A ~$40–60 build that outperforms a $400 air purifier.
The Corsi–Rosenthal Box uses a simple combination of MERV-13 furnace filters and a box fan to clean indoor air of respiratory aerosols, dust, wildfire smoke, and more — and it does it remarkably well.
What it is
A homemade air cleaner with a serious track record.
This air cleaner is a homemade system designed to lower the levels of respiratory aerosol particles — including those that carry the virus that causes COVID-19 — in indoor air. It also reduces dust, wildfire smoke, pollen, and other fine particles.
These air cleaners are being used in homes, schools, and other locations around the world. They are not HEPA air cleaners in the traditional sense, but recent testing at UC Davis and elsewhere has shown that the Corsi–Rosenthal Box can be more effective than commercial HEPA air cleaners costing many times more.
The key is surface area. Four MERV-13 filters arranged in a cube give the fan far more filter area to draw through than a typical single-filter purifier, dramatically increasing clean air delivery rate (CADR) without straining the fan.
Dirty air is drawn in through all four filter sides.
Clean air is pushed up through the fan and back into the room.
Safety
A few simple precautions to keep in mind.
Fan speed
Low or medium is the sweet spot.
The air cleaner can be noisy on the highest fan setting. Running it on low or medium gives you most of the benefit at a fraction of the noise.
On the lowest setting, the system has been shown to be comparable in effectiveness to a commercial HEPA air cleaner — while also extending the life of your filters significantly.
Quiet and effective. Comparable to a HEPA purifier. Best for bedrooms or quiet spaces. Filters last the longest — up to 5–6 months.
Higher clean air delivery rate than low. Still acceptably quiet for most living spaces. Good balance of performance and noise.
Maximum performance but noticeably loud. Best for short bursts when air quality is particularly poor (wildfire smoke events, etc.).
Filter life
Watch the color — your filters will tell you when it's time.
The air cleaner will eventually collect particles — mostly in the form of dust — and will need to be taken out of service. How long this takes depends on how long it runs each day and how dusty the environment is.
The white filter media will gradually discolor from white to light brown to gray as particles collect. If the build-up becomes so thick you can no longer see the white filter media, it's time to replace the filters.
Don't wait too long — heavy particle build-up strains the fan motor, reduces airflow, and lowers effectiveness. Changing filters before they're fully clogged extends the life of the fan.
High speed, dusty location: 1–2 months
End of life
How to safely dispose of the filters and reuse the fan.
Before handling spent filters, leave the unit unplugged for two to three days to allow any particles to settle.
Wear a mask before removing the filters. The collected particles can become airborne when disturbed.
Carefully remove all duct tape and set the box fan aside. The fan can be reused with new filters.
Remove the tape connecting at least two filters. Carefully fold them up and place the filters and cardboard base into a large plastic garbage bag.
Wash your hands immediately after handling spent filters, out of an abundance of caution.
♻️ Reuse the fan — just buy new filters.
The box fan is the most expensive part and can be reused indefinitely. When your filters are spent, simply purchase new 20"×20"×1" MERV-13 filters from a hardware or home improvement store and build a fresh cube.
MERV-13 filters are widely available online and in stores for $10–18 each. Four filters plus a roll of duct tape and you're back up and running for well under $100.
Full build instructions: fanfilterbox.com/build-guide
Important note
This is a supplement, not a substitute for a mask.
The Corsi–Rosenthal Box is not a substitute for high-quality masks. Use of this, or any other air cleaner, should be a supplement — not a replacement — for wearing a mask.
Use of a high-quality mask, particularly N95 and KN95, is the most effective way to protect your respiratory system. An air cleaner improves the overall air quality of a shared space, but does not directly protect your airways the way a well-fitted mask does.
Further reading
Helpful links and resources.
Ready to build one?
The whole build takes about 15 minutes and costs ~$40–60. No tools, no special skills.