AIR DISINFECTION: Schools without air purifying disinfection devices have more than 3 times as many #COVID19 cases as the schools that do.
CDC recommends them for schools too —but only two kinds—portable HEPA filters (no ionizers!) and upper room UV.
https://montreal.ctvnews.ca/quebec-schools-without-air-purifiers-have-3-to-4-times-more-covid-19-cases-says-dad-running-citizen-count-1.5400739
CDC recommends them for schools too —but only two kinds—portable HEPA filters (no ionizers!) and upper room UV.

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
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