If you need indoor air filtration and fast and in a specific area, and if you’re at all handy, there’s a DIY solution that can save you $1000 or more. The Corsi-Rosenthal (CR) Box was developed primarily by Richard Corsi, an indoor air quality specialist and Jim Rosenthal, a filtration company CEO during the COVID19 pandemic. The CR box consists of a simple box fan with filters taped all around it without air gaps. If the filters are taped securely and properly, the box fan pulls and “forces” air through the simple filters. In this case, the sum is more powerful than the parts and impressive filtration is achieved, as using multiple filters reduces airflow resistance due to the greater surface area.
Check out the video above where Green Builder Matt Hoots of Sawhorse Inc. shows how to build a CR box. Matt wanted to make sure that dust levels were appropriate for occupants and a sensitive pet, for an ongoing project that was just about complete. Matt also had concerns that a flooring materials choice by the client may have led to higher VOC levels at this time while the product cures.
The CR story is a positive example where people invented and then shared a helpful product in a time of need, and where many citizen scientists then tried out the product and spread the word to others. After it was clear that COVID was an airborne illness, air purifiers were tougher to get, and many residents, schools, and small businesses couldn’t afford expensive equipment. Richard Corsi and Jim Rosenthal showed that the airflow from a simple box fan, when surrounded by MERV 13 filters, could filter at near HEPA levels for a heck of a lot less money. Some good news is that box fans are readily available and are not expensive, and yet they offer good power for the investment.
MERV stands for Minimum Efficiency reporting value. MERV runs on a scale of 1 to 30 where higher numbers mean the filters can trap and remove ever smaller particles. MERV 13 and higher is considered high efficiency filtration. A MERV 13 filter will capture more than 90% of larger particles, such as pollen pet dander, dust mites, lint and most household dust. MERV 13 filters capture more than 85% of medium sized particles, which might be bacteria, respiratory aerosols, mold spores and fine dust. MERV 13 will catch about half of small particles, like viruses, smoke, the finest dust particles, and combustion particles. Higher filter levels exist but Above MERV 13, filters create airflow resistance and can strainHVAC system motors. So these are typically not practical for most home environments. Even to achieve MERV 13 filtration, some systems will require a retrofit - so a CR box may be a cheaper option, and/or a temporary option, if it can work for your space.
The CR box idea spread in 2021 through word of mouth and due to the fact that the instructions were easy enough. This is a great example of open source ideas and community sharing. Eventually in 2022 and 2023, both the CDC and EPA determined that these DIY cleaners performed as promised and beyond. The boxes were included in school COVID mitigation guidance documents. Some schools and communities even hosted mass community builds of the boxes.
Why didn’t these “kits” become more mainstream?
The thing is, the boxes work. They can save money and are great options for several situations that might be urgent and/or temporary and where saving money is a desirable enough outcome in addition to achieving the result. If you’re worried about looks, the boxes move, and you can certainly use them in less traveled spaces like a basement.
A Corsi-Rosenthal Box is helpful in many situations by improving indoor air quality (IAQ) in specific areas of a building. The CR box will reduce airborne dust and other particles. In particular, a CR box can help when pollutants are present that a MERV 13 filter can capture. Examples of situations where the CR box can help reduce irritants and pollutants include:
A key point is that CR boxes can help make clean air more accessible and equitable. This is something one can build - and not have to buy. And it can have a long life with filter changes every few months, depending on the volume of dust and other irritants and pollutants. In the case where a home already has mechanical ventilation, the CR box can reduce the load of an existing mechanical HVAC system by addressing a particular area.
The boxes compete well in practice with retail equipment, but note they generally aren’t whole home solutions. That said, you can clear one area and move the CR box around as needed. They do compete with other stand-alone area solutions. A fairly simple IAQ monitor can help you determine how the box is working and whether an when you might want to move it to another area if needed.
What you will need:
(These instructions paraphrase information from the video, which will be easier to follow.)
In the video, Matt shows a fairly reduction of PM 2.5 from an original 52 to 29, 12, and then down to 4 in 3 hours. He also tests for VOCs over a new gym floor in another room (the client chose the material!). The home has excellent mechanical ventilation, including an ERV. But the CR box can show what’s causing current challenges with the higher irritant and pollutant levels of particulates that come with construction.
Have you built a Corsi-Rosenthal box? What irritants or pollutants were you trting to reduce, and what was the situation? How did it go? Are there any success stories or lessons learned you can sharre?
It's impressive what a simple box fan can do as part of a Corsi-Rosenthal box.
Taping the filters together.
Creating a square or cube out of the filters.
A clever use of the box fan packaging. 
Sealing the box fan to the top of the Corsi-Rosenthal box.
