Tips on building a Corsi-Rosenthal Box for an airborne pandemic

Corsi-Rosenthal box materials

I just built a Corsi-Rosenthal Box. You might be thinking “what?” Well, it’s a simple and inexpensive DIY air filtering device that helps remove airborne viruses, wildfire pollution, pollen, dust, etc. from indoor air. Of particular note: the filters used are good enough to remove COVID-19 aerosols from contaminated air. Also, it’s incredibly easy to build and inexpensive. My Corsi-Rosenthal Box took less than an hour to make using the readily available supplies shown above. And it cost just $94 plus a dollar or two for duct tape.

That compares favorably with the $230 sleek Coway Air Purifier, shown below, that I purchased at the start of the COVID-19 pandemic. Especially since the latter is rated at ~250 cubic feet per minute (CFM) while mine provides ~350 CFM.

My air filter is 40% of the price of the Coway yet provides 40% more ventilation!

To be fair, mine is larger — and uglier. Here it is!

Even though I’d never made one before the air cleaner took less than an hour to put together. If I did it again, I think I could build one in less than 30 minutes.

Materials, tools, and basic construction

Corsi-Rosenthal box materials
All the materials I used are shown above. Here’s the list:

The only tools I needed were a Swiss army knife, scissors, a pencil, and a loop of string. (Oh, and a thumbtack.)

Because there are already good resources available for choosing components and building a Corsi-Rosenthal Box, I’m not going to replicate them here. Instead, here’s what I found most helpful:

These resources cover everything you need to know to build inexpensive and effective air cleaners, plus a little interesting history of how they came to be.

Tips on building a Corsi-Rosenthal Box

While building my box I learned some little details that aren’t covered in the core resources above. None of them are vital, but they might help you. Here they are:

Making the box as square as possible

Corsi-Rosenthal box taped filters

How should the four filters be duct-taped together? See the picture above, which shows the seam arrangement I used to create a square shape. I found that 2″ duct tape worked well for the entire project.

You probably don’t need extra cardboard!

I purchased my fan and filters online, and both got shipped in boxes that are the perfect size to provide the cardboard you need. I used the Lasco fan box for the box base (see below), as well as four pieces needed to seal the corners of the fan mounting and the fan shroud (see next two sections). Very little cardboard was left!

Corsi-Rosenthal box bottom cardboard

Note that I sealed the bottom of the box along the filter edges and added four additional short strips of duct tape at each corner to reinforce the construction.

Creating four cardboard corner arcs to seal the fan mounting and the fan shroud

Here’s how I marked up the other side of the Lasco fan box to create four corner arcs that sealed the fan mounting and the fan shroud (see next section).

I placed the fan in the center of the cardboard and drew around its edge, creating the outside pencil line as shown below.

After removing the fan, I used a tape measure to find the center of the cardboard and inserted a thumbtack. Then I made a loop of string to draw a circle to cut out for the air exhaust portion of the fan shroud. (Look at the earlier photo of my finished unit to see what the fan shroud looks like.) The right length for the loop will depend on the fan you use. Here are the optimum fan shroud openings (with the 3M Filtrete 1900 filters I used) for two common fans:

So for my Lasco fan, I used a 15″ loop of string to draw a central circle with a 7.5″ radius.

After marking the cardboard I cut the four fan seal corner arcs from the cardboard corners with my Swiss Army knife and duct-taped them in position as shown below.

I then cut out the fan shroud.

Final assembly

All that remains at this point is to place the fan on the top of the box and seal it to the filter edges and the corner cardboard arcs with duct tape. Finally, tape the edges of the fan shroud to the fan.

You’re done! Here, again, is the finished unit.

This Corsi-Rosenthal Box is quiet, even on maximum fan speed, and noticeably more powerful than my commercial Coway unit. I live in a very airtight home — my CO₂ meter shows significantly higher readings when just the two of us are there. So I am happy to have this second unit available when folks visit. Building it was fun and easy. Recommended!


An affordable air quality tool for meeting planners

air quality tool meter CO2
The COVID-19 pandemic has devastated the in-person meeting industry. Though it took too long to recognize that COVID-19 spreads via air transmission, we finally have effective procedures (vaccine mandates, masking, air quality standards, and social distancing) to reduce infection risk at in-person meetings. Now, meeting planners can add an affordable air quality tool to their site visits.

How can you determine air quality at a prospective venue?

Look around the room at an in-person event and you’ll see if masking and social distancing are taking place. We can implement vaccination mandates using third party vendors such as, CLEAR Health Pass, Safe Expo, and others. But how can we determine the air quality at a prospective venue?

Currently, we don’t know how to detect airborne COVID-19 viruses. (This is likely to be true for a long time. We still have no test for airborne tuberculosis bacterium (TB) transmission two centuries after identifying TB as a distinct disease.)

Luckily, under conditions I’ll outline below, we can obtain useful information about a venue’s air quality by using a device that measures a proxy for air pollution: carbon dioxide (CO₂).

People breathe in air, typically containing about 0.04% CO₂. They breathe out a mixture of gases containing about 4 – 5 % CO₂. People with COVID-19 co-exhale respiratory aerosols containing the SARS-CoV-2 virus.

If an occupied building space has effective ventilation, the occupants’ excess exhaled CO₂ is quickly diluted with fresh air, and the CO₂ level in the air remains close to normal values. Measuring the level of CO₂ in the air can, therefore, tell us whether effective ventilation is present or not.

Here are the generally accepted standards for CO₂ levels:

~400 parts per million (ppm) – Normal outdoor air level.
400 ~ 1000 ppm – Typical value level indoors with good ventilation.
1,000 ppm – the OSHA/ASHRAE recommended maximum level in a closed room.
> 1,200 ppm – Poor air quality – requires ventilation to the room.
2,000 ppm – This level of CO2 typically produces a significant increase in drowsiness, tiredness, headaches, lower levels of concentration, and an increased likelihood of spreading respiratory viruses.

Until recently, meters that measure CO₂ levels in the air cost hundreds of dollars. (Some models with especially accurate sensors or the capability to measure other air pollutants still do.) But today we can buy an affordable air quality tool — a hand-held CO₂ meter for under $100. The one I just purchased (illustrated above) cost $80, and there’s a wide variety to choose from (for example, from here or here).

My 3.27″ (diameter) x 1.26″ (depth) meter measures CO₂ levels from 0 – 5,000 ppm. It can run on standby for 18 hours, supports USB charging, and includes a battery level indicator and temperature and humidity readings. While its specifications omit accuracy, inexpensive CO₂ meters are typically reliable within ±100 ppm. This is good enough to provide a decent estimate of the air quality in an enclosed space.

My unit shows a concentration of ~350 ppm CO₂ outside my rural Vermont home, which was built tightly. In my home office, the level increases to about 450 ppm, and rises to about 525 ppm if I’m sitting next to the unit for a while. Slightly cracking open a window quickly brings down the reading.

I haven’t had time to explore other buildings yet, but am looking forward to seeing what I find out when I do.

Is a CO₂ a proxy for indoor air quality in occupied spaces?

Can measuring CO₂ levels give us a useful indication of indoor air quality?

The answer is a qualified yes. It depends!

First of all, we need to measure CO₂ levels in occupied spaces. A meeting planner doing a site visit should take CO₂ readings in occupied meeting rooms, restaurants, hotel lobbies, etc. Taking measurements in empty spaces will only show high readings if the building ventilation system is grossly inadequate (with CO₂ infiltrating from other areas.) Also bear in mind that increasing the number of occupants in a space increases the likelihood that an infectious person will be present and the number of people possibly infected. Doubling occupancy can thus cause a four-fold increase in risk of transmitting COVID-19.

Second, there are sources of CO₂ that are not related to human exhalation but will increase meter readings. A common source is combustion emissions such as gas stoves, which can significantly increase CO₂ levels. Pets can also increase CO₂ levels, though animals are unlikely to be sources of the COVID-19 virus. Such sources will cause increased levels of CO₂ without increasing the incidence of COVID-19 transmission.

Finally, air treatment options, such as MERV 13 or better filtering, or possibly ultraviolet-C radiation, may reduce the prevalence of active COVID-19 aerosols. When venues employ these mitigation strategies, CO₂ levels will not be decreased. Of course, if a venue has deployed these preventative measures, they will surely inform you about them when asked!

Due to these factors, you shouldn’t rely solely on measurements of CO₂ levels to determine whether a space is ventilated enough to mitigate transmission risk.

However, a simple CO₂ meter like the one I now own can be an effective air quality tool, providing valuable information to anyone who wants to investigate the air quality of occupied spaces at venues, hotels and properties, restaurants, and other meeting locales. I’ll be bringing mine when I travel, and I encourage you to do this as well!

More information on the relationship between CO₂ levels and COVID-19 exposure

If you’d like to learn more about the relationship between CO₂ levels and COVID-19 exposure risk, here are some useful references:

And here are some less technical media articles on CO₂ meters:

Feedback Frames—a low-tech tool for anonymous voting

Feedback+Frames+open+with+coins Jason Diceman is developing a novel tool for anonymous voting — Feedback Frames.

Unlike high-tech audience response systems, Feedback Frames are refreshingly low-tech (no computers, clickers, smartphones, power, or technical support required). One graphic explains the tool:



Although I tend to prefer public and semi-anonymous techniques for the participatory voting I use extensively in my facilitation practice, Jason’s approach is a refreshing alternative to the complex (and typically expensive) high-tech ARS methods routinely used for anonymous voting at meetings.

Jason created an even lower-tech (free!) tool Idea Rating Sheets in 2004 (originally called “Dotmocracy”) to “make it easier to find agreements in large groups”. He has been a Senior Public Consultation Coordinator for the City of Toronto since 2010. Jason is working to crowdfund his invention. His FAQ should answer all your questions. Visit his website to learn more.

Convenings 2.0: Connecting adult learning, communication strategies and event logistics to build stronger relationships

Convenings 2.0

What’s not to like about Convenings 2.0?

“The W.K. Kellogg Foundation is a relationship-based and an event-based organization. We love to bring our grantees together so they can learn, network and share best practices.”
Sterling Speirn, President and CEO, W.K. Kellogg Foundation

Hot off the press! You can download for free the beautifully designed Convenings 2.0 report (click on the report cover above). Convenings 2.0 describes a wealth of thoughtful approaches, proposals, and standards for meetings hosted by the W.K. Kellogg Foundation for its grantees. Containing extensive research by Carol and Mike Galle, and Sharon McMurray of Special D Events, I believe this document deserves wide circulation to associations, foundations, and event and association professionals.

Thirty-five subject matter expert contributors from foundations, the meeting industry, and adult learning/academia contributed to the report, including my friends and colleagues Mitchell Beer, Michelle Bruno, Sandy Heierbacher, Carolyn Ray, Maarten Vanneste, and myself. The bibliography includes books and reports by Joan Eisenstodt, Jackie Mulligan, Maarten, myself, and many others.

The report scope includes meeting and event logistics, knowledge management, integrated communications, technology, foundation considerations, design and execution, communication and branding, a set of twenty-one recommendations, and an outline of design, execution, and evaluation process, with an appendix covering adult learning theory and its application to meetings and a glossary. Apposite quotes are sprinkled throughout the fifty-two pages.

There’s something for everyone here, folks. It’s well worth reading!