Four filters, a box fan and tape
Four or five HVAC particulate air filters form a cube, with a box fan drawing air through them. Duct tape seals the seams. Engineers Richard Corsi and Jim Rosenthal designed it during the COVID-19 pandemic to lower the risk of infection by cutting the levels of airborne viral particles (opens in a new tab) indoors.
Clean air for a tenth of the price
A 2022 study (opens in a new tab) found that the five-filter design delivers 600 to 850 cubic feet per minute of clean air, depending on fan speed, for roughly a tenth of the cost of commercial air filters.
Which fan
Caution
EPA and UL tested (opens in a new tab) box fans made since 2012, which have fused plugs and thermal cutoffs, with clean filters and with filters loaded with smoke and dust. None overheated or showed a fire hazard. Fans made before 2012 were not tested; EPA does not recommend them for these boxes, or running one unattended or while sleeping.
Watch someone build one
The videos below walk through the build, one of them with co-inventor Richard Corsi.
Videos
Science in Action: How to Build a Corsi-Rosenthal Box (opens in a new tab)
Learn how to build a Corsi-Rosenthal box. The device was created to provide significant reduction in the amount of virus-laden, aerosol particles that are in the air. Follow along as Dean Richard L. Corsi, co-inventor, shows you the major components of this device, and how to build one.
The research
14 papers chosen for this practice, each with its source and summary. After them, every study in the database that mentions inexpensive air filtration.
The review highlights the critical role of heating, ventilation, and air conditioning (HVAC) systems in controlling the spread of COVID-19, emphasizing the need for improved air filtration and ventilation strategies to ensure indoor air quality and reduce infection risks.
Key points
- HVAC systems can contribute to the spread of COVID-19 if not properly managed.
- Indoor air quality is crucial; ventilation can dilute airborne pathogens.
- Upgrading filters to MERV-13 is recommended to reduce virus transmission.
- Continuous operation of HVAC systems is essential for maintaining air quality.
- Outdoor air circulation before and after occupancy can mitigate risks.
- Portable HEPA air cleaners can supplement ventilation in poorly ventilated spaces.
- Avoid recirculation of air in HVAC systems to prevent airborne transmission.
- Regular maintenance of HVAC systems is necessary to ensure effectiveness.
- Implementing UVGI can enhance air disinfection but should not replace filtration.
- Social distancing and mask-wearing are still critical alongside ventilation strategies.
Air filtration and SARS-CoV-2 (opens in a new tab)
Effective air filtration is crucial in preventing the spread of COVID-19, as SARS-CoV-2 can remain airborne longer than previously thought, making it essential to understand and properly utilize air filter technologies like HEPA filters to enhance indoor air quality and health.
Key points
- Air filtration is crucial in controlling the spread of COVID-19.
- Proper understanding of air filtration principles is essential for effective use.
- SARS-CoV-2 can remain airborne longer than previously thought.
- HEPA filters are recommended for effective air filtration.
- Masks and air filtration systems are vital in managing COVID-19 spread.
- Environmental conditions affect the viability of SARS-CoV-2 in aerosols.
- Air pollution is linked to higher COVID-19 mortality rates.
- Effective air filtration can improve health outcomes for those exposed to COVID-19.
Characterizing the performance of a do-it-yourself (DIY) box fan air filter (opens in a new tab)
The DIY Corsi-Rosenthal Box air filter, which uses MERV-13 filters and a box fan, effectively reduces indoor air pollution and offers a cost-efficient alternative to commercial air purifiers.
Key points
- DIY Corsi-Rosenthal Box air filter effectively reduces indoor particle concentrations.
- Clean air delivery rate (CADR) increases with fan speed, reaching up to 850 ft³/min.
- Cost-efficient: CADR is <$0.072 per ft³/min, much lower than commercial HEPA filters.
- Noise level ranges from 58 dB (low speed) to 67 dB (high speed).
- Low-cost sensors can provide reliable performance measurements for DIY air filters.
Improving indoor air quality through proper ventilation, filtration, and monitoring can significantly reduce the risk of COVID-19 transmission and other respiratory illnesses.
Key points
- Proper ventilation and air filtration are crucial to prevent COVID-19 spread indoors.
- Disinfecting surfaces is less effective than improving air quality.
- COVID-19 primarily spreads through inhaling aerosols, not surface contact.
- Regularly replacing indoor air can dilute virus concentration.
- HEPA filters and high MERV-rated filters are recommended for effective air purification.
- Monitoring CO2 levels can indicate air quality and ventilation effectiveness.
- Germicidal UV lights can help kill airborne viruses but must be installed correctly.
- Economic costs of infections can outweigh the costs of implementing air quality controls.
Corsi-Rosenthal Box: How to Make an Air Filter With a Box Fan (opens in a new tab)
A homemade air filter made from a box fan, cardboard, tape, and furnace filters, known as the Corsi-Rosenthal box, may be an effective and affordable way to improve indoor air quality by removing pollutants and viruses like COVID-19, according to early research.
Key points
- A good filtration system can help protect you and your family from allergens, indoor pollutants like smoke, and viruses like COVID-19.
- Richard Corsi, PhD, of the University of California-Davis and Jim Rosenthal, CEO of Tex-Air Filters, have created a type of homemade air filter that costs less than a third of the price of many HEPA devices.
- Some early research suggests these DIY air cleaners, made from cardboard, tape, furnace filters, and a box fan, might be more effective in some ways than HEPA purifiers.
- Experts say they could offer a simple solution to improve air quality in our homes, classrooms, and workplaces, which aren’t always well-ventilated.
- One study looked at how well Corsi-Rosenthal boxes removed chemicals from the air in an academic building at Brown University. In the 2-month period when the boxes were used, concentrations of certain PFAS (chemicals found in cleaning products and textiles) went down by 40%-60%. At the same time, concentrations of phthalates (from personal-care products and building materials) went down 30%-60%.
- According to the Environmental Protection Agency (EPA), the best ways to improve indoor air quality are to remove sources of pollutants (like cleaning chemicals or cigarette smoke) and ventilate your space with outdoor air.
COVID-19 and importance of air filtration (opens in a new tab)
Air filtration, avoidance of recirculation of air, enhanced particle filtration by suitable air filters, and air disinfection can control COVID-19 infection transmission in indoor spaces, and the use of HEPA filters, UVC light, and natural ventilation are effective methods for reducing the spread of the virus.
Key points
- Effective ventilation, avoidance of recirculation of air, enhanced particle filtration by suitable air filters and air disinfection can control COVID-19 transmission.
- Opening windows can increase natural ventilation and dilute indoor air, reducing infection spread.
- Indoor air purifiers with high efficiency particulate air (HEPA) filters can filter contaminated indoor air, but must be replaced regularly.
- Far-UVC light is recommended for single rooms without air conditioning, while a combination of HEPA filter and UVC light can reduce bacterial and virus load significantly in air-conditioned rooms.
- The supply or outdoor air should be filtered by a filter and then mixed with the recirculated air and supplied to the room through the HEPA filters.
- The ASHRAE recommends HVAC systems with a minimum efficiency of Minimum Efficiency Reporting Value-13 (MERV-13) to filter even very small particles.
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DIY box fan filters – Corsi-Rosenthal box (opens in a new tab)
The Corsi-Rosenthal box is a cost-effective DIY air filter made from MERV13 furnace filters and a box fan that can help clear indoor air from airborne virus particles, wildfire smoke, pollen, dust, and more.
Key points
- DIY box fan filters can help clean indoor air from airborne virus particles, wildfire smoke, pollen, dust, and more.
- The Corsi-Rosenthal box is a cost-effective and efficient air cleaner.
- The box fan filter can be built using a box fan and MERV13 filters.
- There are various modifications and designs available for the box fan filter.
- The box fan filter has been used in classrooms and homes to improve indoor air quality.
Using a relatively low-cost “do-it-yourself” air filter (Corsi-Rosenthal Box) can reduce indoor air concentrations of certain chemicals, including PFAS and phthalates, which are commonly found in office building materials and products, but may result in slightly higher sound levels.
Key points
- A low-cost “do-it-yourself” air filter (Corsi-Rosenthal Box; CR Box) reduces exposure to several lower-volatility phthalates and sulfonated PFAS previously found in office building materials and products.
- Concentrations of seven PFAS and five phthalates were 28-62% lower while CR Boxes were deployed.
- Concentrations of five PFAS and one phthalate increased 23-44% during the intervention period, but the 95% CI of most of these estimates included the null.
- Daytime sound levels increased 5.0 dB when CR Boxes were operating.
Effectiveness of HEPA Filters at Removing Infectious SARS-CoV-2 from the Air (opens in a new tab)
HEPA filters can effectively remove infectious SARS-CoV-2 from the air, providing insights into the proper use and performance of air filtration technologies to prevent the spread of COVID-19.
Key points
- HEPA filters can effectively remove infectious SARS-CoV-2 from the air.
- HEPA filters with an antiviral agent coating have a capture ratio comparable to conventional HEPA filters.
- Proper use of HEPA-filtered air cleaners can help prevent the spread of COVID-19.
Corsi-Rosenthal boxes, a DIY indoor air filter, were found to be effective in reducing a wide range of volatile and semi-volatile organic contaminants, according to a study that employed non-targeted and suspect screening analysis using high resolution mass spectrometry.
Key points
- Corsi-Rosenthal boxes, a do-it-yourself indoor air filter, may have potential co-benefits of reducing indoor air contaminant levels.
- Do-it-yourself Corsi-Rosenthal boxes are an effective means for improving indoor air quality by reducing a wide range of volatile and semi-volatile organic contaminants.
A study has shown that biocide treated air filters can effectively kill bacteria, fungi and viruses, including SARS-CoV-2, and maintain their structure and filtration function while also being durable and maintaining antimicrobial activity throughout their operational lifetime, making them a potential tool to prevent airborne transmission of respiratory diseases in public spaces such as public transport.
Key points
- Antimicrobial and anti-viral coated air filters can be effective in preventing the spread of airborne pathogens.
- The study was funded by the Wellcome Trust ISSF award, Royal Academy of Engineering Award, and an MRC Proximity to Discovery award.
- The authors acknowledge financial support for this work by the Engineering and Physical Sciences Research Council (EPSRC) Grant codes: EP/P029868/1 and EP/S021434/1.
- The article is licensed under a Creative Commons Attribution 4.0 International License.
- The study was conducted by researchers from the University of Birmingham, University of Nottingham, and NitroPep Ltd.
Indoor Air Changes and Potential Implications for SARS-CoV-2 Transmission (opens in a new tab)
Increasing ventilation and filtration in indoor spaces can help reduce the risk of airborne transmission of infectious agents, including SARS-CoV-2, and should be considered as a fundamental part of risk reduction strategies.
Key points
- Increasing air changes per hour and air filtration can help reduce the risk of airborne transmission of respiratory infectious diseases like SARS-CoV-2.
- Higher ventilation and enhanced filtration are often overlooked but fundamental parts of risk reduction strategies that could have benefits beyond the current pandemic.
The Homemade Air Purifier That’s Been Saving Lives During the Covid-19 Pandemic (opens in a new tab)
The Corsi-Rosenthal box, a simple and inexpensive air purifier made from furnace filters and a box fan, has been effective in reducing airborne transmission of Covid-19 and is an example of how grassroots innovation and citizen engineering can have a significant impact in times of crisis.
Key points
- The Corsi-Rosenthal box is a simple, inexpensive, and effective air purifier that has been used to fight Covid-19.
- It consists of furnace filters and a box fan, and can be assembled with minimal technical expertise and $60 to $90 in materials.
- Corsi-Rosenthal boxes are more effective than placing a single filter in front of a box fan, and can supplement existing ventilation systems in shared spaces.
- The Corsi-Rosenthal box was developed through a grassroots effort that drew on the knowledge and efforts of a wide range of contributors from different backgrounds and organizations.
- Grassroots innovation has promise, but can also face challenges such as lack of technological sophistication and resources, rules and regulations, and sustaining effort.
Ventilation in Buildings (opens in a new tab)
Proper ventilation, including increasing outdoor air delivery, filtration, and air cleaning, can help reduce the concentration of viral particles in indoor spaces and lower the risk of exposure to COVID-19.
Key points
- Indoor air movement and dilution of viral particles through mechanical or nonmechanical means can reduce the airborne viral concentrations and the overall viral exposure to occupants.
- Ventilation system upgrades or improvements can increase the delivery of clean air and dilute potential contaminants.
- Implementing multiple building-level mitigation strategies at the same time is consistent with CDC’s layered approach and will increase overall effectiveness of ventilation interventions.
- Ventilation interventions can reduce the spread of disease, but they will not eliminate risk completely.
- Improving air circulation and cleanliness can help reduce viral particle concentration.
- Aim for 5 or more air changes per hour (ACH) of clean air to help reduce the number of germs in the air.
- The ventilation interventions listed above come with a range of initial costs and operating costs.