As climate change makes wildfires more extreme, people are increasingly living with smoke. The number of people residing in areas that experience at least one day of dangerous smoke pollution per year has increased 27-fold over the last decade, according to research led by Stanford scientists.

A DIY air purifier called a Corsi-Rosenthal box can help clean the air in your home, making it safer for you and your family to breathe. The boxes cost under $100 to build, less than half the price of commercial air filter units.

The design was originally created by Richard Corsi, the dean of engineering at the University of California, Davis, and Jim Rosenthal, the CEO of filter manufacturer Tex-Air Filters, to help reduce the spread of COVID-19. The Corsi-Rosenthal box uses four air filters and one box fan. To save space and money, some people build a filter with a box fan and a single air filter.

Listen: Breathe easier during wildfire season with a box fan and single air filter

https://traffic.libsyn.com/secure/climateconnections/CX230116.mp3

I don’t live in an area with many wildfires, but I built a Corsi-Rosenthal box to help my friends gather safely during the pandemic. Here’s how I did it.

Step 1: Get your supplies

To build a Corsi-Rosenthal box, you’ll need:

  • One 20-inch box fan
  • The cardboard box the fan comes in
  • Four 20×20-inch MERV 13 filters
  • One roll of duct tape
  • Scissors or a box cutter

The Environmental Protection Agency recommends using a newer-model box fan, as they have better safety features. Fans made before 2012 pose fire risks. I found my supplies at my local hardware store.

Step 2: Duct tape the filters together to form the sides of a cube

The side of the filters should have arrows on them. Make sure that the arrows point to the cube’s center. Tape over the full length of the corner where the filters meet to create a seal. A well-sealed Corsi-Rosenthal box ensures that air is forced to go into the box through the filters before being pushed out of the fan.

Step 3: Cut a square of cardboard and tape it to the bottom of the filter cube

The base of a typical Corsi-Rosenthal box is a piece of cardboard. I cut my cardboard square out of the box the fan came in. For extra filtration power, some people put a filter as the base of the cube. If you choose to make that variation, you’ll want to raise the cube so it doesn’t sit directly on the floor.

Step 4: Place the box fan on top of the cube and identify gaps

The box fan will sit on top of the four filters. Place it there and check the corners for holes that air can get through. Cover these gaps with small pieces of cardboard.

Step 5: Tape the box fan to the top of the filter cube

Place the box fan on top of the cube and make sure that the air is blowing up, out of the box. Once again, you’ll want to tape over all of the gaps to ensure that air can’t get into the cube without first going through the filters. I also taped a few inches down the electrical cord to make sure there were no gaps at the place where the cord met the fan.

Step 6: Breathe easier

Combine your new Corsi-Rosenthal box with other safety precautions, such as keeping windows tightly sealed. If your home is drafty, the Environmental Protection Agency suggests designating one room to be the “clean air room.” Keep your Corsi-Rosenthal box in that room and spend as much time as you can there.

Read: How to protect yourself from wildfire smoke

The Environmental Protection Agency tested DIY air filters for safety and found that “none of the scenarios tested posed any observable fire hazards.” But you should make sure that your fan meets safety standards for electric fans (look for a UL or ETL safety marking). You should also follow the box fan manufacturer’s instructions, which may include not leaving children unattended when the fan is in use, avoiding use of an extension cord, and not using a damaged or malfunctioning fan. And make sure that you have working smoke detectors throughout your home.

During a heavy smoke event, you may need to change out the filters on your Corsi-Rosenthal box. If you see large particles accumulate, the filters darken in color, or smell smoke odor coming from the fan, replace the filters.

Samantha Harrington

Samantha Harrington, director of audience experience for Yale Climate Connections, is a journalist and graphic designer with a background in digital media and entrepreneurship. Sam is especially interested...