Microns refer to the size of the particles that a filter can stop from getting through. The lower the number, the more contaminants a filter can stop. Some filters that are for more viral toxins like total dissolved solids have a rating as low as 0.1 Microns. This is some of the best filtering available as any organisms larger than 0.1 Microns cannot sneak through. A 100-Micron filter will be able to stop any sediment or organism that is 100 Microns or larger.