That's because box fan are not designed as static pressure fan. They are a airflow fan for the purpose of moving much air as possible with minimal resistance. S

That's because box fan are not designed as static pressure fan. They are a airflow fan for the purpose of moving much air as possible with minimal resistance. Static pressure designed as forced pull/push air, it have slightly thick blade and have a different motor for it. The air purifier uses the static pressure fan because of the filter media. Take computer fans for example. There are two common case fan type, the airflow and the static pressure. The airflow fan is for the tower case, the static pressure one is for the radiator (water-cooling kit).