FiltersFast Compatible Replacement for Hunter 30931 HEPAtech-Filter- Hunter 30931 The 30931 filter fits the HEPAtech 251 - 30251 air purifier. Approximate Dimensions: 16" x 14" x 1-1/4" Electrostatically-charged filter that captures 99.9% of particles as small as 0.1 microns. The 30931 should be replaced every six months.