The Corsi-Rosenthal Box is easy to make with a box fan, four filters and a whole lot of duct tape. This one was constructed in spring 2023 for allergy and wildfire season. Bailey Jo Josie /

The Corsi-Rosenthal Box is easy to make with a box fan, four filters and a whole lot of duct tape. This one was constructed in spring 2023 for allergy and wildfire season. Bailey Jo Josie /

Prepare for summer smoke and wildfire now, says DOH

For those on a budget, DIY home air filters are an option that will last through the smoky season.

  • By Bailey Jo Josie bailey.jo.josie@rentonreporter.com
  • Thursday, June 15, 2023 3:39pm
  • Northwest

In the wake of the recent Canadian wildfire smokes making the air quality drop to unhealthy on the North American east coast, Washingtonians are reminded of the now-annual looming threats of wildfires and smoke in our own backyard.

For Smoke Ready Week, observed June 12-16, the state Department of Health (DOH) urged the public to begin preparations for smoke that can make air unhealthy to breathe.

“We are expecting to have above normal fire activity in Washington by July,” said Kaitlyn Kelly, an air quality policy specialist for the DOH. “We have an outlook for below normal precipitation and that’s going to last into fall, so conditions are trending to a long fire season.”

The DOH says that when smoke arrives back to the Pacific Northwest, it’s important to reduce exposure by staying up to date on the forecast and air quality index, limiting time outside, and keeping indoor air as clean as possible. This can be done with an HVAC system with MERV 13 filter, a HEPA portable air cleaner or a homemade box fan filter.

Having a way to filter the air in the home is essential and the DOH wants to remind the public that, after several days, smoke can enter homes and buildings through leaky gaps in windows and doors.

“We no longer have smoke-free summers in Washington,” said Kelly. “It’s important to know what is in wildfire smoke and why it can be bad for your health.”

Smoke from wildfires contains fine particles and gases including carbon monoxide. Exposure to smoke can cause a number of health problems ranging from minor to severe.

The DOH warns that those with pre-existing conditions, infants and children, pregnant individuals, adults 65 and older, and people with heart and lung disease can be most sensitive to wildfire smoke. Minor symptoms can include headaches, stinging eyes, runny nose, and trouble breathing.

To stay updated on wildfires, air quality, the forecast, and health information on the WA Smoke Blog. For more information on how to protect yourself from wildfire smoke, visit the DOH’s Smoke from Fires webpage.

How to make a box fan air filter

Named for Richard Corsi and Jim Rosenthal, this do-it-yourself air filter is an easy way to help keep the air in the home clear of wildfire smoke, pollen, dust and airborne virus particles. You will need:

  • one — 20-inch by 20-inch box fan, preferably a newer one
  • four — 20-inch by 20-inch by 1-inch MERV 13 furnace filters
  • one — fan shroud made from cardboard (like from a newly purchased box fan)
  • one — fan bottom made from cardboard
  • scissors and/or box cutter
  • a lot of duct tape

Depending on space in the home and preference, the box fan can either be placed on top of the Corsi-Rosenthal Box or on the side. (Reporter’s note: When I built mine, I placed the fan on the side so I could also use it as a regular fan.)

Set up the box fan(with the front of the fan facing outward), cardboard bottom and the four filters in a way where all components line up into a perfect square. For a better idea of what it needs to look like, go to cleanaircrew.org/box-fan-filters/.

Begin duct-taping the components together, making sure there are no gaps and that the duct tape is not covering up the filters or keeping the fan from being able to turn on. Allow an exit spot for the box fan’s cord.

Once the components are connected, go over all corners and awkward spots with more duct tape. Duct tape fan shroud to the front of the fan.

When in use, try and keep your box in the center of the room with doors and windows closed. If the box cannot be placed in the center of the room, place it far enough away from the walls to allow airflow. For large rooms, a second air filter box may be needed.

After some time (six months or shorter, depending on how much smoke or particles are filtered through), the filters will need to be replaced. Just cut the old ones and the duct tape away from the box fan and replace them with the new filters and tape. Remember to properly dispose of the filters.


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A simple illustration of how to put together the Corsi-Rosenthal Box, this version has the fan on the side but the box can also be built with the fan sitting on top. Illustration by Amanda Hu.

A simple illustration of how to put together the Corsi-Rosenthal Box, this version has the fan on the side but the box can also be built with the fan sitting on top. Illustration by Amanda Hu.

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