... the first circulation. While some dust and bacteria remain in the returning air, in the second the afterward circulations, all the remained ... for removing the Sulfid, BHC, NH3, HCHO, and bad smell in the air. TypePrincipiumWindageEfficiencyCostfilter screen laminar flowstylelow/mid/high filter screens ... windageFilter 95% dust, bacteria(tiny to 0.01)normal cleaning with water, repeated use, low cost