2) "The number of cases that [schools with air purifiers] reported, on average, per school versus where there's no... air quality measures... is three to four times fewer cases," he said.
3) Here are the specific CDC indoor air quality guidance for school safety.

Many countries have not adopted them. But portable HEPA filter are <$10/student one time investment if one is used per classroom. That’s not a lot. And cost in subsequent years are very low too. https://twitter.com/DrEricDing/status/1365678603212648461
4) School classrooms need at least 5 optimal air exchanges per hour via either natural ventilation or via HEPA filters or Safe upper room UV. Most classrooms have just ~2. Not enough. https://twitter.com/drericding/status/1364063590370713600
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