A micron, short for micrometer, is a unit of measurement used mainly to describe ultra tiny things. A micron is actually 0.000039 of an inch. The term micron and the symbol ยต used to represent the micron have been used by the air purification industry for years. It's a great way to show how small of particles the purifiers can actually remove. For comparrison:
Micron size is important for purification purposes. You must know what size the purifier removes in order to know how effective it is.