A novel fan that operates as an air moving device as well as a filter has been investigated. The unit utilizes a brush disk, instead of bladed impellers, within a centrifugal fan casing and is known as a 'mop fan'. The performance of the mop impellers as fans was tested and their characteristic curves were found to be similar to these of centrifugal fans. The characteristics of the mop fan in terms of airflow rate, pressure, and efficiency were tested using a standard method, BS 848. The tests allowed mops of different fibre number and fibre diameter to be constructed and so optimize mop fan performance. Copyright (C) 2001 John Wiley & Sons, Ltd.