A portable fan includes a casing having an air inlet and a first connector, and an air outlet having a second connector. A filter unit includes a third connector, which is substantially the same as the second connector, for co-operating with the first connector to removably connect the filter unit to the casing, a filter which is located upstream from the air inlet when the filter unit is connected to the casing, and a fourth connector, which is substantially the same as the first connector, for co-operating with the second connector to removably connect the air outlet to the filter unit. This can allow the air outlet to be connected either directly to the casing, or to an optional filter unit which is connected to the casing.