From the early days of the COVID pandemic, Corsi-Rosenthal (CR) boxes thrust filtration into the spotlight of indoor air quality (IAQ).
While all high-efficiency filtration lowers viral load by mechanically removing airborne particulates, these DIY boxes popularized high-performance filtration with affordable, easy-to-source materials (furnace filters and a box fan).
In commercial settings, the primary metric for filtration performance is efficiency. Specifically, first-pass efficiency: the percentage of particles of a specific size range that are removed with one pass through the filter.
This focus on efficiency is why HEPA filters have been the gold standard in the IAQ industry.
It’s an easy pitch for a sales team: Our filter is lab-tested to remove 99.97% of particles, including COVID!
The problem is that a cheap CR box significantly outperforms many HEPA filters in real-world tests.
What! How? Why!
Because focusing on efficiency misses half of the equation. Clean air delivery rate (CADR) is what really matters: that’s efficiency times airflow.
Wha- why does airflow matter?
Because a filter only cleans air that physically moves through it. Higher airflow means a unit processes more air per hour.
It’s like a kid and an adult stealing from a cookie jar. Sure, the adult can hold more cookies in one hand, but those kid hands are relentless— the kid grabs one cookie at a time but they do it again, and again, and again.
HEPA is the adult. A high removal rate but they don’t reach into the jar as often.
Note to new parents: this is why we put the cookie jar out of reach.
No. Well, maybe. Sometimes.
CR boxes are a DIY solution that’s affordable and accessible. They’re great for spot filtration during periodic, high-risk events like wildfire smoke, sick family members, creating in-house isolation rooms, etc.
They’re cheap and effective.
But they’re also large, noisy, and inconvenient– those drawbacks are manageable for short periods but deal breakers for a long-term solution in a commercial space.
One of the lasting legacies of the COVID pandemic will be the public’s interest in IAQ. We’re at the edge of a paradigm shift in realizing the importance of clean air in our communities. A similar shift happened when cholera and other waterborne diseases changed how we test, monitor, and treat water.
Improving water quality was required for public health.
We need to think the same way about clean air.
But to provide clean air at scale, we need affordable products with standardized, replaceable parts with low noise, high CADR, and good designs to operate conveniently in different spaces.
We need C.L.E.A.N. filters.
I see what you did there. Clean air. Cute.
Components, Location, Efficiency, Airflow, and Noise.
That’s the measure of a good filter.
Most standalone HEPA filters excel in one category. The original CR boxes are competent in three.
High-end residential HVAC covers all five but low-end systems barely do one.
Listen, I love acronyms as much as the next person, but what does it *mean*?
At its most basic, filtration is simply airflow through a filter to remove particles. That covers a range from CR boxes to engineered HEPA systems.
All of those filters have four (4) key components: frame, electronics, fans, and the media filter. Each component varies from proprietary and custom to open-source and off-the-shelf.
CR boxes are popular because the components fall into the latter category. Every part is available at your local hardware store.
Proprietary and custom parts are better, right? RIGHT?
For the filter company, yes. It ties consumers to one company for higher ongoing costs with expensive replacement filters and repairs.
It’s a captive market.
The antithesis is the residential furnace filter market with standard sizing and competition driven by the consumer’s ability to switch brands.
Commercial filter replacements can be 10 times more expensive than residential filters.
The ideal filter uses commodity components with replaceable parts and residential furnace filters to reduce costs while maintaining performance.
Schools have different use patterns and requirements than houses, small businesses, or medical treatment rooms.
CR boxes are good options in houses because noise levels and floor space aren’t as valuable. Many homes have central HVAC systems that easily handle the filtration requirements for low occupancy.
Schools are high-occupancy spaces with lots of traffic, limited floor space, and less patience for noise.
The physical placement in a room is also important.
Ah, so what is the best place for a standalone filter?
That’s easy– it’s the place that doesn’t interrupt anyone in any way.
See, a partially obstructed or poorly placed filter has a reduced CADR or creates dead zones with poor distribution of clean air in the room.
But that’s nothing compared to inconvenience. An inconvenient filter gets unplugged, and an unplugged filter has a CADR of zero.
This is the “hot take” section but I feel a sense of conviction because of research by Jim Rosenthal, David Elfstrom, and Joey Fox.
The industry holds first-pass efficiency as the gold standard for filter performance.
But that wisdom is wrong.
And it leads to the wrong solutions.
The removal rate of particles (i.e. the underlying goal of filtration) is driven by CADR not efficiency.
So do all HEPA filters have bad CADR?
Not at all! Hospitals and operating rooms with high-risk, contagious diseases use engineered HEPA systems to achieve 99.99% first-pass efficiency with high airflows.
HEPA filters are also great solutions in vacuum cleaners, powered air-purifying respirators, and extreme conditions like capturing radioactive particles from research lab exhausts.
But a standalone HEPA filter with low airflow has poor CADR and underwhelming performance because it doesn’t process enough air.
I’m convinced. It sounds like we shouldn’t care about efficiency at all.
Intuitively we understand the need to stay above a minimum efficiency threshold; a lonesome box fan doesn’t clean the air. After all, CADR equals efficiency times airflow so we need a non-zero removal rate to make it work.
I get it, a baby doesn’t steal any cookies from the cookie jar.
MERV-11 is the lower limit. Below that, a filter has minimal ability to catch respiratory particles. I mentioned low-end HVAC systems earlier: a standard furnace includes a MERV-8 filter. For our purposes, that’s a CADR of zero.
The other side of the CADR equation is airflow. Airflow is easy to increase but needs to be balanced with noise levels.
Standalone HEPA filters are plagued by noisy operation. They usually have three or more fan speeds and the performance is tested on the highest speed. That produces a great CADR for marketing brochures but it doesn’t reflect real-world performance.
Does airflow really matter as much as efficiency?
Totally. And it’s easier to hit.
Tests from Jim Rosenthal show that MERV-13 with higher airflow significantly outperforms HEPA filters.
The downside to airflow is noise.
The best-case scenario for a noisy filter is getting switched to a lower fan speed. The worst-case is getting turned off or unplugged.
In the fight between noise and airflow, consumers choose quiet operation over performance.
Indoor environmental quality (IEQ) wins because the annoyance (noise) has an actionable fix with immediate satisfaction. That easily beats the invisible benefits of IAQ.
Filters get tested for noise, don’t they?
Yes, but the results are a range based on the different fan speeds and marketing materials focus on the lowest speed for noise levels.
Filters need to be tested for CADR and noise at the fan speed that’s used in the field (the lowest speed).
The burgeoning community science market for CR box development gave us a brilliant solution: PC fans.
PC fans are performance-optimized, quiet, commoditized, and single-speed.
Check, check, check, and check.
We need to adjust our lens for high-quality filters, shifting from efficiency to effectiveness.
But, aren’t those the same thing?
Efficiency is a single variable. Effectiveness reflects the full scope of the CLEAN framework that’s rooted in real-world performance, occupant behaviours, and accessibility.
An effective filtration system uses replaceable components with affordable filters to ensure constant air cleaning with dependable results. It’s a system with low noise and high CADR that operates in various settings without disrupting occupants.
We’ve been focusing on the wrong metric for healthier buildings.
Particulate filtration happens one way or another. Either communities, companies, and individuals invest in effective solutions or that role falls to our lungs.
On average, we spend 90% of our time indoors.
Imagine that. When you turn 40, you’ve spent a combined 36 years in buildings.
And now consider the compounding benefits of fewer sick days, lower absenteeism, and increased productivity from cleaner air. School boards, businesses, and countries that prioritize clean air will see world-changing benefits over time.
The next time you’re buying or recommending an air filter, make sure it’s CLEAN so we can take definitive steps toward cleaner air and healthier people.
