DIY air purifier: the Corsi–Rosenthal box for haze
Updated 3 min read
A Corsi–Rosenthal box is a cube of four pleated air filters with a box fan on top. The fan pulls air in through all four filters and blows it out of the top. It takes about 15 minutes to build, and in lab tests it cleans smoke as well as, or better than, many commercial air purifiers.
Richard Corsi proposed the idea in August 2020, and Jim Rosenthal, head of a Texas filter company, built and tested it. It was designed for wildfire smoke, which is the same kind of particle as our haze.
How well it works
The US Environmental Protection Agency (EPA) tested several versions against wildfire smoke in a test chamber. CADR is the rate of clean air delivered; higher is better.
| Build | CADR |
|---|---|
| Fan with one MERV 13 filter (1 inch) | about 190 m³/h |
| The same, with a cardboard shroud | about 265 m³/h |
| Fan with two filters and a shroud | about 445 m³/h |
| Four-filter box with a shroud | about 680 m³/h |
A separate study (Dal Porto and colleagues, 2022) measured a five-filter version at 1,000 to 1,400 m³/h depending on fan speed, at roughly a tenth of the cost per unit of clean air of commercial purifiers. To judge whether that is enough for your room, see our air purifier guide.
What you need
- A 20-inch (about 50 cm) box fan: the square kind with a flat frame.
- Four pleated air filters, 20 × 20 inches and 1 inch thick, rated MERV 13. Asian suppliers often use other ratings: look for F7, or ISO ePM1 50% or higher, which are roughly equivalent.
- Cardboard for the base and shroud, and strong tape (duct tape or packing tape).
How to build it
- Stand the four filters up as the walls of a cube. The airflow arrows on the filter frames must point into the cube.
- Tape every seam so there are no gaps. Air that slips through a gap is not filtered.
- Close the bottom with a sheet of cardboard.
- Set the fan on top, blowing up and out of the box, and tape it tight to the filters.
- Cut a cardboard shroud to cover the four corners around the fan blades. The EPA found this adds about 40% more clean air at no cost.
Its limits
- It removes particles such as PM2.5 and PM10, but not gases or smells: there is no carbon filter.
- It is louder and bulkier than most purifiers.
- Dust-loaded filters are almost completely ineffective. Replace them when they turn dark grey or brown.
- A single HEPA filter strapped to an ordinary fan has not been tested the same way. HEPA is denser, so an ordinary fan pushes less air through it; filter area matters more than per-pass efficiency.
Safety
- Use a fan with the SIRIM and Energy Commission (ST) label, ideally with overload or thermal protection.
- Do not use an extension cord, and do not leave an older fan running unattended or while you sleep.
- In UL's tests in the US, no build caught fire, but motor temperatures rose most when the filters were full of dust. One more reason to change them on time.
Sources: US EPA research on DIY air cleaners for wildfire smoke (Holder et al., Indoor Air, 2022); Dal Porto et al., Aerosol Science and Technology 56(6), 2022; UL Chemical Insights report on DIY air filtration (2021).