You can build an effective air filter for about $35 using a box fan, furnace filters, duct tape, and a piece of cardboard. The most popular design, called a Corsi-Rosenthal box, cleans air at rates that match or beat commercial HEPA purifiers costing $200 to $550. It takes roughly 30 minutes to assemble with no special tools.
What You Need
The standard build uses these materials:
- 4 MERV-13 filters (20″ × 20″, 2-inch depth)
- 1 box fan (20-inch, with a UL or ETL safety certification and manufactured in 2012 or later)
- Duct tape
- Cardboard (the box the fan shipped in works perfectly)
- Scissors
MERV-13 filters capture at least 50% of particles in the 0.3 to 1.0 micron range and 85% or more of particles between 1.0 and 3.0 microns. That covers wildfire smoke, pollen, dust, mold spores, and many airborne respiratory droplets. You can find these filters at most hardware stores or online for about $8 to $12 each.
Step-by-Step Assembly
Every filter has an arrow printed on its frame showing the intended airflow direction. Before you start taping, identify these arrows. They should all point inward, toward the center of the box you’re about to build.
Build the Four-Sided Frame
Stand the four filters on their edges to form a square, like the walls of a box with no top or bottom. The arrows on each filter should point inward. Run duct tape along every seam where two filters meet, taping both the inside and outside edges. You want airtight seals so air is forced through the filter material rather than leaking through gaps.
Add the Cardboard Floor
Cut a square of cardboard to fit the bottom opening of your filter box. Tape it on securely from all sides. This sealed base forces all air intake through the four filter walls. The fan packaging is the right size if you’re using 20-inch filters.
Mount the Fan on Top
Set the box fan on top of the filter cube with the airflow blowing upward, away from the box. The fan pulls air in through the four filter walls and pushes clean air out the top. Tape the fan to the filter frame on all four sides, again sealing any gaps. Some builders also add a strip of cardboard or weatherstripping between the fan and the filters if there’s a slight size mismatch.
That’s it. Plug it in and you have a working air purifier.
How Well It Actually Works
A peer-reviewed study published in Science of the Total Environment compared nine DIY purifier designs against three commercial HEPA units. The results were striking. A basic Corsi-Rosenthal box using 2-inch MERV-13 filters and a box fan on its lowest speed delivered an estimated clean air delivery rate (CADR) of 293 cubic feet per minute. That’s more air cleaned per minute than a Coway Airmega 300 running at maximum speed (282 cfm), a unit that retails for around $549.
Thicker filters performed even better. Setups using 4-inch MERV-14 filters hit 322 cfm, and 5-inch MERV-16 filters reached 405 cfm, though at higher cost. The New Jersey Department of Environmental Protection found similar numbers, with both two-filter and four-filter DIY designs exceeding 400 cfm for fine particles (PM2.5).
The total cost for the basic build runs about $35. Even the highest-performing DIY version topped out at $120, still less than half the price of mid-range commercial purifiers.
Noise Levels
On its lowest speed setting, the basic MERV-13 Corsi-Rosenthal box measured about 52 decibels. That’s roughly the volume of a quiet conversation or a running refrigerator. For comparison, a Coway Airmega 300 at max speed hit 59 decibels, while cheaper HEPA units like the Airthereal AGH550 reached 66 decibels at full power.
The DIY box has a real advantage here: because four large filters give the fan less resistance than a single dense HEPA filter, you can run it on low and still move a lot of air. Commercial units often need to run at higher, louder speeds to deliver comparable airflow. If you plan to use yours in a bedroom overnight, the lowest fan setting is usually comfortable for sleeping.
Is It Safe?
The main concern people raise is whether strapping a filter to a fan motor could cause overheating. The EPA commissioned Underwriters Laboratories (UL) to test this directly. They ran box fans with various filter configurations, including clean filters, smoke-loaded filters, and dust-clogged filters, and measured temperatures on every fan component.
None of the scenarios caught fire. External fan surfaces stayed near room temperature when filters were attached. Internal components did get warmer, but all measured temperatures remained below maximum acceptable safety thresholds. Even in an extreme test where both sides of the fan were completely blocked, temperatures stabilized after about 20 minutes and stayed safe through a continuous seven-hour run.
The one important caveat: use a fan manufactured in 2012 or later. Newer models include a fused plug and other safety features that older fans lack. Look for the UL or ETL certification label on the fan. Fans built before 2012 were not tested and carry known fire risks.
Where to Place It
Set the unit on the floor or a low table in the room where you spend the most time. Keep doors and windows closed while it runs so it’s cleaning recirculated air rather than fighting a constant influx of outdoor pollution. If you’re using it during a wildfire smoke event, one box can reasonably handle a single room. For larger spaces or whole-apartment coverage, building two is more effective than trying to crank one fan to high speed.
Position it so the fan exhaust (the top) isn’t blowing directly at a wall or into a corner. A few feet of clearance above the fan lets the clean air circulate through the room.
When to Replace the Filters
For MERV-13 filters in a residential setting, replacement every 30 to 90 days is the general guideline. The actual timeline depends on how often you run the unit and what’s in your air. Homes with pets, smokers, high humidity, or active wildfire smoke will clog filters faster. You can visibly inspect the filters by pulling back the tape on one corner: if the filter material looks dark and matted with debris, it’s time to swap them out.
Keep a spare set of filters on hand, especially if you’re building this for wildfire season. A clogged filter doesn’t just stop cleaning the air effectively. It also makes the fan work harder, increasing noise and motor strain. Fresh filters are cheap compared to the cost of running a degraded unit for weeks.
Ways to Improve the Basic Design
The four-filter cube is the standard, but you can modify it based on your priorities. Upgrading from 2-inch to 4-inch or 5-inch filters increases both airflow and filtration efficiency because thicker filters have more surface area for trapping particles. The tradeoff is cost and size.
Some builders add a cardboard shroud or collar between the fan and the filter box to improve the seal. Others mount the fan with bungee cords instead of tape, making filter replacement easier. If you find the vibration annoying, setting the whole unit on a rubber mat or folded towel dampens the buzz against hard floors.
For the filters themselves, make sure the arrows all point inward and that you’re not accidentally installing any filter backwards. A backwards filter still works, but it wears out faster because the denser side faces the incoming air instead of catching large particles on the pre-filter layer first.