Medtex™ F1 Filter PM2.5 Dust Filter (5-layer) WhiteThis filter has been tested by the Research Unit of Applied Electric Field in Engineering (RUEE) according to the ASTM F2299 03 standard. The test results show an average filtration efficiency of 99. 54% for 0. 3 micron particles. The filter has a 5 layer filtration system: Layers 1 and 5 Spunbond Nonwoven: These filters provide reinforcement for filtering large particles. Layers 2 and 4 Meltblown Nonwoven: These filters are for filtering small