How to make a box fan air filter for wildfire smoke

You need a box fan, a filter and some tape

The EPA's basic design uses:

• A 20-by-20-inch box fan, made in 2012 or later, with a UL or ETL safety mark
• A 20-by-20-inch MERV 13 furnace filter, 1 or 4 inches thick
• Clamps, duct tape or bungee cords
• A 20-by-20-inch piece of cardboard for a shroud (optional, but it helps a lot)

MERV 13 matters. The EPA says the filter's MERV rating strongly affects how much clean air the fan delivers, and it recommends MERV 13 for the very small particles in smoke. Our HVAC filter guide explains the ratings.

Here's how to build the basic version

1. Attach the filter to the back of the fan, the side that pulls air in.
2. Point the arrow on the filter's edge in the direction the air flows, toward the fan.
3. Seal the edges between the fan and filter as well as you can.
4. For the shroud, cut a circle the size of the fan blades out of the cardboard and tape it over the front of the fan, so it covers the corners.

Run it on the highest speed as much as you can, with windows and doors closed.

A Corsi-Rosenthal cube uses 4 or 5 filters

The Corsi-Rosenthal box, or cube, puts more filter area on one fan. 4 or 5 filters are taped to the fan to form a cube. For the 5-filter version, the EPA says to set it on blocks so air can get in through the bottom. The design caught on during the COVID-19 pandemic as a low-cost way to cut indoor transmission risk.

A simpler step up is the wedge: 2 filters taped into a triangle with cardboard, against the fan.

Here's what the EPA's tests found

EPA researchers tested designs in a room-sized chamber filled with smoke from smoldering pine needles. They measured each one's CADR (clean air delivery rate), the standard measure for air purifiers. Higher means it cleans the air faster.

The shroud added about 40% at no cost. AHAM suggests a smoke CADR equal to the room's square footage for wildfire smoke. By that rule, the basic fan fits a room of about 110 square feet, and the cube fits one of about 400.

The EPA says well-built DIY cleaners can perform about as well as commercial units, and the shroud and multi-filter designs did better than higher-priced commercial ones in its lab. Filter condition matters. With heavily loaded filters, the DIY units were almost completely ineffective.

The DIY units were also louder and drew more power than a commercial cleaner with similar CADR.

Use a newer fan, and follow the fan's rules

The EPA worked with UL's Chemical Insights Research Institute to test 5 box fan models with clean filters and with filters loaded with smoke and dust. In the harshest test, both sides of the fan were blocked for 7 hours. No fan caught fire, and temperatures stayed below the UL 507 safety standard for electric fans.

The tests covered only a few models, so the EPA can't promise every setup behaves the same. Its safety rules:

• Use a box fan made in 2012 or later. Newer fans have fused plugs and thermal cutoffs. The EPA says fans built before 2012 weren't tested and have known fire risks.
• Look for a UL or ETL mark, which shows it meets the UL 507 standard.
• If you must use an older fan, don't leave it running unattended or while you sleep.
• Don't use an extension cord or a damaged fan.
• Don't leave children alone with it running.
• Make sure your smoke detectors work.

It's cheap to build

In the EPA study, the box fans tested ran about $24 to $55 at April 2022 prices. CARB's 2020 fact sheet put a premium 1-inch MERV 13 filter at up to about $25. Many people already own a box fan, so the filter may be the only cost.

The study found the Corsi-Rosenthal cube gave the most clean air per dollar, followed by the wedge and then a single 4-inch filter. The cube costs more up front and takes more space. A single 4-inch filter is easier to build and fits a small room.

Change the filter when it turns gray

During smoke, the EPA says to check the filter daily. When it turns dark brown or gray, or smells like smoke, replace it. Keep spares on hand, and change it again when the smoke event ends. The Puget Sound Clean Air Agency's rule of thumb is to replace filters after 3 months of continuous use, or sooner if they look dirty.

Use it where you spend your time

Put it in the room you use most, toward the middle and away from walls and drapes. The Puget Sound agency says running it in a window cuts filtration, and in a large room you should keep it near you. Our smoke guide covers setting up a clean room.

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