Black Berkey filters act in a 'mechanical' way by retaining pollutants in the millions of micro channels that compose it and also thanks to other layers, such as the ion exchange membrane or activated carbon. For the purely mechanical part, it is estimated that the filtration fineness is below 24 nanometers (0.024 microns) which corresponds to the size of certain viruses that are effectively filtered such as MS2 or Fr Coliphage.