
When wildfires or other air quality emergencies strike, having access to clean air is essential for your health and well-being. (Be sure to check out your area’s Air Quality Index). The Corsi-Rosenthal box is a simple, low-cost DIY air purifier that can help improve indoor air quality using everyday materials. The box was invented by Richard Corsi, the dean of engineering at the University of California, Davis, and Jim Rosenthal, the CEO of filter manufacturer Tex-Air Filters. Following the guide, “Science in Action: How to Build a Corsi-Rosenthal Box” published by UC Davis Engineering, we will help walk you through the building process step-by-step to help you stay prepared and breathe more easily during emergencies.


This box is made with simple materials found in your local hardware store. The box fan pulls air through the filters on the sides and blows out clean air. This has been proven effective in reducing exposure to airborne particles, including virus-containing particles like COVID-19 and harmful wildfire smoke.
Section #1: Gather Materials
Materials List:

- 4 Air Filters: 3M MPR 1900 (20″ x 20″ x 1″or 20″ x 25″ x 1″) or MERV 13 (20″ x 20″x 2″ or 20″ x 20″ x 1″ or 20″ x 25″ x 2″ or 20″ x 25″ x 1” [2-inch preferred])
- 3M MPR 1900 20” x 25” x 1” can be found on Amazon in a 2-pack for $43.64
- 3M MPR 1900 20” x 20” x 1” can be found at Lowe’s in a 1-pack for $22.97
- *Preferred* MERV 13 20″ x 20″x 2″ can be found at Walmart in a 4-Pack for $55.98
- MERV 20” x 20” x 1” can be found at Walmart in a 2-pack for $22.58
- MERV 20″ x 25″ x 1″ can be found at Home Depot in a 1-pack for $18.97
- *Preferred* MERV 20” x 25” x 2” can be found at Home Depot in a 3-pack for $47.99


- Box Fan’s Cardboard Box or Any Available Cardboard

- Pair of Scissors

Utility Knife
- 25mm Metal Body Snap-Off Knife can be found at Home Depot for $14.25
- Westcott Safe Edge Finger Loop Ceramic Safety Cutter, Green at Office Max for $7.99
- Nimble – The One Finger Safety Cutter on Amazon for $11.95


- Tape Measure
Section #2: Form the Cube

1. Identify the Filters: Locate the four rectangular air filters, each with a long and short side.
- Understand Orientation of the Filters:
- Hold one filter so that its long side is vertical (standing upright) and its short side is horizontal (facing outward). This is the position for all filters in the box-like structure.

- Remember: There will be arrows on the filter. Feel for raised or textured markings where the arrow is printed. Ensure that they are facing inward when forming your box shape.

2. Start Forming the Box Shape:
- Position the Filters:
- Place the first filter flat on the surface in front of you with the long side facing upward and away from you.
- Position the second filter vertically on the left edge of the first, so it forms a right-angle corner to the left. The two filters should meet along their long sides.
- Place the third filter vertically on the right edge of the first filter, and mirror the position of the second filter on the left.
- Place the fourth filter across the top, connecting the tops of the second and third filters.

3. Continue Building the Cube:
- Gently trace along the edges of the filters to ensure they meet at right angles and form a symmetrical, box-like shape.
- Secure the Structure:
- Use duct tape to firmly wrap around and seal each of the four connecting edges on the outside to hold the cube-like shape in place.


Section #3: The Base

- Prepare the Base:
- Use one side of a sturdy cardboard box placed under the cube’s base.
- Cut the cardboard along the outer edges of the cube to fit snugly at the base of the cube structure.

- Attach the Base:
- Place the cardboard piece under the cube.
- Duct tape the cardboard on the outer corners to all four sides of the cube to secure it firmly.

Section #4: The Fan
- Position the Fan:
- Place the fan on top of the cube with the airflow direction facing upward, so air blows out of the top.

- Seal the Fan:
- Use duct tape to seal all edges where the fan meets the cube to prevent air leakage.
- Ensure the corners are securely sealed.

- Seal Additional Openings:
- Check for any holes or gaps on the sides of the fan housing.
- Seal these off with duct tape to maintain optimal airflow efficiency.

Section #5: The Shroud
- Cut the Shroud:
- Use a sturdy sheet of cardboard and a utility knife.
- Cut around the outside border of the fan to fit snugly over the top of the fan.
- Measure and cut a circular hole in the center of the cardboard (approximately 15 inches in diameter for a Lasko Box Fan).
- Tip to Measure a Circle:
- Use a string that is measured 7.5 inches in length tied to a pencil on one end and taped to the center of the cardboard on the other. Follow this YouTube video for further instructions.

- Attach the Shroud:
- Place the shroud over the top of the fan.
- Secure it by taping all four sides horizontally to the fan edges with duct tape.


- Plug in the device and place in the middle of a room:
- You should place a Corsi-Rosenthal box in the middle of a room on a table or cabinet to allow for efficient airflow.
- Avoid placing it on the ground, in corners, or against walls.
In times of poor air quality, a Corsi-Rosenthal Box can be a simple and effective tool to help protect your indoor environment. By following these steps, you can build a cost-effective air purifier to improve air quality and support healthier breathing during emergencies. Stay prepared, stay safe, and breathe easier knowing you have a reliable solution at hand. For more resources check out CFILC’s Disability Disaster Access & Resources Program which assists individuals with disabilities and older adults in disaster readiness and recovery.