Question: Why are filters described in terms of “microns?

Answer:

Filters are commonly described by the size of the pores in the material that makes up the given filter. This stated pore size is the maximum size particle that will flow through a filter. A micron is 1/1,000,000th of a meter or approximately 0.00004 inches. Manufacturers rate filters based upon this pore size, however there is no industry standard technique to express the ratings. Some manufacturers/retailers claim filter ratings expressed using “nominal” numbers. Others claim filter ratings using “absolute” numbers. Many (most) don’t tell you which rating system they are claiming. For example, a 5 micron absolute filter removes a very high percentage (approaching 100.0%) of particles 5 microns or larger; while a 5 micron nominal filter will remove approximately 85% of particles 5 microns or larger.