Thread: How do we make the air in classrooms safer?
All you need to know about HEPA filters,CO2 monitors and ventilation. Contents: basic ideas; how to with links to resources; caveats; links to studies / evidence (that I'll add to cos takes more time) 1/
All you need to know about HEPA filters,CO2 monitors and ventilation. Contents: basic ideas; how to with links to resources; caveats; links to studies / evidence (that I'll add to cos takes more time) 1/
Airborne respiratory diseases (TB, Covid etc) are all transmitted the same way: pathogen is in tiny particles that we breathe out / give off when speaking. They're small enough to stay in the air like smoke; you get more if you’re closer but eventually the whole room is smoky 2/
People breathe them in, and get infected. More particles are given out through talking, shouting or singing. Because the virus is in these same particles, it doesn’t matter what variant of covid it is. nature.com/articles/s4159… 3/ 
So if we can stop people breathing in respiratory particles from someone who’s infected, we can stop airborne diseases spreading. There are 4 ways to do this: masks; fresh air (ventilation); filtering (HEPA); and sterilising the air (UV) 4/
Masks are good. Blue (“surgical”) > cloth > nothing. Mainly stop wearer unwittingly infecting others, so good if everyone wears them. (Stops particles in jet from mouth/nose). Well fitting FFP2 / FFP3 even better, protect wearer too; can be reworn.
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https://twitter.com/trishgreenhalgh/status/1477606074001801218?s=20
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However, masks, while important, are not enough for long periods of time in the same room (like in schools) and can’t be worn when eating and drinking. Plus there are no masks in UK primary (elementary) schools and they’re not a (nice) long term solution. So... 6/
2nd option, flush the old exhaled particle-filled air out with fresh air ("ventilation") - mechanically through building ducts, or naturally by opening a window. 7/
Window ventilation depends on weather & can be hard to judge. To know if need more fresh air, we use CO2 to estimate amount of respiratory particles built up in the air (hard to measure) - we breathe both out, and they're both removed by ventilation.
https://twitter.com/UnchartedWorlds/status/1478433051038588929?s=208/
Outdoor fresh air is 400 ppm CO2 (global warming!). Above 800 ppm, >1% of air in room has been breathed out, so you want to get fresh air in by opening windows. Quick blast to refresh if too cold to leave open. More good tips:
https://twitter.com/CathNoakes/status/1474048759071924225covidstraighttalk.org/indoorwindtunn… 9/
If you have mechanical ventilation in the building (many newer build unis & schools will) check whoever manages your building: have set it to maximise fresh air (not “recycled"). Can check all OK with CO2 monitor too. 10/
If you can't get enough fresh air (can’t open the windows enough / too cold) so CO2 usually > 1200ppm by end of lesson: you can also *filter out* the infectious aerosols from the air. Do this with HEPA ("high efficiency particulate air [filter]"). 11/
Many home "air purifiers" are HEPA. They're plug in portable boxes the size of a bin, that suck air in the sides & blow filtered air out the top. You need 2 or 3 to a classroom, dotted around (see later). [NB they don't remove CO2 so CO2 monitors won't show the HEPA benefit]. 12/
HEPA is well established technology, that removes all particles in the 0.1-10 micron size range, which includes the exhaled particles carrying airborne diseases (also pollen, smoke, particulate pollution etc). "H13 HEPA" removes 99.95% of the particles 14/ cibsejournal.com/technical/unde…
This useful site finds commercial HEPA air cleaners & calculates how many needed for your classroom (with further explanations) cleanairstars.com/hepafilters/ 14/
Typical unit £50-£200 varying in how much filtered air you get. (My one above was reduced to £35)
Typical unit £50-£200 varying in how much filtered air you get. (My one above was reduced to £35)
Note: official UK govt recommended models are Dyson and Camfil, but these are ££ for the clean air they deliver. Only things that matter are clean air rate (CADR), cost, & not too noisy (resource above includes that). So better to get several less expensive units in the room. 15/
Final option: ultraviolet light kills virus in airborne particles: in sunlight, or units fitted indoors (widely used for measles and tuberculosis) cibsejournal.com/technical/lead… Needs specialist fitting and high ceilings (for safety) but is good for large rooms (canteens, gyms) 16/
With high covid rates, you want clean air *and* masks. When face to face unmasked, ventilation / HEPA / UV won't stop infection: if you're opposite a smoker, an open window won't help too much if they're breathing smoke into your face. Masks (esp.FFP2) provide that protection 17/
Fresh, clean or sterilised air is extremely good at reducing transmission and works on *any* variant, and other airborne infectious diseases too. It's a good long-term investment, and it’s also *very do-able*. Let’s fund it & let’s do it. /END
Caveat: clean air is an extra layer. Face to face unmasked = still high risk (esp indoors). Distancing also helps (as with smoker analogy). And without distancing or masks, clean air may still make difference, eg infecting only 1 or 2 classmates, not 10. But not a magic bullet.
Advice on positioning your HEPA units
https://twitter.com/adsquires/status/1478354191584268288?s=20
Evidence for effectiveness in schools and other settings (to be added to)
https://twitter.com/adsquires/status/1478342930641440771?s=20
Political / practical limitations (which is why I didn't include) but I agree with this:
https://twitter.com/mjb302/status/1478812431828799490?s=20
How to calculate how much clean air (per person) you're already getting from ventilation using your CO2 sensor (and therefore how much you need to make up using HEPA)
https://twitter.com/adsquires/status/1470677561629974537?t=tsRAaJOyL0irf7zqteqzFg&s=19
You *can* have open windows AND HEPA. In fact you *should* do both, if your windows aren't delivering anough fresh air. This from @mjb302, who's doing the @AAirDStudy looking at HEPA in Addenbrookes Hospital, Cambridge.
https://twitter.com/mjb302/status/1478840081658130439?t=BDi0tA52yc1hnbEnlxgn0A&s=19
You can find HEPA units on Amazon (/other shops). Search for "HEPA air purifier" & look for "True HEPA" or "H13 HEPA". Don't get ones with ozone / ioniser. Ignore quoted room area, look for their Clean Air Delivery Rate (CADR) and how noisy (dBA). Then use cleanairstars.com/hepafilters/
Thinking of upgrade to FFP2? Do it! It's these masks (slightly thicker, usually white, pic below). £1 each (in pk 10), each lasts weeks. My experience wearing to lecture: as comfortable as cotton, and apparently easier to hear too:
economist.com/the-economist-…
https://twitter.com/BethBromley10/status/1478851131656835078?s=20
economist.com/the-economist-…
Window ventilation can be augmented by putting fans in windows blowing fresh air in / old air out. A team in Germany has gone further, developing a low-cost duct system you can make for <£200 from common components from DIY store
https://twitter.com/Paul_Briley/status/1478859676309676032?t=aJ3nYVsBewil3HQWGhm-6Q&s=19
DIY filtration devices (filters + fan + box) are also very effective: "Corsi-Rosenthal boxes" (after @CorsIAQ & @JimRosenthal4). Links to resources, "how to"'s etc airbon.co.uk/post/diy-air-p…. In UK @PlasticFull has done a lot on this.
For people who may not read the instructions on your HEPA box carefully (you know who you are!): the filter often comes inside a plastic bag in the device, and you have to take it out of the plastic bag and re-insert it, or it won't do any useful filtering...
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