Corsi-Rosenthal box: a DIY air cleaner for wildfire smoke

What you need

The parts list is short and comes straight from EPA's DIY air-cleaner guidance. The classic build is four filters plus a fan arranged as a cube; a simpler one-filter version also works if you only have a single filter, just with lower airflow.

• One ~20-inch box fan — made after 2012 and UL- or ETL-listed (see the safety note below; this is not optional).
• MERV-13 furnace filters — four 20×20×1-inch filters for the cube, or one for the simple version. MERV-13 is high enough to capture the fine smoke particles; equivalent ratings are FPR 10 or MPR 1500 on brands that use those scales instead.
• Tape — to seal the filters to each other and to the fan so air can't leak around them.
• Cardboard — for the base of the cube and for a shroud over the fan face (EPA notes a shroud improves airflow).

Note the arrows on filters: each has a printed airflow direction. They all point inward, toward the fan, because the fan pulls air through the filters and blows the cleaned air out the top.

How to build a Corsi-Rosenthal box

The four-filter cube takes about fifteen minutes. Work on a flat surface and keep the filters' airflow arrows pointing in toward where the fan will sit.

1. Form the walls. Stand four filters on edge and tape them together into a square tube (a cube open at top and bottom), airflow arrows pointing inward.
2. Seal the base. Tape a piece of cardboard across the bottom so air can only enter through the filter faces, not the base.
3. Mount the fan on top. Set the box fan on the open top, blowing upward and out, and tape it down all the way around so there are no gaps.
4. Add a cardboard shroud. Cut a cardboard ring or square that covers the fan's corners and leaves only the round blade area open. EPA notes this shroud improves airflow, so the fan moves more clean air.
5. Run it in the room you occupy. Place it in the room where people spend the most time, keep windows and doors closed, and let it run.

The one-filter version is even simpler: tape a single MERV-13 filter to the intake side of the box fan (arrow pointing toward the fan), add a shroud, and run it. It moves less air than the cube but still helps.

How well does it work?

This is the question worth answering honestly, because a DIY cleaner is only useful if it actually moves clean air. EPA tested box-fan-and-filter units against wildfire smoke and reported their performance as a clean-air delivery rate (CADR) — how much particle-free air the unit produces per minute.

EPA's finding, relayed: a well-built DIY unit can deliver clean-air rates in the range of many commercial HEPA air purifiers. We give that as EPA's range rather than a single headline number, because the exact figure depends on the fan, the filters, and how well the box is sealed — and we won't invent a precision EPA doesn't publish for a generic build.

One caveat EPA is clear about: a DIY air cleaner complements, it doesn't replace, the basics. Keep windows and doors closed, and cut indoor smoke sources (don't fry, vacuum, burn candles, or smoke indoors during a smoke event). The box lowers what's already in the air; it can't keep up if more smoke keeps pouring in. For the wider set of protective steps, see wildfire smoke and health.

Corsi-Rosenthal box vs. a commercial air purifier

A DIY box and a store-bought purifier do the same job — pull particles out of the air — and EPA's testing shows a good DIY unit competes on clean-air rate. The honest trade-offs are about cost, looks, and convenience, not whether it works.

Cost. A DIY box is typically far cheaper than a commercial unit, since it's just a fan and furnace filters. We won't quote a price tied to any product — filter and fan prices vary — but the gap is large enough that many households build one per room.

Noise, size, and looks. A commercial purifier is quieter, tidier, and smaller; a box fan cube is loud and bulky. Filters on both need replacing — swap the DIY filters when they look visibly dirty or after heavy smoke exposure.

If you're weighing a commercial unit instead, the number that matters is CADR matched to your room size. Our air purifier for wildfire smoke guide walks the EPA CADR sizing method so you buy enough clean-air capacity for the space — the same logic that makes the four-filter cube outperform a single filter.

Corsi-Rosenthal box FAQ

Is a Corsi-Rosenthal box safe to run?

Yes, with the right fan. EPA's guidance: use a box fan made after 2012 (UL or ETL listed) so it shuts off if it tips over or overheats. Don't leave any fan running unattended for days, and keep the intake filters clear.

Which MERV rating should the filters be?

MERV-13, per EPA. That rating is high enough to capture the fine smoke particles (PM2.5). Equivalent labels on other scales are FPR 10 and MPR 1500.

How long do the filters last?

There's no fixed date — it depends on how much smoke they catch. Replace the filters when they look visibly dirty or after a heavy smoke event. During a bad wildfire-smoke stretch they load up faster than in clean air.

Does a DIY air cleaner really work against wildfire smoke?

EPA's research found box-fan-and-filter units can meaningfully reduce indoor PM2.5 from smoke, with clean-air rates in the range of many commercial HEPA purifiers. It works best alongside closed windows and reduced indoor smoke sources — it complements those steps, it doesn't replace them.