Best reply on the thread was: Now show us what 20-30 students in a single classroom does to air quality. I went to a brand new high school in 1992. There was a phenomenon of a few students passing out here and there. It was discovered after the first quarter that no one had opened the vents to the outdoors and the building was circulating the inside air around.
There are several comments about personal AQI sensors. Can anyone recommend an affordable one? Mostly for scientific curiosity.