THE Corsi/Rosenthal Box FAQ

box fan filter FAQ

See also our main DIY box fan filter page


Table of Contents

The Corsi/Rosenthal Box Hall of Fame

We all need inspiration – and these are our heroes! Some to inspire because they are super-efficient. Some to inspire because they are simply gorgeous.

We love them all because they mean clean lungs!

Click here to enter the Hall of Fame!


Read Me First


Before reading any farther, even with 2 or 3 portable air cleaners in your space, everyone must wear a mask (more on masks, kids’ masks and reusable elastomeric respirators).

Like getting closer to a cigarette smoker, respiratory aerosols get thicker in concentration as you get closer to people. We give off respiratory aerosols when we breathe, talk, yell, laugh, and sing.

Regarding lunch and breaks – of course masks come off, so the risk goes up. Here are many ideas on how to handle those times.

Introduction.

You are probably interested in the DIY Air Cleaner known as the Corsi/Rosenthal Box. Conceived by Dr. Corsi, then built by Jim Rosenthal, Chairman and Chief Executive Officer of Air Relief Technologies, Inc. and past president of the National Air Filtration Association (NAFA).

To be crystal clear about appearances of Conflict of Interest, this website is put together by Citizen Scientists who have been learning about Indoor Air Quality (IAQ) from the multitude of IAQ experts, engineers, and scientists on Twitter, amongst other places.

While Dr. Corsi and Jim Rosenthal are referred to, and cited frequently, that same information has been either vetted by other IAQ experts or obtained from them separately.

Sections

  • Build – everything necessary just to get building.
  • “Not sure” – the FAQ’s that dive into why reducing the amount of SARS-COV-2 laden aerosols are important. And whether or not portable air cleaners do that – especially the Corsi/Rosenthal Box.
  • Implementation – important details on how to use – like where to place it in the room.
  • Safety – addresses safety concerns of all sorts.
  • General Questions – addresses most everything not in the above categores.
  • Countries – not all countries have the same materials as in the US.

I need some sort of letter and tactics to overcome objections.

Here’s such a letter.

Below is a brief preview of the 17 page document, and addresses every concern we have seen since the Corsi-Rosenthal Box was brought to fruition.

How to use such a letter? Possible ideas:

  • Just emailing it to the concerned individual.
  • Better would be to find parents “…who have working relationships with district admin and/or school board members? It helps to meet with them or call them individually. I sent a letter to our superintendent and cc’d the admin folks I know. They are working on getting CR boxes now.” (Quote from DaleAnn Baker.)

How NOT to use such a letter? Some people are all about videos. That’s ok – meet them where they are at.

What if you experience push-back after the letter, or you anticipate a hard push-back? Sometimes, it helps to gather evidence of poor ventilation.

  • Get a CO2 monitor like an Aranet. Put it in your kid’s backpack pocket (this twitter thread has some ideas) – ideally, you want it where your kid is.. Track the ventilation for a couple of days. (Very important – HEPA/ DIY air cleaners will NOT lower CO2, as gas molecules are too small to be captured by the filters.).
  • What is the value of measuring CO2? This excellent Washington Post article , posted at archive.is (so all can read it), explains all.
  • If the CO2 is showing as high, then you can present that as an indicator of the ventilation as being insufficient.
    • What is considered high? Above 920 ppm (or 500 above baseline).
      • This is discussed by Dr. Dustin Poppendieck here.
        • Prepandemic times:
          • For 24 students & 1 teacher, roughly 550 ppm over the outside air (usually 420 ppm – you can find this just by putting the CO2 monitor outside the classroom). So, 970 ppm.
          • For 34 students & 1 teacher, roughly 900 ppm over outside air (1320 ppm).
          • But these are NOT those times.
      • Pandemic Recommendations – roughly 500 max over baseline. 920 PPM in most locations.
        • The Belgian Government recommends no more than 900 PPM total – so roughly 400 to 500
        • Harvard Healthy Buildings initiative (Page 27, 5-6 ACH is roughly 920 PPM as per Dr. Poppendieck).
  • Some Talking points, from this spreadsheet developed by David Elfstrom., as discussed here.
    • At 800 PPM (Parts Per Million), 1% of what everyone breathes is from other people’s lungs – which might have Covid in them. (Though some of it would be from your own lungs.).
    • At 1000 PPM, 1.5%
    • At 1200, 2.1%.
    • At 1500, 3%
  • Dr. Linsey Marr, respiratory virus transmission/aerosol scientist tracked the CO2 in her school. Make sure to read the comments.
  • Additional Resources on this topic:

I just want to buy an air cleaner.

Please see our air cleaner/filter buying guide.

Also these tweets are two excellent sources – both from Indoor Air Quality Experts.

This is Dr. Shelly Miller – who authored this tweet – and co-authored the tools she mentions here.

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This is Marwa Zaatari, who regularly helps decipher the complex air cleaner market to get you the best bang for your buck. Her pinned tweet is the best place to find an amazing graphical way of finding a great air cleaner.

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Build

5 Filter Build – The Original Corsi/Rosethal Box – slightly raised as the bottom is a filter.

End goal:

Shopping List – the below links may quickly no longer be valid, so including a description of each item.  No affiliation with any company whose products I have listed:

Build time!

Tape the boxes together- very important to point the arrows on the edges of the filters, inwards.

Tape the fan to the front, windy-side out.

Make sure there aren’t any seams where air can get through. The idea is that the fan pulls the air through the filters.


Next, we are going to make a fan shroud. Directions here.

With the shroud done, you need some clearance under it. See below…see how the box is off of the ground?

“These are 20” strips cut from “pallet” protectors. But any reasonably sturdy material will work. The completed box is very light.”

So, try folding some cardboard in 1/2 to make a triangle, or cut a 1″x1″ board to fit.

The clearance is great below this build – though it would be best to have the fan on top pointing up.  With the fan pointing down, there’s more of a chance of dried respiratory aerosols being blown back up into the air.

This build is terrific because it’s build on casters!!!!  With school colors for the tape!

4 Filter Build – Also called the Compretto Cube. Bottom is cardboard – no filter.

There are several different builds below. No matter which one you choose make sure you put a shroud on the front of the fan to enhance efficiency.

Here is a 4 filter build known as the Compretto Cube: hosted by Philip Neustrom.  This diagram is just a sample – very well-done build site!

Here’s another 4 filter build from Don Blair, citizen scientist in the Boston area.  Great Work Don!  Great NPR article on this.

PNG of this poster.

Very handy PDF download here.

Here’s a video on how to do it from This Old House:

Here’s another video build:

Click here to go back to all FAQ’s.

4 Filter Build – KIDS TEACHING KIDS!

Click here to go back to all FAQ’s.

3 Filter Build – The Baskett Box Design, with one side and bottom made of cardboard.

Known as the Baskett Box after the person who came up with this version. We strongly urge you to place it in the middle of the room, but understand that if you place it up against a wall due to space concerns.

Build is located here.


2 Filter Build – The Wedge/ The Triangle – top and bottom are cardboard.

Add title

Click here to go back to all FAQ’s.

Here’s one as a goal – it’s the same triangle filter as in the video, but with a shroud added for optimum performance, and adhesive contact liner layered over the cardboard.  Beautifully done!

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This was what it looked like before the contact paper was added.

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In this one, you’ll notice that the shroud ismade of tape.

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Immediately below is the build thread ( make sure you put a shroud on the front of the fan to enhance efficiency):

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Slightly different build.

Here’s a slightly different build. I like this one for the step by step instructions (make sure you put a shroud on the front of the fan to enhance efficiency):

Video Build

Here’s an alternate build video (make sure you put a shroud on the front of the fan to enhance efficiency):

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1 Filter Build – aka single filter strapped to the back of a box fan.

The single filter version did very well vs commercial air cleaners.

Here’s what the final build can look like with judicious use of duct tape. Pretty, pretty sharp:

Actual Build

 Here’s the actual build from the above Twitter thread. 

Here’s a more simple, slightly less effective single filter build by aerosol scientist, Dr. Huffman.

And here’s a single filter build, but with a 4″ filter.  It should be very close to the 4 filter Corsi/Rosenthal Box in performance.

No matter which build you do – make sure you put a shroud on the front of the fan to enhance efficiency.

With the single filter build, if you have 3 of them – you are going to have great results – over 6 ACH in a classroom:

One last option shown below

 Won’t be as good – but something is better than nothing!

You could make that something even better by replacing the MERV 10 filter it comes with, with a MERV 13-A filter.

Round Fan Build – for countries with no box fans.

Add title

Some countries don’t have box fans, but have round fans.

“Presenting V1.0 accessible, effective and validated DIY filters using round fans inspired by the #CorsiRosenthalBox. They are the result of international collaboration from designers, engineers and advocates in @TheWHN Airborne Group DIY Filter Team.”. Source here.

For the type of fan, “For gym fans, anything between 80-120w 16-18″ is best. Aim lower for quietest operation. These designs and numbers were tested using a 18″ 45 cm gym cage fan. Best not to go above 120w. Will likely be running setting 1 or 2. Highest power flow is large but unstable & noisy.”. Source.

“Calculated based on 62% filter efficiency but we have particle measurements for Aerostar brand. We’re building and testing in UK,Spain,Canada,Switzerland. “. Source.

“Using ISO 5801 pressure balanced calibrated flow test. Similar to described in this paper .” Source.



4 Filter Build

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1 Filter Build

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1 Filter Build Video

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2 Filter Build (Wedge Shape)

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Some creative ideas:

4/ 5 Filter Build

A most excellent idea is this one, by ChurchAndCoVid : “The plywood (OSB) is 21”x21” (53 cm). Fan diameter is ~16” (40 cm) This could go on top of 4 merv-13 filters or a frame with polypropylene sheet.”

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This is from here – an excellent build site hosted by Philip Neustrom.

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A round fan that might work with a box combination:

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A round fan video build below, found by David Elfstrom:

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Cómo construir unos filtros de aire de bajo costo? Recursos aquí!

Translations by Emily Barnes Franklin – thank you!

Click here to go back to all FAQ’s.

____________________________________

How to build the shroud


Next, we are going to make a fan shroud..a fancy way of saying we are going to block off the square corners, so just the round fan is exposed.  This was discussed here by Jim Rosenthal.

Short version – the round fan does not reach the square corners, so there’s a gap there, and is not efficient. David Elfstrom, “At the tips of the fan blades there is negative pressure which sucks air in through the front (outlet) of the fan. By covering up this area, more air will flow through the filter on the inlet side and noise is reduced.”

As you can see below, the green arrows are where the air is flowing out of the fan, and the red arrows are where the negative pressure is sucking air back in the opposite way. Picture from here.

When you make the shroud, you will be putting it on the outside of the fan. On the side that the air is blowing from. And you only need one shroud, not one on the inside AND the outside. Just the outside.

You could do this with tape:

Or cardboard.

So – there have been two fans that have had their fans optimized by David Elfstrom. They are here. After this, will show you how to find the shroud size for your specific fan.

All others, you could can use tissue paper to determine how big the hole in the middle should be. As the fan is on, pull a piece of tissue across the front of the fan, slowly. You will very quickly see the part of the tissue that is being sucked back INTO the fan. You’ll want to cover that part with the shroud. In the picture below, you’ll see the green arrows up, and red arrows down – the red arrow down is the part you want to cover.

After determining the size, draw a circle on your cardboard. If you draw an “X”, corner to corner, that will give you the middle. Then, you can use this technique with a pin and string to draw that circle.

Some people put contact paper over their cardboard parts – like the shroud.

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Type of Fan

  • 20″ Box Fan – very important that the On button/ Speed Control is on the outer edges of the fan – or on the front where air comes out.  You do not want it on the back where air comes in. 
    • Lasko 20″ Box 3-Speed Fan
    • Utilitech 20-in 3-Speed Indoor Box  Fan  – these  also  come  in  black.

Safety note: the box fans have been tested and found to more than up to the task.  With that said, as an additional note, it might be good to get one with a fused plug, which the Lasko and Utilitech fans have for sure.

Type of Tape

Any type of duct tape is recommended. Keep in mind that you can use the duct tape color to accessorize your DIY Air Cleaner. Please see the Gallery for lots of creative fun ideas.

Type of Filter

20″x20″x1″ or 20″x20″x2″ MERV 13 filters are recommended. MERV 13-A filters are available here.  Why 13-A? Because they are top of the line.

If MERV 13-A filters are not available, the next filters recommended are MERV 13 filters from 3M .  Specifically, 3M 1900 filters are excellent (the people on the thread are IAQ experts) .

These can gotten at:

  • Lowe’s
  • Home Depot
  • Walmart
  • Ace Hardware
  • etc.

What is not recommended?

At this time, nothing. There was an issue with FilterBuy’s filters sourced from Amazon, but that turned out to be a bad batch and has been resolved.

Last tip – if you are making the 4 or 5 filter box, you could make the sides out of 20″x30“x1 filters – and get a huge gain in filter area. You can find 20″x30” filters at Walmart. Tip courtesy of ocnj of Twitter.

Lowe’s also has 20″x25“x1 – and same deal.

The Record Player Build

This build by Amanda H, has a printed out record label in the middle. This build is simply amazing! Click through to see it in action!!!

She was kind enough to provide the files so that you could recreate this build as well. Thank you, Amanda!!!

File is located:

Where can I find the original Build instructions in the Mega Corsi/Rosenthal thread?



Not sure? These are the FAQ’s for you.

Seeing is believing

Comparison of air filtration/ventilation at a restaurant table. The smoke machine (generates aerosols like we breathe, but visible) flicked on every 15 seconds, video sped up 10x. With ventilation, smoke is continuously cleaned. Without, it builds up. In an unventilated space, you’re breathing exhaled air from other people.

In this video from Roger Yang (thank you!), you can see the HEPA and HVAC at work on the left. Nothing on the right. Air cleaning works….

Here’s another video showing a CorsiRosenthal Box head to head with a HEPA cleaner.

DIY Air Purifier ($110CAD) vs Germ Guardian 3-in-1 HEPA Tower ($189CAD).

The smoke machine was blasted for 30 seconds with both purifiers on high. Thank you, Amanda!

✅CR Box never got that foggy because it was cleaning at high speed the whole time. All cleared by 55 seconds!!

✅ The commercial product, also cleaned – but got super foggy because it could not keep up. At 55 seconds, just a soupy mess still. Took until 3:31 to clear.

Why use an air cleaner? The air doesn’t look dirty, and I can breathe easily?!

SARS-COV-2, the virus that causes CoVid, is actually carried by respiratory aerosols. Portable air cleaners (also called air purifiers and air filters), work by pulling air through high-quality filters.

As the air cleaner pulls air in with the fan contained in it, the filters attached to it clean the air. Details on how the filters work below*.

Respiratory aerosols (emitted when we breathe, talk, laugh, yell, sing, cough, sneeze) carry SARS-COV-2 and are extremely small. The ones most likely to carry SARS-COV-2 are too small to see.

Roughly 85% of respiratory aerosols are 5 microns or smaller. High-level tweetorial about this study.

Just to give you an idea of how small, here are 50 micron aerosols floating:

<blockquote class="twitter-tweet">
<p dir="ltr" lang="en">Much discussion lately in aerosol/disease transmission communities about the “5 micron cutoff” where droplets supposedly fall to ground w/in 1-2 m. <a href="https://twitter.com/jljcolorado?ref_src=twsrc%5Etfw">@jljcolorado</a> and <a href="https://twitter.com/linseymarr?ref_src=twsrc%5Etfw">@linseymarr</a> has suggested ~50 microns.</p>
Here’s some video evidence for that. 50 micron droplets wafting in lab... <a href="https://t.co/5SrE7GeKDF">pic.twitter.com/5SrE7GeKDF</a> — Ryan Davis (@MicroLevitator) <a href="https://twitter.com/MicroLevitator/status/1283556047471378432?ref_src=twsrc%5Etfw">July 16, 2020</a></blockquote>
<p><script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

*How the filters actually clean the air.  The first thing to know is that while the filter appears to be flat, it is actually a 3 dimensional matrix of interwoven fibers.

Those fibers work via the following mechanisms:

  • Inertial impaction – Occurs when a particle encounters a filter fiber due to the inertia of the particle.
    • Example: A large dust particle is unable to change direction of flow due to its inertia, so it impacts the fiber and becomes attached to it.
  • Interception – Occurs when a particle follows a the air flow happens to come in contact with the surface of a fiber.
    • Example: An intermediate dust particle that readily follows the airflow stream comes in contact with a filter fiber.
  • Diffusion – Occurs when particles do not follow the airflows as readily and are governed by random particle motion (Brownian motion of small particles).
    • Example: A small particle, such as certain exhaust particles, encounter filter fibers at random.
  • Electrostatic attraction – Occurs when an electrostatic charge on the filter fibers are present.
    • Example: Static charge on a fabric creating static cling.

In terms of filtration, the most important filtration mechanics pertaining to everyday use are: Intertial impaction and Interception.

The larger particles that encounter the filter tend to impact onto the exterior surface of the filter.

The smaller particles that follow airflow streamlines tend to impact and intercept within the depth of the filter.

Here’s a video from the NYTimes showing how the N95 masks catch viruses.  These are the same as how air filters work.

SARS-COV-2, the virus that causes CoVid, is actually carried by respiratory aerosols. Portable air cleaners (also called air purifiers and air filters), work by pulling air through high-quality filters.

As the air cleaner pulls air in with the fan contained in it, the filters attached to it clean the air. Details on how the filters work below*.

Respiratory aerosols (emitted when we breathe, talk, laugh, yell, sing, cough, sneeze) carry SARS-COV-2 and are extremely small. The ones most likely to carry SARS-COV-2 are too small to see.

Roughly 85% of respiratory aerosols are 5 microns or smaller. High-level tweetorial about this study.

Just to give you an idea of how small, here are 50 micron aerosols floating:

<blockquote class="twitter-tweet">
<p dir="ltr" lang="en">Much discussion lately in aerosol/disease transmission communities about the “5 micron cutoff” where droplets supposedly fall to ground w/in 1-2 m. <a href="https://twitter.com/jljcolorado?ref_src=twsrc%5Etfw">@jljcolorado</a> and <a href="https://twitter.com/linseymarr?ref_src=twsrc%5Etfw">@linseymarr</a> has suggested ~50 microns.</p>
Here’s some video evidence for that. 50 micron droplets wafting in lab... <a href="https://t.co/5SrE7GeKDF">pic.twitter.com/5SrE7GeKDF</a> — Ryan Davis (@MicroLevitator) <a href="https://twitter.com/MicroLevitator/status/1283556047471378432?ref_src=twsrc%5Etfw">July 16, 2020</a></blockquote>
<p><script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

*How the filters actually clean the air.  The first thing to know is that while the filter appears to be flat, it is actually a 3 dimensional matrix of interwoven fibers.

Those fibers work via the following mechanisms:

  • Inertial impaction – Occurs when a particle encounters a filter fiber due to the inertia of the particle.
    • Example: A large dust particle is unable to change direction of flow due to its inertia, so it impacts the fiber and becomes attached to it.
  • Interception – Occurs when a particle follows a the air flow happens to come in contact with the surface of a fiber.
    • Example: An intermediate dust particle that readily follows the airflow stream comes in contact with a filter fiber.
  • Diffusion – Occurs when particles do not follow the airflows as readily and are governed by random particle motion (Brownian motion of small particles).
    • Example: A small particle, such as certain exhaust particles, encounter filter fibers at random.
  • Electrostatic attraction – Occurs when an electrostatic charge on the filter fibers are present.
    • Example: Static charge on a fabric creating static cling.

In terms of filtration, the most important filtration mechanics pertaining to everyday use are: Intertial impaction and Interception.

The larger particles that encounter the filter tend to impact onto the exterior surface of the filter.

The smaller particles that follow airflow streamlines tend to impact and intercept within the depth of the filter.

Here’s a video from the NYTimes showing how the N95 masks catch viruses.  These are the same as how air filters work.

But my lungs are tough! 

Sure, they are.  They have to be to enable you to breathe.  But – if you catch CoVid, you literally drown in the mucus created in your lungs.

Speaking of drowning – your lungs process over 2,000 gallons of air, per day.  That’s a swimming pool of air.  

How many respiratory aerosols could be in that?

Very rough math (sorry)…if a respiratory aerosol is 5 microns, that would be 1.51416e+18 aerosols processed.  1,514,160,000,000,000,000.

Potentially, 1.5 quintillion 5 micron aerosols.  

Your lungs handle all of that. How big are they? One of the largest organs in your body.  Not as big as your skin which can be cleaned with hand sanitizer, but close.  About the size of a tennis court if laid out flat.

Your lungs have 1,500 MILES of airways in them – imagine driving from Chicago to Las Vegas.  Now – think….if even as little as 1 SARS-COV-2 virion landed on that roadway, you could get CoVid. 

The true amount of inoculum (amount required to get you infected) is unknown, but – you get our point. 

You use those lungs to breathe 14 times per minute – over 17,000 breaths per day.  And with Delta?  Only takes one breath.

<iframe class="giphy-embed" src="https://giphy.com/embed/x6DriQrWUmEBq" width="480" height="286" frameborder="0" allowfullscreen="allowfullscreen"></iframe>

How do I know air cleaners even work?

Because they have been studied, and found to be very effective at removing aerosols from the air: 

  • Classrooms:   “…our study shows that box fan air cleaners can serve as an effective low-cost alternative for mitigating airborne transmission risks in poorly ventilated spaces.”
  • Hospitals (preprint):  “A single bed hospital room equipped with an excellent ventilation rate (∼ 14 air changes per hour) can clear the aerosols in 20 minutes. However, with the addition of two air cleaners, the clearance time became 3 times faster (in 6 minutes and 30 seconds).”  
    • “What? How can there be an 14 ACH ventilation rate, but only 3 ACH to clear the aerosols? 14 ACH is every 4 mins?”
    • Answer: “An ACH is the rate to remove 63% of the air (not 100%). It requires 5x this time (5 x – 1/ACH (ie lifetime / residence time) to clear 99% (ie remaining = 0.37 ^ 5 < 0.01) of the air. Think of a drop of red dye in a sink – easy to remove the first 50% really long to remove it all.”
  • Hospital (published): “With 2 small domestic air cleaners in a single patient room of a hospital ward, 99% of aerosols could be cleared within 5.5 minutes.”
  • A systemic review found:” Portable HEPA filter systems have been shown to be another effective tool in reducing viral load.”
  • CDC MMWR: “Portable HEPA air cleaners offer a simple means to increase the filtration of aerosol particles from a room without modifying the existing building ventilation system…”
  • FORD engineers did a study on a single filter version, called the Scrappy Filtration Kit..

The following Federal Agencies recommend them (along with many state public health agencies):

  • CDC directly recommends them: “Use portable high-efficiency particulate air (HEPA) fan/filtration systems to enhance air cleaning (especially in higher risk areas such as a nurse’s office or areas frequently inhabited by people with a higher likelihood of having COVID-19 and/or an increased risk of getting COVID-19).”
  • ASHRAE recommends them
  • EPA recommends them.
  • The Corsi/Rosenthal Box as discussed in the recent “Delta Schools Airborne” Webinar.
  • Media: NPR, This Old House,

If these air cleaners are so great, why haven’t any large organizations rolled them out?

They have!

At least 20,000 by the Ontario, Canada government – in classrooms, gyms, libraries with no mechanical ventilation.  As approved by their Chief Medical Officer of Health.

Toronto School District itself.

New York City, but there’s a note of caution there.  Experts have indicated that NYC did not make the best choice.

Won’t a portable air filter (cleaner) just move the air with contaminated aerosols around to infect more people?

Per Dr. Poppendieck “I actually love this question because it shows the person has thought about what we intuitively understand about our indoor environment.  We generally don’t feel the air move indoors.  Humans can sense air movement down to about 0.1 m/s (0.3 ft/s).  Below that, we generally consider the air to be “still.” Hence, there is a natural assumption that the air on the other side of the classroom won’t reach me.  But most of us have also observed particles moving in a sun beam in a room we feel is “still.”  So we intuitively think air is still, even through we have observed it moving. 

In fact, in most indoor environments air is moving around 0.05-0.1 m/s (0.15-0.3 ft/sec).  This means that if an airborne particle traveled in a straight line it could travel the length of a classroom in roughly three to five minutes. That speed is without taking into account inputs that enhance air movement (a person coughing, a person walking, the heat rising from every person, HVAC airflows, settling, thermal gradients due to sunlight). 

You already know that this is true, just think about how long it takes you to smell a cigarette that someone is smoking from across the room.  A room is much like a mixing bowl full of ingredients for making cookies.  Once we add a new ingredient to the batter and start mixing, some of the new ingredient will stick to the edge of the bowl (like the corners of a room), but most of it will evenly mix throughout the batter relatively quickly. 

But what about plumes forming when a portable air cleaner is on? Well, there will be plumes of aerosol containing viruses moving throughout the room without any filtering device present due to the all the things in a room that enhance airflow. Remember air is not still, even if you can’t feel it moving. Airborne viruses are already moving around the classroom in a time frame that is faster than the length of a one hour class.  So even without a portable air filter, everyone in the classroom will likely be exposed to the airborne viruses by the end of the class.

A portable air cleaner will slightly increase the air velocity and mixing in the room (although you won’t likely feel air going into a portable air cleaner unless you are less than ~0.5 m or 1.5 ft from the intake), while also reducing the average aerosol containing virus load in the room by 10 % – 40 % (1 micron) over the duration of the class. 

So will portable air cleaners in a classroom make the airborne virus problem worse?  Probably not since the room is fairly well mixed in the time frame of a class even without an added fan (air cleaner) and the air cleaner will reduce the average concentration of aerosols that one breaths in.  Will portable air cleaners keep me safe from COVID19 while teaching?  We can never be completely safe while not in isolation. However, portable air cleaners are one layer of a risk reduction approach to make teaching in the time of airborne viruses safer.”

Source:  http://poppendieck.com/IAQ/COVID19%20FAQ.html

Does the Corsi/Rosenthal Box work?

Yes – it removes aerosols from the air.  How well does it work?  Very well.

Dr. Marwa Zaatari, Chief Science Officer at Blue Box Air & ASHRAE Associate Member

“It is the most cost effective purifier on the chart.” – this evaluation is from Dr. Zaatari, who regularly publishes an easy-to-understand chart that simplifies the often complex air clean marketplace.  It should be noted that this was the single filter version – so the 2/ 4/ 5 filter versions should perform even better.

One large caveat – this is with the shroud on the fan. 


UCDavis Western Cooling Efficiency Center found it delivers an equivalent performance to commercial air cleaners that cost 3x as much.

<blockquote class="twitter-tweet"><p lang="en" dir="ltr">Check out our new testing on DIY portable air cleaners. We found that the <a href="https://twitter.com/CorsIAQ?ref_src=twsrc%5Etfw">@CorsIAQ</a>/ <a href="https://twitter.com/JimRosenthal4?ref_src=twsrc%5Etfw">@JimRosenthal4</a> Box performs similarly to residential portable air cleaners in terms of estimated clean air delivered &amp; costs about 3x less. <a href="https://t.co/i0jvLPhkSF">https://t.co/i0jvLPhkSF</a> <a href="https://twitter.com/UCDavisCOE?ref_src=twsrc%5Etfw">@UCDavisCOE</a> <a href="https://twitter.com/ucdavis?ref_src=twsrc%5Etfw">@ucdavis</a> <a href="https://t.co/RC04VPi0Lj">pic.twitter.com/RC04VPi0Lj</a></p>&mdash; UC Davis Energy (@ucdavisenergy) <a href="https://twitter.com/ucdavisenergy/status/1432774552745091079?ref_src=twsrc%5Etfw">August 31, 2021</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

And here is the final brochure/ PDF documenting all of their work.


CBC Business Marketplace evaluated a MERV 11 single filter version of the Corsi/Rosenthal Box

The $70 single filter MERV 11 version easily outperformed the $800 Dyson Pure Hot & Cool. Even more impressively, this was with NO shroud which increases performance by 47%, and a MERV 11 filter instead of a MERV 13 filter.


The Built Environment Research Group at the Illinois Institute of Technology found these excellent results for the 4 Filter version of the Corsi/Rosenthal Box

“… (T)he Built Environment Research Group has been independently evaluating the efficacy and potential for adverse consequences…” And they evaluated this version in September 2021.

<blockquote class="twitter-tweet"><p lang="ro" dir="ltr">We tested a version of a Corsi-Rosenthal box with MERV 13 filters: <a href="https://t.co/zxBwCGsD6l">https://t.co/zxBwCGsD6l</a><br>CADR intuitively increased with particle size:<br>166 CFM for 0.09-1 µm<br>321 CFM for 0.5-3 µm<br>464 CFM for 5-11 µm<br>More air cleaner testing here: <a href="https://t.co/ONJkSXxAra">https://t.co/ONJkSXxAra</a><a href="https://twitter.com/JimRosenthal4?ref_src=twsrc%5Etfw">@JimRosenthal4</a> <a href="https://twitter.com/CorsIAQ?ref_src=twsrc%5Etfw">@CorsIAQ</a></p>&mdash; BERG@IIT (@built_envi) <a href="https://twitter.com/built_envi/status/1435328519471501320?ref_src=twsrc%5Etfw">September 7, 2021</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

Why does it work so well?

Jim Rosenthal, past president of the National Air Filtration Association and chief executive officer of Tex-Air Filters, discusses why the Corsi/Rosenthal Box works so well. It’s a fascinating article, but if you have no time:

  • The sheer amount of air flow from the CR Box is about 700 CFM (on only medium speed) versus the $900 HEPA filter it was compared with (300 CFM on maximum). 
  • It must be emphasized that Jim Rosenthal called for further research into this.
  • Here’s a graph of how it performed:
  • Yes – at least as good as a $900 HEPA filter.
    • Though to be crystal clear, the CADR for the MERV 13 filters is still being measured.  
    • Above is with MERV 11 filters.  The one you will make will have MERV 13 filters, so should be better.


Dr. Richard Corsi discusses the Corsi/Rosenthal Box

In this NPR Article, Dr. Corsi reported, “”People are now reporting 600 cubic feet per minute in clean air delivery rates. That’s phenomenal. That’s begentter than a lot of the more expensive HEPA-based portable air cleaners,”

Speaking of Dr. Corsi – in this video he discusses the benefits of the Corsi/Rosenthal Box.

This is anecdotal – and you have to have masking as well as air cleaning!

<blockquote class="twitter-tweet" data-conversation="none">
<p dir="ltr" lang="en">We built 18 of these for a small school here in East Rochester. Classes have been going 5 days a week all school year without any outbreaks! <a href="https://t.co/sp4dWWOOSj">pic.twitter.com/sp4dWWOOSj</a></p>
— Matthew Bowers (@ROCPassiveHouse) <a href="https://twitter.com/ROCPassiveHouse/status/1371863229144690688?ref_src=twsrc%5Etfw">March 16, 2021</a></blockquote>
<p><script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script></p>

Regarding putting these in classroom – I thought kids don’t transmit CoVid?

They get it and transmit it. 

They have ACE2 receptors, which SARS-COV-2 targets.  And just like adults, they have a thermal plume wafting SARS-COV-2 up to you.  Basically, our body’s own heat forms a never-ending plume of air movement upwards because heat rises.

“While there has been no systematic study on human thermal plume in the COVID-19 context, an existing body of evidence suggests that it generates constant uprising airflows around the human body and transports aerosols from lower regions into the breathing zone.”

And

” Under the constant uprising airflows, the vertical transport and human inhalation of indoor aerosols would be particularly enhanced when the emission source is located at lower horizons in proximity to the human occupant (Rim et al. 2009).”

” To summarize, there is ample evidence indicating that thermal plume creates constant uprising airflows around the human body and therefore has the ability to transport abundant quantities of airborne droplets and particles from lower regions to the human breathing zone.”

Some more interesting bits of info from that study:

  • We might be breathing in microplastics: 
    • “Vianello et al. (2019) utilized a breathing thermal manikin with respiratory patterns and metabolic heat similar to those of adult males to assess the influences of thermal plume on the human inhalation of indoor microplastics. The study found that thermal plume continuously transported microplastics from lower regions of the room into the breathing zone of the sedentary manikin. “
  • For the office:
    • “For sedentary individuals, moving their body closer to desk can partly block the uprising thermal plume and reduce its velocity entering the breathing zone (Melikov 2015; Licina et al. 2014). “
  • “Generated by body-to-air temperature gradients, human thermal plume creates persistent uprising airflows along the human boundary layer, typically as a laminar-to-turbulent flow with varying thicknesses from 2–4 mm to 200 mm and a maximum velocity of 0.2–0.3 m/s, which is comparable to the designed air velocity in various buildings with mixed ventilation under the current design criteria. “

Implementation

Where should it/they be placed in the room?

A simple question, but can be a complicated answer.

  • Try not to place next to a wall if possible.  You want air flow.
  • If you know who’s infected, next to them (typically placement in a patient’s hospital room).
  • If you are in a classroom, you will want 2.  Place one in the center of the room, even if you have windows open.  Place the other in the middle of the room, but up near the teacher. 

From http://poppendieck.com/IAQ/COVID19%20FAQ.html

I just bought a portable HEPA air cleaner.  Where should I put it in the classroom?  To be effective, portable air cleaners need to get as much of the air in the room through the filter as quickly as possible.  Think about trying to beat an egg in a square pan.  If you try to beat the egg with your mixing device in the corner or on an edge, the egg in the far side of the pan will not get stirred very quickly.  However, if you place the beating device in the middle of the pan, the egg will mix faster (but still not perfect in the corners). 

Likewise, in a classroom, you want to place an air cleaner as close to the middle of the room as possible, elevated up off the ground.  This will maximize the amount of air moving through the HEPA filter. Make sure you place the air cleaner in a location where it won’t fall onto someone and secure the cord to prevent any tripping hazards.

Most portable air cleaners draw air in from the sides and exit on the top.  Many Do-It-Yourself filters made with box fans and filter furnaces exhaust air to the side. These DIY designs should not be placed on the floor as the exiting air may resuspend particles from the floor that may contain viruses.

Operate your portable HEPA cleaner on its highest setting. The higher the setting, the more air that moves through the filter and lower the virus concentrations will be in the air. Hence, highest setting equals lowest risk for people in the classroom. Lower settings maybe desirable due to fan noise, but be aware they will increase risk as less air is moving through the filter.”

Why should it be pointed up at the ceiling?  I saw a study that said pointed down is better.

That study did not include the risk of reaersolization (blow back up in the air)  of dried respiratory aerosols that will have settled on the ground.   You will give a little in efficiency to gain safety.

http://poppendieck.com/IAQ/COVID19%20FAQ.html

Should I add 2 filters on top of each other? (No)

As discussed here, it decreases performance. The 1″ filters are fine – although you would get even better performance with 2″ filters. Just not 1″ filters doubled up.

Electricity – how much does it use?


This is with the Utilitech 20″ Box fan – 58 Watts when cranked to max, as measured by Jim Rosenthal.  The measurements below are for:

  • No filter attached.
  • 1 filter attached – 1″, 2″, 4″
  • Box filter – the full Corsi/Rosenthal Box attached.
This image has an empty alt attribute; its file name is E9vEYI6XoAgg2C2.png

This with the Lasko 20″ Box Fan – 89 Watts when cranked to max.

This image has an empty alt attribute; its file name is E9vEnJ6XoAMPncH.png

Exactly. The fan on high for the Corsi/Rosenthal box used 54 watts. The ELGIT air cleaner (also known as a Banker’s Lamp) used 60 watts. Just in case someone doubts the data – here are the pictures. https://t.co/jpK81qqa2F pic.twitter.com/ysLQKGvSmh — Jim Rosenthal (@JimRosenthal4) August 26, 2021

David Elfstrom also measured both with the shroud on the single filter version, which makes the fan operate more efficiently.

Utilitech/Hurricane with 13.5″ shroud: 350 CFM, 52 W! 

Lasko with 15″ shroud: 350 CFM, 74 W !

This image has an empty alt attribute; its file name is shroud-300x169.png

Sources:

Optimization so far, with 3M Filtrete 1900 20x20x1

Lasko with 15″ shroud: 350 CFM, 74 W

Utilitech with 13.5″ shroud: 350 CFM, 52 W!

Actual flow will be slightly higher when measured at outlet with backpressure compensation feature in the flow capture hood. /14 — David Elfstrom (@DavidElfstrom) August 16, 2021

Optimization so far, with 3M Filtrete 1900 20x20x1

Lasko with 15″ shroud: 350 CFM, 74 W

Utilitech with 13.5″ shroud: 350 CFM, 52 W!

Actual flow will be slightly higher when measured at outlet with backpressure compensation feature in the flow capture hood. /14 — David Elfstrom (@DavidElfstrom) August 16, 2021

How long do the filters last?

Always refer to the manufacturer’s specifications.  But a general rule of thumb is:

  • If used during smoke season, 3 months.
  • If during regular usage, 6 month.

Additional details on MERV 13-A filters which for sure will are the ones that last that long.

How expensive is the Corsi/Rosenthal Box?


You can build one of these for just under $100, and it will help significantly remove respiratory aerosols that contain SARS-COV-2 from the air. We recommend at least 2 of them in a normal sized classroom. $200.

Keep in mind – something is better than nothing.  Even if you did the single filter build, which would be about $36 with the filter AND fan, it would help significantly reduce aerosols in the room.  

And you could save up to buy the rest of the filters, later.

How loud are these?

Roughly 51 DBA..This is more quiet than a refrigerator.

How do I throw out/ dispose of the filters?

Put on your N100 elastomeric respirator (more comfortable than N95). Does not have to be N100 – just better masks work…better.

Carefully, pick up the Corsi/Rosenthal box and gently take it outside.

Carefully cut the filters apart, gently peel it back to get the fan off.

Separate the filters and gently place in a garbage bag.  Do not just shove them in as they might have virus in them and don’t want the garbage bag blowing the virus back on you. 

 Tie the bag tight.  Toss into garbage.

Make new box!

 Why not spray disinfectant on it?  

 Because by the time the garbage bag degrades the virus will have been inactivated.

But – this was for the original WT strain of SARS-COV-2.  Who knows what it might be for Delta.

Here are two estimates that indicate the filter environment (moving air, etc) might lead to quicker inactivation.

Since we don’t know 100% either way, just be careful.

Safety

Do we need to wear masks even though we have a couple of air cleaners?

I’m sorry, you do still need to wear a mask.  Why? 

For the same reasons Health Care Workers continue to get infected in hospitals which have excellent ventilation with 6 Air Changes Per Hour.

Ventilation cleans up far-field aerosols (>6 feet away), but cannot clean the air of aerosols that have just been emitted by the infected person (near-field aerosols < 6 feet away).

An example to help explain – you  are in a room with someone smoking a cigarette and an air cleaner.  You can see the smoke being removed from the air by the air cleaner.  But if the smoker blows smoke in your face, the air cleaner cannot clean it in time.

Another point:

“We put a HEPA air cleaner in our classroom.  Do I have to wear a mask in the classroom? Yes.  Think about a used diaper in a room.  What is the best way to keep the room from stinking?  It is not to open a window or put in a filter.  It is to remove the diaper from the room.  This is one of the fundamental pillars of indoor air quality: source reduction. 

Wearing a mask, even a cloth mask that isn’t perfectly fitted, will reduce the number of aerosols that get into the air from a contagious individual.  When we talk, we expel more particles that are airborne than when we are just breathing.  When we talk loud or sing, we expel even more.  Hence, even the teacher should wear a mask. Masks with clear mouth shields should be sought if people who need to lip read are present. 

The goal is a layered risk reduction.  We need to take as many actions as we can (increase ventilation, HEPA air cleaner, masks, etc.) to reduce the risk as none will completely eliminate it. Wearing masks will reduce average airborne particle load (1 micron) in the classroom by roughly 30%.”  Source.

Here’s a video that shows aerosols in general – and how plexiglass does NOT stop aerosols.

Electricity – are these fans a danger with the filters attached?

No:

HVAC – someone said it will mess up the HVAC of our building?

This could not be farther from the truth.

Air cleaning is additive – not subtractive.

For example – hospitals have excellent air cleaning.  In this preprint of a study – “A single bed hospital room equipped with an excellent ventilation rate (∼ 14 air changes per hour) can clear the aerosols in 20 minutes. However, with the addition of two air cleaners, the clearance time became 3 times faster (in 6 minutes and 30 seconds).”  Explanation of why .

Don’t forget – the following agencies recommend portable air cleaners.

  • CDC directly recommends them: “Use portable high-efficiency particulate air (HEPA) fan/filtration systems to enhance air cleaning (especially in higher risk areas such as a nurse’s office or areas frequently inhabited by people with a higher likelihood of having COVID-19 and/or an increased risk of getting COVID-19).”
  • ASHRAE recommends them
  • EPA recommends them.

ASHRAE is available to consult with.

They can be emailed COVID-19@ashrae.org

Clickable link to that email address.

How to keep little fingers out of the filters? Or – can students touch the filters?

It is recommended not to – the virus might be on the filters – but It is far more likely that airborne transmission will occur.

The odds of transfer from the filters to the hands and then to the mucosal regions of eyes, mouth, or nose are very, very low.

  • The filter is actually a 3 dimensional matrix, the virii might not be on the top level. 
  • This is a very inhospitable environment for viruses.  The constant air movement should result in quick virus inactivation.
  • For infection to happen, you would have to have transfer of infectious virus from the filter to the hand.  Then, from the hand to the face’s mucosal regions (eyes, nose, mouth).  Possible, but extremely unlikely.
    • Especially when you consider finger print ridges vary in width from 100 µm, for very thin ridges, to 300 µm for thick ridges. You would have to be extraordinarily unlucky for the 1 -5 µm respiratory aerosols to be land square on a ridge. Since there are valleys as well, which take up 50% of the space on your finger (eliminating that 50%). Below a microscopic close-up of the the fingertips.
    • You might object and say – what about norovirus? True, but different virus. For CoVid, there are just count-on-one hand few reports of fomite infection. Whereas there are 100’s of airborne transmission.

But there are some options below to prevent touch. These which will result in a less efficient air cleaner.  How much less efficient? Impossible to tell without access to specific testing equipment.

Here are some ideas if you are dead-set on this:

This one is very well done – but would result in an even great inefficiency of the air cleaner.  (It is welded together).

General Questions

Where can we learn more about Arizona State University’s partnership with local schools?

Here is a great write-up of how to develop such a program. Here is their link tree to see how you can donate to them.

Wouldn’t it work better if we added UVC bulbs under the shroud like hospitals do?

This is a great question!  Two part answer.  

First part is that it’s actually two different technologies.  Bulbs are usually used to clean items in a sealed container in a hospital.

Hospitals use UVGI to clean the air.  And it can be extremely effective.  Here’s respiratory virus transmission expert, Dr. Don Milton (MD, PhD) …” min 20 to max 1000 equivalent ACH. ”  Hospitals have excellent ventilation at 6 Air Changes Per Hour, so 20 ACH is phenomenal.  But, it’s an entirely different technology than bulbs.

2nd part, per David Elfstrom, IAQ expert, the bulbs will damage the filters and the air moves too fastSame basic response from Jim Rosenthal, also IAQ expert.

We can only open our windows, but not if the children have to wear a coat? Natural Ventilation…

Quick Tip if your windows and doors open – at the VERY LEAST, open during breaks and lunches. Source:

Professor Cathy Noakes:

“If its cold, crack the windows rather than opening them fully and then open them wider to air the room periodically say at break times. The colder it is, the smaller the gap you need for ventilation. And let them layer up too.

Ventilation is driven by pressure differences and they are created by temperature differences (hot air rises) and wind. So for the same size gap you get more flow when there is a bigger temp difference between indoor and outdoor or there’s a higher wind speed.

So a fairly small gap in winter can give you as much ventilation as fully open window in summer. It is hardest to ventilate when its a warm still summer day with no difference in temperature and no wind.”

Some recommended ways to help with natural ventilation from the CIBSE Natural Ventilation Chair.

Original Source is here – 41 pages of great information. But if you have no time, here’s the most relevant 4 pages to you or me, the common person. Some samples below:

Openable windows and vents should be used more than normal, as long as security is
considered and the open windows do not cause a hazard to anyone moving outside. If
possible windows should be open at least 15 minutes prior to room occupation.


“In cooler weather even a small opening can deliver significant ventilation flows, and this can minimise risk to occupants of the space.”

“Where there are both high-level and low-level openable windows in a room, it is
recommended to open the high-level windows during cooler weather
in the first instance; incoming air will be warmed as it flows down into the room, thereby reducing cold draughts.”

Do you have any lesson plans to incorporate this into school?

  • Here’s a lesson plan from Safe Ed For all.
  • A presentation by Jim Rosenthal that contains some great slides you could use.
  • Some middle, elementary, and high school lesson plans from CleanArAir. The links are affiliate links.

Besides that – here are a collection of fun lesson ideas for your consideration.

This was conceived as a combination of physics (aerosols), biology (lungs), and math (large number conversions).  All parts are meant to be used separately or together.  

For now, just going to lay out the experiments, if you write out introductions and want to contribute a write-up such as “What are aerosols”, please contact @lazaruslong13 on Twitter.

Here’s a very cool demonstration of some of the physics involved:
https://clarkvangilder.com/2021/08/27/covid-aerosol-physics/

Aerosols Introduction

After discussing, show this video of 50 micron aerosols.  Afterwards, you can have them visualize dust motes caught in a sunbeam in their living room – those are approximately 50 micron aerososl as well.

Fun Aerosol Demonstration Option 1

Smoke machine plus garbage can vortex…demonstrating aerosols, and Bernoulli’s Principle

OR

Fun Aerosol Demonstration Option 2 – this one HAS to be outside.

Cheap Aerosol Demonstration – Get a little flour and blow into the air – those are aerosols.  

Lungs (Biology)

From here.  Materials required are here – the lung demonstration is very well laid out on their web page.

And while doing the above experiment, or before, you can talk about about fun lung facts from the below two articles.  What you see is just a sample of the fascinating knowledge in them.

After talking about the gallons breathed, you could segue into…

Math 

Your lungs process over 2,000 gallons of air. Per day. Very rough math follows  (please feel free to contact me to get it corrected!)

If a respiratory aerosol is 5 microns, that would be 1.51416e+18 aerosols processed. 1,514,160,000,000,000,000. Potentially, 1.5 quintillion 5 micron aerosols.

So, we don’t want  a bunch of aerosols in our lungs, so what  do we do?  Clean the air.

Corsi/ Rosenthal Box Science

Country Specific

France – no parts sourced there yet.

Switzerland


<blockquote class="twitter-tweet"><p lang="en" dir="ltr">Corsi-Rosenthal Style Air Cleaner with 5 Camfil F7 Filters, Swiss Made 🇨🇭 /cc <a href="https://twitter.com/DavidElfstrom?ref_src=twsrc%5Etfw">@DavidElfstrom</a> <a href="https://twitter.com/JimRosenthal4?ref_src=twsrc%5Etfw">@JimRosenthal4</a> <a href="https://twitter.com/CorsIAQ?ref_src=twsrc%5Etfw">@CorsIAQ</a> <a href="https://t.co/oQigkrjXb5">pic.twitter.com/oQigkrjXb5</a></p>&mdash; Markus Leutwyler 🇨🇭 (@twtomcat) <a href="https://twitter.com/twtomcat/status/1433246570837946368?ref_src=twsrc%5Etfw">September 2, 2021</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

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