Box fans have very low static pressure so airflow drops, very rapidly, as the filter starts to accumulate dust and its resistance to airflow goes up. This is m

Box fans have very low static pressure so airflow drops, very rapidly, as the filter starts to accumulate dust and its resistance to airflow goes up. This is mitigated by using four filters - constructing a frame that holds them top and sides - and the box fan goes on the bottom, with legs to hold it off the floor. Box fans are also very noisy. The box-fan design was intended mostly for people in wildfire areas who needed something to mitigate smoke inside their homes.