A micron is one millionth of a meter (0.00003937 inches). By contrast, a human hair is about 100 microns in diameter.
It's important to understand what kind of micron rating you're using when selecting a filter bag. For historical reasons most filter material is assigned a micron rating based on approximately 80% efficiency. Filters that remove 80% of all particles larger than 50 microns in a single pass are rated 50 microns.
Filter bags made out of monofilament mesh or micro-fiber are given absolute ratings. A filter bag with an absolute rating will capture 99% of its rated particles in a single pass.
Filters are sometimes measured in U.S. Wire Mesh instead of microns. Wire mesh measures the number of wires (or threads) per linear inch, not the size of the holes between them. As the number of wires per inch goes up, the size of the holes (in microns) goes down. There is no simple formula to convert between microns and wire mesh because it would have to account for changing wire diameter.
Standard Filter Micron Ratings and Materials
*Nylon mesh is also available in 45, 125, and 175 microns.