Maintenance procedure · Written to ASD-STE100 Issue 9 · Formatted to ANSI Z535

Build a Box Fan Air Cleaner for Wildfire Smoke

Attaching a MERV 13 furnace filter to a box fan, EPA's tested single-filter and Corsi-Rosenthal box designs, and the certified-fan safety rule they depend on.
ApplicabilityA box fan with a UL or ETL safety mark and a 2012 or later date, matched to a standard furnace filter. Not for oscillating, tower, ceiling, or industrial fans, and not a substitute for a commercial air cleaner when one is available and affordable.
Task timeat least 15 min (31 of 37 steps estimated) editorial estimate
PersonnelOne
Revisionv1 · 2026-08-04
BEFORE YOU BEGIN

Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.

Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.

Summary

Wildfire smoke that gets indoors is a real health risk, and a commercial air cleaner is not always available or affordable during a smoke event. The Environmental Protection Agency (EPA) tested a low-cost stand-in: taping one or more furnace filters to a box fan. EPA's lab testing found a well-built version reduces wildfire smoke particles about as well as some commercial air cleaners, and safety testing by Underwriters Laboratories found no fire hazard in the box fan and filter combinations it tested.

This page covers EPA's basic single-filter design and the higher-performing four-filter "Corsi-Rosenthal box," the fan age and certification rule the safety testing is built on, and how often to change the filter during a smoke event. It does not replace medical advice, an evacuation order, or the repair of a faulty fan — see "Where this page stops" below for those.

Necessary equipment

ItemNote
Box fan, 2012 or newer, UL or ETL markedUse a fan with a UL or ETL mark and a 2012 or later date.
Furnace filter, MERV 13 or higherMatch the filter width and height to the fan opening.
Duct tapeUse a wide strip of tape to seal the filter to the fan.
Sturdy cardboardUse a flat piece as large as the front of the fan.
Scissors or a utility knifeCut the cardboard shroud.
Tape measure or rulerMeasure the fan opening and the blade circle.
3 extra furnace filters materialUse four filters in total for this design.
Leg supports (blocks, bricks, or cans) materialUse sturdy supports if you add a fifth filter.

Specifications

Wildfire-smoke CADR: one 1-inch MERV 13 filter, no shroud
Manufacturer or standard verified
111 ± 1 CFM

Source: Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors · tier 1 · accessed 2026-08-04

Wildfire-smoke CADR: one 1-inch MERV 13 filter with a cardboard shroud
Manufacturer or standard verified
156 ± 4 CFM

Source: Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors · tier 1 · accessed 2026-08-04

Wildfire-smoke CADR: one 4-inch MERV 13 filter with a cardboard shroud
Manufacturer or standard verified
248 ± 15 CFM

Source: Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors · tier 1 · accessed 2026-08-04

Wildfire-smoke CADR: two 1-inch MERV 13 filters with a cardboard shroud
Manufacturer or standard verified
263 ± 22 CFM

Source: Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors · tier 1 · accessed 2026-08-04

Examine the fan

Estimated 1 min

NOTEThis build needs a newer, certified fan.
Minimum box fan age and certification for a DIY air cleaner
Manufacturer or standard verified
2012 OR NEWER, WITH A UL OR ETL SAFETY MARK

Source: DIY Air Cleaners · tier 1 · accessed 2026-08-04

WARNING

USE ONLY A FAN WITH A UL OR ETL MARK AND A 2012 OR LATER DATE.
Older fans do not have this tested safety feature. An older fan can catch fire.

  1. Find the manufacturer's label on the back of the fan.
    <1m
  2. Find the UL or ETL mark and the model year on the label.
    <1m
  3. If the model year is before 2012, do not use this fan.
    <1m
  4. Examine the cord and the housing for damage.
    <1m
  5. Do not use a fan that has a damaged cord or housing.
    <1m

Choose a filter

Estimated 2 min

NOTEA thicker filter cleans more air than a thin filter. A higher MERV number also cleans more air.
Recommended furnace filter efficiency rating for wildfire smoke
Manufacturer or standard verified
MERV 13 OR HIGHER MERV

Source: DIY Air Cleaners · tier 1 · accessed 2026-08-04

Furnace filter size for a standard 20-inch box fan
Manufacturer or standard verified
20 IN BY 20 IN, 1 TO 4 IN DEEP inches

Source: DIY Air Cleaners · tier 1 · accessed 2026-08-04

Filter size for a box fan that is not 20 by 20 inches
Unverified — do not rely on this value
MEASURE YOUR OWN FAN. BUY A FILTER THE SAME WIDTH AND HEIGHT AS THE FAN OPENING.

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

CADR increase from a 4-inch filter compared with a 1-inch filter
Manufacturer or standard verified
123 percent

Source: Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors · tier 1 · accessed 2026-08-04

  1. Measure the front opening of the fan.
    <1m
  2. Buy a furnace filter that matches this width and height.
    1m
  3. Choose a filter with a MERV rating of 13 or higher.
    <1m
  4. Choose a filter that is 1 to 4 inches deep.
    <1m
  5. Find the airflow arrow on the filter frame.
    <1m

Build the basic air cleaner

Estimated 7 min

NOTEThis design uses one filter and a cardboard shroud.
CADR increase from adding a cardboard shroud to a single-filter design
Manufacturer or standard verified
40 percent

Source: Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors · tier 1 · accessed 2026-08-04

  1. Place the filter flat against the front of the fan.
    <1m
  2. Point the filter's airflow arrow toward the fan.
    <1m
  3. Attach the filter to the fan frame with tape, clamps, or a bungee cord.
    2m
  4. Seal every gap between the filter and the fan frame with tape.
    1m
  5. Cut a flat piece of cardboard the same size as the front of the fan.
    1m
  6. Mark a circle in the middle of the cardboard, sized to the fan blades.
    1m
  7. Cut the circle from the center of the cardboard.
    1m
  8. Tape the cardboard over the filter, so the hole is over the fan blades.
    1m

Build a Corsi-Rosenthal air cleaner

NOTEThis design uses four filters. It filters more air than the basic design. A fifth filter on the bottom filters even more air. Raise the box on sturdy legs so air can flow under it.
Wildfire-smoke CADR: four 1-inch MERV 13 filters with a cardboard shroud (Corsi-Rosenthal box)
Manufacturer or standard verified
401 ± 31 CFM

Source: Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors · tier 1 · accessed 2026-08-04

  1. Stand four filters on their edges to form a square box.
  2. Point every airflow arrow toward the inside of the box.
  3. Tape the four edges together where the filters meet.
  4. Place the fan on top of the box, front side up.
  5. Tape the fan to the top of the box.
NOTICE

USE STURDY SUPPORTS UNDER THE LEGS IF YOU ADD A FIFTH FILTER.
Blocks, bricks, and cans work well.

  1. Cover the bottom of the box with a flat piece of cardboard.

Position and run the air cleaner

Estimated 1 min

  1. Place the air cleaner in the room where people spend the most time.
    <1m
  2. Keep the air cleaner away from walls and furniture.
    <1m
WARNING

DO NOT USE A FAN BUILT BEFORE 2012 WITHOUT A PERSON IN THE ROOM.
Do not run this fan while you sleep. An unattended older fan can catch fire.

NOTICE

FOLLOW THE FAN MANUFACTURER INSTRUCTIONS FOR SAFE USE.
Do not leave a child unattended near the running fan. Do not connect the fan to an extension cord. Make sure the smoke detectors in the home are working.

  1. Connect the fan to a wall outlet.
    <1m
  2. Set the fan speed switch to high.
    <1m
  3. Run the air cleaner when people are in the room.
    <1m

Maintain the filter

Estimated 4 min

Filter check frequency during a wildfire smoke event
Manufacturer or standard verified
EVERY DAY

Source: Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors · tier 1 · accessed 2026-08-04

  1. Do a check of the filter every day during a wildfire smoke event.
    <1m
  2. If the filter has a dark brown or gray color, replace it.
    1m
  3. If the filter has a smoke smell, replace it.
    1m
  4. Remove the used filter gently.
    <1m
  5. Do not shake the filter.
    <1m
  6. Put the used filter in a garbage bag.
    <1m
  7. Wash your hands after you handle a used filter.
    <1m
  8. Store some spare filters near the air cleaner.
    <1m

Terminology

This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.

DIY box fan air cleaner
Also called: Corsi-Rosenthal box, box fan air purifier, homemade air purifier, furnace filter fan purifier, smoke box fan, box fan filter purifier

When to use a professional

If you have heart or lung disease, talk to your health care provider about wildfire smoke. If a commercial air cleaner is available and affordable, use it instead of this design. If smoke symptoms are severe, get help from a doctor right away.

If you cannot stay cool or keep smoke out of the home, then seek shelter elsewhere. Follow the local evacuation orders. This page does not cover fan repair. Take a damaged fan to a repair shop, or replace it.

Common questions

What is a Corsi-Rosenthal box?

A Corsi-Rosenthal box is the nickname for a do-it-yourself air cleaner built from four or five furnace filters taped into a cube with a box fan on top. EPA lab testing found this four-filter design removes wildfire smoke particles faster than a single-filter box fan. EPA also reports that a well-built DIY air cleaner, of this or a simpler design, can perform similarly to a commercial portable air cleaner at removing smoke particles.

Is a homemade box fan air purifier safe to run?

EPA and Underwriters Laboratories tested several box fan models with filters attached, including an extreme case with both sides of the fan blocked for seven hours; none of the tested scenarios caught fire. EPA's own guidance says fans built before 2012 were not tested and pose a known fire risk, so it recommends building a DIY air cleaner only with a newer, UL- or ETL-marked fan.

Can I use an old box fan I already own?

Only if it is certified. Look for a UL or ETL mark and a 2012 or later date on the fan's label. EPA says fans built before 2012 were not tested for this use and pose a known fire risk; if you use one anyway, never leave it running unattended or while you sleep.

Does a box fan air cleaner work as well as a store-bought air purifier?

A well-built one can perform similarly, according to EPA-funded research, but performance varies from unit to unit depending on how tightly the filter is sealed and how it is built. A commercial air cleaner with a published clean air delivery rate (CADR) is more consistent from unit to unit, so EPA recommends using one instead whenever it is available and affordable.

What size and rating of furnace filter do I need?

For a standard 20-inch box fan, EPA's own build uses a 20 by 20 inch furnace filter, 1 to 4 inches deep, rated MERV 13 or higher. If your fan is a different size, measure its front opening and buy a filter that matches.

How often do I need to change the filter?

Check it every day during a wildfire smoke event. Replace it when it looks dark brown or gray, or when it smells like smoke. Outside a smoke event, replace it whenever it looks dirty.

What else helps besides a DIY air cleaner?

Close windows and doors, and set an HVAC system with a fresh air intake to recirculate mode. Keep an N95 respirator mask on hand for the times you have to go outside in smoke. See Choosing a Disposable Respirator for a similar filter-rating system, and Choosing an Air Cleaner by CADR if you decide to buy a commercial unit instead.

Sources

Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.

  1. Tier 1DIY Air CleanersUnited States Environmental Protection Agency · accessed 2026-08-04 · re-verification due 2027-01-31
  2. Tier 1Research on DIY Air Cleaners to Reduce Wildfire Smoke IndoorsUnited States Environmental Protection Agency, Office of Research and Development · accessed 2026-08-04 · re-verification due 2027-01-31
  3. Tier 1Wildfires and Indoor Air Quality (IAQ)United States Environmental Protection Agency · accessed 2026-08-04 · re-verification due 2027-01-31
  4. Tier 1Guide to Air Cleaners in the HomeUnited States Environmental Protection Agency · accessed 2026-07-28 · re-verification due 2027-01-24

Revision history

Current version 1 · updated 2026-08-04 · next review due 2027-08-04

  1. v1 · 2026-08-04Initial draft, built from EPA's DIY air cleaner guidance and EPA/UL safety testing.

Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.

Comments and corrections

Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.

Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.

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