Edge Collective
DIY Air Purification Designs
Notes on the Corsi / Rosenthal / Comparetto DIY Air Purifier design -- build instructions, performance evaluation, references[Update 2021-08-02 NPR web piece https://www.npr.org/sections/back-to-school-live-updates/2021/08/26/1031018250/does-your-kids-classroom-need-an-air-purifier-heres-how-you-can-make-one-yoursel]
[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 2021-09-20: https://www.courier-journal.com/picture-gallery/life/wellness/health/2021/09/16/trinity-students-donate-air-purifiers-ascension-help-fight-covid/8338612002/ ]
[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 ]
The 'Corsi / Rosenthal / Comparetto' DIY Air Purifier Design
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.
Image files:
- SVG: corsi_header.svg
- PDF: corsi_header.pdf
- PNG: corsi_header.png
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:
- MERV-rated filters are a relatively well-understood, extensively-tested, and widely deployed technology; good estimates exist for what percentage of particles of various sizes are typically removed by such filters. Once air flow through the filters is determined, determing an estimated 'clean air removal rate' is straightforward. This is a big advantage of the Corsi 'airbox' design, when compared with some of the proprietary, more obscure designs available on the commercial market.
- In addition to MERV-13 filters, MERV-12 and MERV-11 are also effective filters, capable of removing a smaller but still very significant percentage of virus-laden particles from the air. Use them if MERV-13 filter are unavailable, or too expensive.
- One-, two-, or three-filter box, as well as five-filter box designs are also possible and can be effective. The more filters in the box design, the greater the air flow rate through the box, and the lower the strain on the fan motor.
Caveats
- Be careful not to touch, shake or otherwise disturb the filters after the box has been used. The filters can effectively trap virus-laden particles, retaining them on their surface; by shaking them you can dislodge the particles, sending contaminated particles into the local air. Use best practices (e.g., N95 masks and great care) when moving or disposing of the filters -- place in large, sealed plastic bag before putting in dumpster;
- Replace filters approximately every 6 months or so;
- Positioning two 'airboxes' too close to one another in the room might result in the filtered-air output of one of them being taken in immediately into the other one -- a 'short circuit' -- minimizing the effectiveness of the additional box;
- Some locations in the room might be more effective at filtering air than others -- e.g., if you have an open window through which air is being ventilated outdoors, it's best not to position the box right in front of the open window -- because you're filtering air that won't be used inside the room;
- Best practice is to place e.g. two air boxes a few feet away from the walls, and avoiding corners -- so that they are filtering maximal amounts of air
Misc Notes
Fantastic FAQ on design
https://cleanaircrew.org/boxfanfilterfaq/
Ideal placement of purifiers
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
Electrostatic / MERV-13 issues over time
https://twitter.com/JohnSemmelhack/status/1313858509667213312?s=20
Sourcing build materials
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.:
How much can they help?
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).
For comparison:
- Hospital patient rooms are often kept at 6 ACH;
- Hospital airborne isolation rooms are kept at 12 ACH or greater;
- Hospital operating rooms are kept at 20 or more ACH;
- ASHRAE recommendations for school classrooms are usually around 3 ACH (actual classrooms are usually less);
- Ventilation in a typical room in a home is less than 1 ACH.
- The Harvard School of Public Health has recommended an ACH of 6 or greater for COVID-19 risk mitigation (note: this recommendation was made before recent, more transmissable variants, like Delta).
 |
|---|
| 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. |
Files:
- SVG: corsi_impact.svg
- PDF: corsi_impact.pdf
- PNG: corsi_impact.png
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.
References / Materials
Placement of box in room
https://twitter.com/JimRosenthal4/status/1434248340930191363?s=20
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.
Folks building boxes
30 Corsi / Rosenthal boxes being built for school: https://twitter.com/v8juice3/status/1434220361000828933?s=20
Guidance on communication with school administration re: pedagogical benefits, safety concern
- Example correspondence regarding pedagogical benefits: https://twitter.com/JimRosenthal4/status/1434114488618323971?s=20
- filters mitigate build-up of dust on motor: https://twitter.com/asbestos/status/1433837001749327877?s=20
- 'persuasian' document: https://twitter.com/LazarusLong13/status/1433881681056829440?s=20
- experience deploying similar setup for years w/out issues: https://twitter.com/LazarusLong13/status/1433860324428447746?s=20
- SF DPH document: https://twitter.com/VanGennepD/status/1433845587636473856?s=20
- Fans are thermally protected, fused: https://twitter.com/DavidElfstrom/status/1433828491632513033?s=20
- Pic of safety plug on fan: https://twitter.com/DavidElfstrom/status/1433837957471997957?s=20
- School boards are cautious b/c of liability: https://twitter.com/JimRosenthal4/status/1434245056827961347?s=20
- Just use when teachers are in classroom and can monitor fan: https://twitter.com/James___Morris/status/1433839583817449475?s=20
- Use a timer: https://twitter.com/crafty_caro/status/1435405716538499081?s=20
Particle sizes
from https://www.cdc.gov/niosh/docs/2003-136/pdfs/2003-136.pdf
Testing
CFM:
https://twitter.com/JohnSemmelhack/status/1313855226844151808
UC Davis evaluation of design
https://twitter.com/JimRosenthal4/status/1432784870179282947?s=20
UL Testing (regards smoke)
Illustrations of build process, with variations
https://encycla.com/Corsi-Rosenthal_Cube
Building a Corsi air box for the first time: timelapse
Video: fully-assembled airbox
Small Poster
Files:
- SVG: corsi_small_poster.svg
- PDF: corsi_small_poster.pdf
- PNG: corsi_small_poster.png
Large Poster
Files:
- SVG: corsi_large_poster.svg
- PDF: corsi_large_poster.pdf
- PNG: corsi_large_poster.png
Spanish version (courtesy Emily Barnes Franklin! @Embarnesey)
This Old House build guide / explanation
Detailed comparison of dust-removal performance
Rationale for 'fan shroud' / improving efficiency of Corsi Box
- Also shows measurements leading to estimate of CFM
Video using anemometer to assess CFM, and demonstrating 'reverse flow at corners' effect:
'Kestrel 1000' anemometer we used to estimate CFM
~ $80 on Kestrel's site: https://kestrelinstruments.com/kestrel-1000-pocket-wind-meter?gclid=CjwKCAjwmeiIBhA6EiwA-uaeFZdalCbfBiA20QclZPCqMZcIu5a5AGOyDmK0euc6eNDWgOzleHluyBoCk6kQAvD_BwE
--
poppendieck.com FAQ on COVID-19 in Classrooms
http://poppendieck.com/IAQ/COVID19%20FAQ.html
Effectiveness of Corsi Air Box -- particulate reduction
- Uses Merv 11 filters
https://www.texairfilters.com/its-all-about-the-air-flow-through-the-filter/
2021-09-07 19:31:18
https://www.built-envi.com/wp-content/uploads/IIT-CADR-Testing-C-R-Box-September-2021.pdf
via:
http://built-envi.com/portfolio/air-cleaner-testing/
via: https://twitter.com/built_envi/status/1435328519471501320?s=20
Corsi video presentation on DIY air filtration
note: reason that you pull through the filter is that it tends to seal, rather than blow apart
Pressure effects
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 ...
Michigan video comparing HEPA to DIY Corsi
https://www.youtube.com/watch?v=kH5APw_SLUU
Energy vanguard instructions
https://www.energyvanguard.com/blog/how-make-high-merv-diy-portable-air-cleaner/
Philly Inquirer Article
Neil Comparetto video
https://www.youtube.com/watch?v=Y7eL2OAnqc8
Tex-air overview
https://www.texairfilters.com/iaq-research-practice-in-action-the-corsi-rosenthal-box-air-cleaner/
Treehugger article
https://www.treehugger.com/build-own-covid-19-air-filter-5081272
Percent effectiveness at capturing particles
https://www.ashrae.org/technical-resources/filtration-and-disinfection-faq
--> 85%
Calculating ACH from CFM
Corsi presentation on air box
https://www.youtube.com/watch?v=MmWQUHswfOs
Recommended ACH for measles
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4400300/
Corsi et al preprint on room-level ventilation assessment
Tweet referencing the publication:
https://twitter.com/vfmcneill/status/1425467992280686593?s=20
Particulate sensing
2021-08-11 09:55:48
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 comparisons:
https://aqicn.org/sensor/pms5003-7003/
https://www.hindawi.com/journals/js/2018/5096540/
https://dalegi.com/2018/06/13/pmsa003-low-cost-air-pollution-sensor-accuracy-an-attempt-at-calibration-in-sarajevo/
https://scholar.google.com/scholar?q=plantower+sensor+models&hl=en&as_sdt=0&as_vis=1&oi=scholart
https://www.sciencedirect.com/science/article/abs/pii/S0269749118316129
https://pubs.acs.org/doi/abs/10.1021/acs.est.8b05174
https://www.tandfonline.com/doi/full/10.1080/02786826.2019.1696015
https://www.sciencedirect.com/science/article/pii/S0160412019321920
https://www.nature.com/articles/s41598-019-43716-3
https://link.springer.com/article/10.1007/s42452-019-0630-1
https://amt.copernicus.org/articles/11/4823/2018/
Thoughts on using Purple Air to assess effectiveness of indoor air sensors
https://twitter.com/Jess_Tryner/status/1425943278759735299?s=20
Critique
"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
Chicken wire shroud
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
https://www.amazon.com/dp/B01MTQVNIC
25 feet -- 3 enclosures -- for $35 -- adds $12 per.
Commercial HEPA models
Medify Air MA-40: CADR given as 224 cfm https://reviewsofairpurifiers.com/medify-ma-40-air-purifier-review/
Aerosol transmission ...
https://twitter.com/jeremychrysler/status/1435077275700109315?s=20
Vaccinated & unvaccinated risk, Delta variant
long covid in kids: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7927578/
Using particle sensors to assess impact of filters
https://twitter.com/dgurdasani1/status/1435520424478183424?s=20
Revision History
2021-08-08 20:48:55
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Replaced 'impact' graphic with a clearer depiction of an example impact of using two air boxes.
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Fixed typo on Home Depot price -- "6 for $30" --> "6 for $130"
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Added caution about replacing filters at approx 6 months
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Use large plastic bag, toss in dumpster, wash hands after
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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
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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.
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Adding warning about bumping / jostling the filters to avoid re-suspension of particulates