With COVID-19, Air Is Both the Problem and the Solution. ""Schools may balk at homemade equipment, but these boxes have been shown to work surprisingly well, “blowing the competition out of the water,” Prather told me.""

...So you’ve got a room, say a classroom, and want to know what to do about the potential SARS-CoV-2 aerosols.

Is the ventilation adequate? To assess this, a nondispersive infrared (NDIR) CO2 sensor can be used. “You want the air in your house [or indoors] to be as fresh as the outdoor air,” Professor Kimberly Prather told me. She is an atmospheric chemist at the Scripps Institution of Oceanography and UC San Diego who studies aerosols. We humans breathe air in, hold onto the oxygen, and breathe out CO2. An NDIR CO2 sensor uses a specific beam of infrared light that CO2 molecules love to absorb. The more CO2 in the detector, the less infrared hits the sensor. The device translates this into the amount of CO2 present in the air in parts per million (ppm). “When people release CO2 into a shared room, that number creeps up. All that means is that you are breathing other people’s shared breaths. We don’t want that.” The CO2 detector cannot tell you if the virus is present. But if the virus is present, large quantities of CO2 means ventilation is bad and those aerosolized viruses are not leaving the room anytime soon. “If I see 1,000-2,000 ppm, I’m out of there. Schools that say ‘our rooms are safe,’ they better prove it, with kids present.”..

...Homemade air cleaners in the shape of a box made out of filters with a fan on top (known as Corsi-Rosenthal boxes, after two respected scientists in the field who came up with them) actually work extremely well, even with furnace filters instead of HEPA filters, and are much cheaper.   Look for filters that are thick and heavily pleated, as they will provide more surface area for filtration. “There are videos of seven-year-olds building them and presenting them at their school,” said Prather. “The kids and parents feel like they’re doing something to help.” Schools may balk at homemade equipment, but these boxes have been shown to work surprisingly well, “blowing the competition out of the water,” Prather told me.

https://www.mcgill.ca/oss/article/covid-19/covid-19-air-both-problem-and-solution