Black Berkey® filters work "mechanically" by trapping contaminents in the millions of microscopic channels they contain, and also thanks to other layers, such as the ion exchange membrane and activated carbon. Solely for the mechanical part, the filtration fineness is estimated to be under 24 nanometers (0.024 microns), which corresponds to the size of some viruses efficiently filtered, such as MS2 or Fr Coliphage.