Prefilters are used to capture sediment and particulate that can clog water filters. While they come in sizes between 1-100 micron, the most popular sizes are 1, 5, 10, 15 and 20.
While prefilters can be ordered in any micron size from 1 to 100, the recommended sizes for a whole house application are 5, 10 or 20-microns. The smaller the micron the faster it can clog, so you can also install up to 3 prefilters in series, starting with the largest to the smallest micron. For instance, install a 20-micron first, followed by a 10-micron and finishing with a 5-micron. Adding even one prefilter will extend the life of all main water filters.
IMPORTANT NOTE about 1-micron Prefilters
A 1-micron prefilter has such small pores that it would need to be changed every month unless you install larger micron prefilters before it. Otherwise it can quickly clog up and diminish your water flow. Plus, if too much water pressure backs up because it is clogged, it could crack the housing. Typically, a 1-micron prefilter is only recommend to remove asbestos or other harmful small particle substances.