Best for
- Wildfire clean-air-room planning
- People comparing DIY designs with portable purifiers
- Anyone sizing a Corsi-Rosenthal box by airflow rather than anecdotes
Watch for
- Assuming every DIY build reaches the same CADR
- Using an older box fan without current safety features
- Treating DIY research as a guarantee for your specific build
EPA measured large differences by design
In EPA-published research, a basic one-inch MERV 13 filter attached to a box fan measured around 111 CFM smoke CADR. Adding a cardboard shroud increased the reported result to about 156 CFM.
The four-filter Corsi-Rosenthal design with shroud was reported around 401 CFM in the same study, showing how filter area and construction can materially change airflow.
Translate CADR into ACH for your room
ACH = CADR × 60 ÷ room volume. A 401 CFM device in a 300 ft² room with an 8-foot ceiling works out to roughly 10 ACH in simple well-mixed-air math.
Real rooms are not perfectly mixed, so use the number as a planning estimate rather than a promise of uniform particle removal everywhere.
Commercial units still have an advantage in standardization
EPA notes that commercial air cleaners with known CADR are generally preferred because their performance is more standardized. DIY units can be effective but vary with assembly and component choices.
Use newer certified box fans with built-in safety features and follow current public-health guidance when building a DIY cleaner.
Use the same basis across brands. Room-size claims, operating-cost assumptions and moisture ratings can use different methods. Normalize the inputs before comparing products.