[Update 2021-08-02 NPR audio piece https://one.npr.org/?sharedMediaId=1030886479:1030886480 ]
[Update 2021-08-17, AM: WGBH piece on the Corsi / Rosenthal box, here: https://www.wgbh.org/news/local-news/2021/08/17/diy-air-filters-for-classrooms-experts-are-enthusiastic-and-a-citizen-scientist-makes-it-easy]
[Update 2021-08-17, PM: there is a fantastic thread on Twitter that collects a lot of useful information on the Corsi / Rosenthal box, here: https://twitter.com/LazarusLong13/status/1425517352624410627. Will incorporate any new details as we have time to digest them.]
[Update 2021-09-22: Larger DIY air purifier poster translated into spanish: https://twitter.com/Embarnesey/status/1435126245474275332]
[Update 2022-02-23: Study of effectiveness of airbox design after long-term use, by Rosenthal: https://twitter.com/SmartAirFilters/status/1496235386892390402?s=20&t=iNsvavgxi10Z_Yn28ZOexQ ]
Several researchers and practitioners, working together and in parallel, have come up with a design for an in-room air purifier which can remove a significant amount of COVID-19 virus from the air.
The design involves making a 'box' out of four 20" MERV-13 filters (the 'sides' of the box), a 20" box fan (the 'top' of the box), and a cardboard (the 'bottom' of the box'). Air flows in through the filter sides, removing particulates of the sizes that can transport COVID-19 particles, and then flows out through the fan at the top.
These devices can be built from parts that cost less than $200 USD, without any special tools.
MERV-13 filters are capable of removing about 85% of viral-laden particulates from air; it's therefore a good assumption that about 85% of the air flow out of the fan at the top of the device is free of COVID-19 virus. Informal tests and analyses using air flow meters and particulate matter sensors indicate that these devices can provide about 600 cubic feet per minute of virus-filtered air to the room.
For medium-sized rooms, the performance of this air purifier design means that installing between one and three of these 'air boxes' can boost the local "air changes per hour" (ACH) to the 'minimum recommended' level of 6 ACH, suggested by the Harvard School of Public Health. (Note: this recommendation was made before the emergence of recent variants; yet greater ventilation rates may now be preferred).
Notes on type / number of filters:
Reference on ideal placements: "Impact of placement of portable air cleaning devices in multizone residential environments" -- for impact of various placements of air purifiers in room -- https://www.researchgate.net/publication/255663019_Impact_of_placement_of_portable_air_cleaning_devices_in_multizone_residential_environments
20 inch MERV-13 filters are commonly used in HVAC / ventilation systems, and are widely available in retail outlets, e.g.:
20 inch box fans are also widely available, e.g.:
Because COVID-19 is an 'airborne' pathogen,improving indoor ventilation is an important way to attempt to mitigate the risk of indoor transmission of the virus.
One way of assessing indoor ventilation for a space is in terms of 'air changes per hour' (ACH) -- the number of times that 'all the air in the room' has been replaced (ideally, by outdoor air).
|Potential impact of Corsi Air Boxes. We estimated that the DIY air filter we assembled has a clean air delivery rate of approximately 600 CFM. Using Equation (1) below, and assuming a room of size 20'x20'x10', such a box will add the equivalent of +3 ACH to the room; two such boxes will add +6 ACH. Some typical ventilation values (in Air Changes per Hour) are depicted for comparison, including the Harvard School of Public Health recommendation of 6+ ACH to help mitigate the transmission of COVID-19 in classrooms.|
We used an anemometer to estimate that one of the 'DIY Air filters' we built is capable of delivering the equivalent of 600 CFM (cubic feet per minute) of 'cleaned' air to a room -- this is also referred to as the "Clean Air Delivery Rate" (CADR). This estimate is based on the cubic feet per minute that flow through the fan on the device, multiplied by 'removal rate' percentage for the filters.
NOTE: the exact CFM / CADR for a DIY device will depend on the actual air flow through the device, which can vary with the construction / type and number of filters / model of fan.
When the air purifier 'cleans' this volume of air and recirculates it back into a room, we can think of this as effectively increasing the ACH in the room.
To estimate the equivalent ACH provided by a DIY air purifier, we use the following equation:
(1) Equivalent ACH = CADR * 60 / Volume_of_room
For example, if a DIY air purifier has a CADR of about 600 CFM, then for a typical room volume -- e.g. 30 foot x 30 foot x 10 foot -- this results in an equivalent ACH of about +3.
The equivalent ACH provided by air purification is 'additive' -- e.g. in the above scenario, two if you put two purifiers in the room that each have an equivalent ACH of 3, it results in an equivalent ACH of 6.
The optimal place is in the center of the room. However, that is not always possible. The units will work effectively as long as they are at least 3 feet away from any wall.
30 Corsi / Rosenthal boxes being built for school: https://twitter.com/v8juice3/status/1434220361000828933?s=20
Spanish version (courtesy Emily Barnes Franklin! @Embarnesey)
note: reason that you pull through the filter is that it tends to seal, rather than blow apart
Discussion in This Old House video https://www.youtube.com/watch?v=aw7fUMhNov8
Thicker MERV 13 means less resistance (because more surface area)
Pleated merv 13 means greater surface area, less resistance, lower average velocity, reduces resistance on fan.
Want air flow arrow pointing in ... cardboard on bottom ...
Fan points up ...
Tweet referencing the publication:
Particulate sensors (for use with filter, for testing)
Adafruit Plantower: https://learn.adafruit.com/pmsa003i
Tex air filters article that uses Dylos sensor: https://www.texairfilters.com/its-all-about-the-air-flow-through-the-filter/
Reddit discussion of low-cost particulate sensors: https://www.reddit.com/r/homeautomation/comments/jo14gs/looking_for_advice_on_an_air_quality_monitor/
"Evaluation of Low-Cost Sensors for Ambient PM2.5 Monitoring": https://www.hindawi.com/journals/js/2018/5096540/
Plantower sensor on Amazon: https://www.amazon.com/DEVMO-Digital-Particle-Concentration-PMS5003/dp/B07S5YX84W/ref=asc_df_B07S5YX84W/?tag=hyprod-20&linkCode=df0&hvadid=459726175633&hvpos=&hvnetw=g&hvrand=2539812408648535115&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9001876&hvtargid=pla-973940804295&psc=1
"COVID-19 Outbreak and Hospital Air Quality: A Systematic Review of Evidence on Air Filtration and Recirculation": https://pubs.acs.org/doi/pdf/10.1021/acs.est.0c03247
Used 90 in of length of chicken wire
MTB Galvanized Hardware Cloth 24 Inch x 25 Foot -1/2 Inch x 1/2 Inch 19GA
25 feet -- 3 enclosures -- for $35 -- adds $12 per.
Medify Air MA-40: CADR given as 224 cfm https://reviewsofairpurifiers.com/medify-ma-40-air-purifier-review/
long covid in kids: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7927578/
Replaced 'impact' graphic with a clearer depiction of an example impact of using two air boxes.
Fixed typo on Home Depot price -- "6 for $30" --> "6 for $130"
Added caution about replacing filters at approx 6 months
Use large plastic bag, toss in dumpster, wash hands after
Added caution about not using multiple of these boxes, don't want too close to one another -- don't want outlet into inlet -- paper by texas at austin -- jeffrey siegal, u toronto & atilla novoselac https://www.researchgate.net/publication/255663019_Impact_of_placement_of_portable_air_cleaning_devices_in_multizone_residential_environments
Added caveat about the CFM / CADR for a given DIY setup being dependent on the quality of the construction and the type of fan; made sure to indicate that our 600 CFM CADR is an example.
Adding warning about bumping / jostling the filters to avoid re-suspension of particulates