1/ Aware that one school district hired a consulting firm to do CO2 measurements in classrooms when no students (at all during the day - U*G*G*H*H) were present and concluded the rooms are well ventilated. This makes no sense, folks. This makes no sense. Wasted money.
2/ Aware that in another case a school district did CO2 "spot checks" for a few minutes in classrooms. This often happens if a firm wants to measure CO2 in all classrooms and is limited in instruments and time.
3/ Time-averaged values during entire occupied periods can be improtant and "spot checks" can be woefully deceiving depending on when collected.
4/ My team has always done measurements for the entire school day, allowing for occupied time averages. In our recent study of high schools we did weeklong measurements to assess inter-day variabilities, etc.
5/ Carbon dioxide measurements can provide some useful information if done right, understanding that they only give us rough information about relative extent of ventilation (but not always).
6/ Carbon dioxide measurements tell us nothing about virus-laden particle control devices (devices that - contrary to another conversation I had recently - do NOT remove CO2 from indoor air).
7/ On the latter point. I could take two identical rooms with the same infected individual in each. In one room I measure 900 ppm CO2 and in the other 800 ppm CO2. The first room might be much safer if it has proven particle control technologies in place & the otehr does not.
8/ If I measure 800 ppm of CO2 in a room with 25 people in it and 800 ppm in a room with 5 people in it (assuming similar supply CO2), then the second room is not nearly as well ventilated as the first room.
9/ In this case, the probability of infectors in the first room (n = 25) is higher due to more people, which leads to higher probability of an infector present, even though CO2 concen are identical. It's also a situation that lends itself to more people becoming infected.
10/ Carbon dioxide is one of the simplest things we can measure and it does provide a rough sense of the quality and extent of ventilation. But it is by no means a perfect measure. Doing so incorrectly or interpreting data incorrectly is easy if not well versed in basics.
Typo: "important"
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