Air filters are measured by the size of the particles they can keep from escaping air compressors. These particles are measured by microns because they are so small. One micron is equal to one-millionth of a meter. When deciding on an air filter, it is important to consider the size of the particles you are trying to filter out of the compressed air. If your application requires filtration of particles smaller than 5 microns, we recommend the .01 micron air filters. If your project does not require filtering extremely small particles, we recommend the 5 micron air filters because these machines and filters require less pressure and tend to last longer.