The Corsi-Rosenthal (CR) Box: An Easy, Affordable DIY Air Purifier for Cleaner Indoor Air

With growing concerns over air quality, particularly in enclosed spaces, finding effective and affordable ways to filter indoor air has become a priority. One solution that’s gained popularity is the Corsi-Rosenthal box, a DIY air purifier that is both cost-effective and capable, when well built and appropriately sized, of reducing airborne particles such as smoke, dust, allergens, and particles that may carry respiratory viruses. Created by air quality experts Richard Corsi and Jim Rosenthal, this device offers an accessible way for anyone to improve indoor air quality. Here’s a look at what a Corsi-Rosenthal box is, why it’s effective, what it’s useful for, and how you can build one yourself.

What is a Corsi-Rosenthal Box?

The Corsi-Rosenthal box is a simple yet powerful air purifier that uses multiple MERV-13 filters and a standard box fan to filter indoor air. MERV-13 filters are commonly used in HVAC systems to trap fine particles like dust, pollen, mold spores, and some respiratory aerosolsTiny droplets that stay airborne and may contain infectious agents.. The setup involves attaching the filters to the sides of the fan to create a cube-like structure, allowing air to pass through the filters and helping remove harmful particles.

Unlike traditional air purifiersDevices designed to remove contaminants from the air to improve quality., which can be costly and sometimes difficult to source, the Corsi-Rosenthal box is designed to be a DIY solution that’s affordable and easy to build using common materials. It can be assembled in about 15-30 minutes and doesn’t require any specialized tools or expertise, making it a popular choice for classrooms, offices, and homes.

Why is the Corsi-Rosenthal Box Effective?

The Corsi-Rosenthal box is effective because it harnesses the power of high-quality MERV-13 filters in a configuration that maximizes airflow and particle filtrationThe process of removing particles from the air or liquids using filters.. Here are some reasons why it’s so effective:

1. High FiltrationThe process of removing particles from the air or liquids using filters. Power: MERV-13 filters are rated for particle capture across the 0.3 to 10 micron range and can reduce many fine particles, including dust, pollen, mold spores, smoke particles, and respiratory aerosolsTiny droplets that stay airborne and may contain infectious agents. that may contain viruses. These filters are especially useful for reducing fine particles that are often found in polluted air or when respiratory illnesses are circulating. ⁴MERVMinimum Efficiency Reporting Value, a scale measuring the effectiveness of air filters. ratings describe a filter’s ability to capture particles between 0.3 and 10 microns; EPA advises choosing at least MERVMinimum Efficiency Reporting Value, a scale measuring the effectiveness of air filters. 13, or the highest rating the system can accommodate, when upgrading filtrationThe process of removing particles from the air or liquids using filters.. CDC/NIOSH also recommends MERV-13 or better for central HVAC systems when compatible. EPA, “What is a MERVMinimum Efficiency Reporting Value, a scale measuring the effectiveness of air filters. rating?,” https://www.epa.gov/indoor-air-quality-iaq/what-merv-rating; CDC/NIOSH, “Improving Air Cleanliness,” https://www.cdc.gov/niosh/ventilation/prevention/air-cleanliness.html.
2. Increased Air Changes Per Hour (ACH)A measure of how often air in a space is replaced within an hour.: The Corsi-Rosenthal box improves air circulation in a room by increasing the rate at which air is filtered. This is measured in air changes per hour (ACH)A measure of how often air in a space is replaced within an hour.. A higher ACH means that the air is cycled through the filters more frequently, reducing the concentration of airborne particles and improving the room’s clean-air delivery as part of a layered risk-reduction strategy. ⁵CDC/NIOSH explains that ventilationThe process of circulating air to improve indoor air quality., filtrationThe process of removing particles from the air or liquids using filters., and air cleaning can provide air changes or equivalent air changes per hour, and that higher clean-air delivery helps reduce viral particles in indoor air. CDC/NIOSH, “How Much VentilationThe process of circulating air to improve indoor air quality. Is Enough?,” https://www.cdc.gov/niosh/ventilation/prevention/aim-for-5.html.
3. Affordability and Accessibility: Compared to traditional air purifiersDevices designed to remove contaminants from the air to improve quality. that can cost hundreds of dollars, the materials for a Corsi-Rosenthal box are often far less expensive than many commercial portable air cleaners, though actual prices vary by location, filter type, and availability. This makes it a highly accessible option for families, schools, and businesses looking for affordable ways to improve air quality.
4. Low Energy Consumption: The device relies on a box fan, which is relatively low in energy usage. When paired with efficient filters, the box provides continuous air filtrationThe process of removing particles from the air or liquids using filters. with minimal power costs.

What is the Corsi-Rosenthal Box Effective For?

The Corsi-Rosenthal box is effective in a wide range of situations and is particularly useful for:

• Reducing Respiratory DropletsLarger droplets expelled when coughing, sneezing, or speaking. and Pathogens: This DIY purifier can help reduce airborne particles that may carry respiratory viruses, such as SARS-CoV-2The virus responsible for the COVID-19 pandemic. and influenza, especially when used alongside ventilationThe process of circulating air to improve indoor air quality., masking, and other layered protections. By trapping respiratory particles, the Corsi-Rosenthal box can help lower airborne transmissionThe spread of pathogens through aerosols or droplets suspended in the air. risk in classrooms, offices, and other shared spaces. ⁶Lindsley et al. found that portable HEPA air cleaners reduced exposure to simulated exhaled aerosolTiny airborne particles or droplets that can carry pathogens. particles, with larger reductions when combined with universal masking. Lindsley WG et al., “EfficacyThe effectiveness of a treatment or intervention under ideal conditions. of Portable Air Cleaners and Masking for Reducing Indoor Exposure to Simulated Exhaled SARS-CoV-2The virus responsible for the COVID-19 pandemic. Aerosols – United States, 2021,” MMWR, PMID: 34237047, https://pubmed.ncbi.nlm.nih.gov/34237047/.
• Improving Indoor Air Quality: The box is effective at filtering out common indoor pollutants like dust, pollen, and smoke particles. This is especially helpful for people with allergies or respiratory issues, as it reduces irritants in the air that can trigger symptoms.
• Filtering Wildfire Smoke: In areas affected by wildfire smoke, a Corsi-Rosenthal box can help filter out harmful particles that enter buildings. Wildfire smoke is rich in PM2.5, fine particulate matter that can penetrate deep into the lungs, leading to health issues. MERV-13 filters in a Corsi-Rosenthal box can reduce these fine particles when the device is well built and appropriately used. ⁷Holder, Halliday, and Virtaranta tested DIY air-cleaner designs against simulated wildfire smoke in a controlled chamber and found that design choices such as filter configuration, shrouds, and filter loading substantially affected clean-air delivery. Holder AL et al., “Impact of do-it-yourself air cleaner design on the reduction of simulated wildfire smoke in a controlled chamber environment,” Indoor Air, PMID: 36437679, https://pubmed.ncbi.nlm.nih.gov/36437679/.

How to Build a Corsi-Rosenthal Box

The Corsi-Rosenthal box is designed to be easy to build with common materials. Here’s what you’ll need and a step-by-step guide on how to assemble it.

Materials Needed:

1. 4 MERV-13 Filters (20”x20” works well)

• These filters are the main component, capturing particles and providing clean airAir free of pollutants, including viruses, bacteria, and harmful particles..

1. 1 Box Fan (20”x20”)

• The box fan will draw air through the filters and circulate it in the room.

1. Cardboard or Foam Board

• Used to create a top cover to prevent air from escaping the structure and ensure it flows through the filters.

1. Duct Tape or Packing Tape

• Tape is used to secure the filters and fan together, creating a sealed box that directs airflow through the filters.

1. Optional: Zip Ties or Bungee Cords

• These can provide additional support to keep the structure stable, but they aren’t strictly necessary.

Assembly Instructions:

1. Arrange the Filters: Place the MERV-13 filters in a square arrangement, with the arrows on the filters pointing inward. This ensures that air is drawn through the filters from all sides. Arrange the filters to create a cube with an open top.
2. Secure the Filters with Tape: Use duct tape or packing tape to secure the filters together along the edges, making sure they form a tight seal so that air is forced through the filters rather than escaping around them.
3. Attach the Box Fan: Place the box fan on top of the cube, with the air intake side facing downward toward the filters. The fan should sit securely on the filters, but you can use additional tape to secure it if needed. Make sure the fan’s airflow direction is set to blow air upward and away from the box.
4. Create a Top Cover: Use cardboard or foam board to create a cover for the top of the box (the side opposite the fan). This cover helps direct air into the filters, maximizing airflow and filtrationThe process of removing particles from the air or liquids using filters. efficiency.
5. Plug In and Test: Once everything is secure, plug in the fan and set it to a medium or high speed. You should feel a strong upward flow of air, indicating that the air is being drawn through the filters and cleaned effectively.

Tips for Optimizing Your Corsi-Rosenthal Box

• Replace Filters as Needed: Over time, filters can become clogged with particles, reducing the efficiency of the box. Replace filters when they become visibly dirty, damaged, or loaded with particles, and follow the filter manufacturer’s guidance; heavy smoke or dust conditions may require more frequent replacement.
• Placement Matters: Place the box in the center of the room or in a location where airflow isn’t obstructed, allowing it to clean as much air as possible.
• Use Multiple Boxes for Large Spaces: In larger spaces or high-traffic areas, consider building and placing multiple Corsi-Rosenthal boxes for optimal air circulation and filtrationThe process of removing particles from the air or liquids using filters..

Final Thoughts: A Simple Solution for Cleaner Air

The Corsi-Rosenthal box demonstrates that effective air filtrationThe process of removing particles from the air or liquids using filters. doesn’t have to be expensive or complicated. This DIY air purifier is a powerful tool for improving indoor air quality, reducing allergens, and even lowering the risk of airborne disease transmission. As air quality concerns grow, particularly with issues like wildfire smoke and respiratory viruses, having a Corsi-Rosenthal box in your home, classroom, or office can make a significant difference in your health and well-being.

With minimal cost and effort, the Corsi-Rosenthal box makes clean airAir free of pollutants, including viruses, bacteria, and harmful particles. accessible, empowering individuals and communities to take control of their indoor environments. It’s an inspiring example of how innovation, combined with simplicity, can provide essential solutions for everyday challenges.