Edge Collective: DIY Air Purification Designs

[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/ ]

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:

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

Electrostatic / MERV-13 issues over time


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:

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.

References / Materials

Placement of box in room


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

Particle sizes

from https://www.cdc.gov/niosh/docs/2003-136/pdfs/2003-136.pdf




UC Davis evaluation of design


UL Testing (regards smoke)


Illustrations of build process, with variations


Building a Corsi air box for the first time: timelapse

Video: fully-assembled airbox

Small Poster


Large Poster


This Old House build guide / explanation

Detailed comparison of dust-removal performance


Rationale for 'fan shroud' / improving efficiency of Corsi Box


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


Effectiveness of Corsi Air Box -- particulate reduction


2021-09-07 19:31:18




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


Energy vanguard instructions


Philly Inquirer Article


Neil Comparetto video


Tex-air overview


Treehugger article


Percent effectiveness at capturing particles


--> 85%

Calculating ACH from CFM


Corsi presentation on air box


Recommended ACH for measles


Corsi et al preprint on room-level ventilation assessment


Tweet referencing the publication:


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 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

plantower comparisons:












Thoughts on using Purple Air to assess effectiveness of indoor air sensors



"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


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 ...


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


Revision History

2021-08-08 20:48:55