Footnote 1 from Fred: "Regarding air filtration in commercial spaces, what I said was generally true for offices, commercial spaces, chemical labs, and medical spaces. Generally there's focus on air quality through outside air intake. However, there is a particular sector that focuses heavily on recirculated air filtration, and that's cleanrooms."
Footnote 2 from Fred: "The CADR to square foot ratio of 2/3 corresponds to 5 air changes per hour. That is to say, an air purifier in the recommended space for its CADR will filter all the air in the space five times per hour, or once every 12 minutes. The "bad" air purifier I mentioned has enough CADR to do 2 air changes per hour in its marketed space of 1100 square feet. It does say "cleans the room in 30 minutes" on its product web page but I wouldn't expect a lay person to know that that is below the AHAM's recommended air cleaning rate. If someone smoked a cigarette in a room and it took an air purifier 30 minutes to make the smell go away I would call that poor performance."