Micron filters remove dirt, debris, and even microscopic particles from water. The pore size of a filter cartridge is measured in microns. The smaller the micron rating, the finer the particulate removed. Which size micron filter you need depends on the size of the smallest particle that needs filtering from your water. A 5-micron filter, for example, removes particles as small as 5 microns. Anything smaller passes through the pores.