Box Fan Filters Provide Cleaner Indoor Air During Smoke Events
Pennsylvania is currently experiencing very poor air quality as smoke from Canadian wildfires drifts across the region. State officials have issued a Code Purple air quality alert in parts of Pennsylvania, indicating air that is "very unhealthy" or even "hazardous" for everyone—not just sensitive groups. Health officials recommend limiting outdoor activities, keeping windows closed, and reducing exposure to smoke particles. (WGAL, 2026).
One low-cost way to improve indoor air quality is a DIY air cleaner known as a Corsi-Rosenthal Box. The device is made from a standard box fan and four or five high-efficiency furnace filters, typically rated MERV-13. When assembled into a cube around the fan, the filters capture fine smoke particles as air is pulled through the box and circulated back into the room (Clean Air Crew, n.d.). The design gained popularity during the COVID-19 pandemic and has since been widely recommended for reducing exposure to wildfire smoke and other airborne pollutants.
Research has shown that these homemade air cleaners can be remarkably effective. A recent study by Rutgers University researchers found that a Corsi-Rosenthal Box built with four MERV-13 filters achieved a Clean Air Delivery Rate (CADR) of about 359 cubic feet per minute, meaning it can rapidly remove fine particulate matter (PM2.5) from indoor air. The researchers reported that using MERV-13 filters increased cleaning performance by about 71% compared with MERV-8 filters, while adding only a modest increase in cost (Baratta et al., 2026).
The study also concluded that the Corsi-Rosenthal Box was "very effective" at reducing PM2.5 concentrations and recommended using MERV-13 filters, operating the fan at its highest speed, and positioning the unit near occupants or pollution sources for maximum benefit (Baratta et al., 2026). Researchers found that the device could reduce high particle concentrations to much safer levels in as little as 20 minutes under test conditions.
Building a Corsi-Rosenthal Box is relatively simple and inexpensive. Detailed instructions, videos, and safety guidance are available from Clean Air Crew.
As wildfire smoke continues to affect Pennsylvania, a DIY box fan filter can provide an affordable way to improve indoor air quality and reduce exposure to harmful smoke particles. While it is not a substitute for medical advice or evacuation during severe smoke events, research suggests it can be an effective tool for cleaner indoor air when outdoor conditions deteriorate (Baratta et al., 2026).
References
Baratta, A., Han, T. T., & Mainelis, G. (2026). Not all boxes are equal: Investigation of parameters affecting PM2.5 particle removal by the Corsi-Rosenthal box. Journal of Aerosol Science, 192, 106731.
Environmental Protection Agency (2026). Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors
WGAL. (2026, July 17). Code Purple air quality alert in effect as Canadian wildfire smoke spreads across Pennsylvania. [wgal.com]
Clean Air Crew. Box Fan Filters (Corsi-Rosenthal Boxes).Â
Image source: Artofit.org: A variation on the box fan with MERV 13 filter air cleaner
