wall of the restaurant blowing in air coming in from ducting, but you cannot see where the duct goes. Suppose your carbon dioxide meter is reading significantly higher than it did outside…
The most likely possibility is that the fans are not drawing in air from outside. Maybe the air conditioning outlet and the inlet are too close to each other. Maybe the incoming air is not missing with the restaurant air. I used to work in clean rooms ages ago, and it isn’t always obvious what the air does.
It could be that the air is being filtered and recirculated. This may mean the air is having any covid aerosols removed, and the carbon dioxide level does not correspond to the covid risk. This is the sort of thing they do in passenger aircraft because it is cheaper to filter and recirculate the air than to pressurise and warm the air from outside. This is unlikely to be economic for any public space. Again, it may not work depending on how the air mixes: there are worries that people spread infections on aircraft.
Public spaces might have a set limit to the carbon dioxide concentration over and above the ambient level outside. This may be a cheap and practical measure that may correlate with how healthy the air is. If you feel ill, you take your temperature. Your body temperature is not a measure of how many infectious bacteria you have in you at that moment, or how infectious you are. You may have just come out of a hot bath or sat in from of a roaring fire. But it is a handy measurement if we keep our common sense about us.
I would add the usual cop-out: I would like to see more research on this topic.
