The invention relates to a tubular filter element for use in an ultra filtration plant. The element is in the form of a tube of woven fabric which is napped on its internal surface. A cellulose filter aid is deposited, on the nap and the resulting filter element has a high collection efficiency and low filter drag making it particularly suitable for use in an air recirculation plant. The air is drawn from a working area and passed through a conventional primary collection unit and then through an ultra filtration unit containing an array of the filter elements. It is then sufficiently clean to be passed back to the working area, preferably through a HEPA filter, and this results in a substantial saving in heat energy which would otherwise be wasted.