I thought a current suspect of diminished classroom performance is CO2, and a typical filter won't help there. I'd like to see a study with CO2 sequestration in classrooms and/or other people-dense indoor settings.
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Or perhaps design buildings with windows that people can open?
At the risk of sounding facetious, I think most people underestimate how much CO2 is produced by 20 odd people in a room. Equally we underestimate how much air is renewed by just opening a window.