Talking about oil and fuel filters and Micron Ratings 

A MICRON (or 1 micrometer) is a length of measurement used by filter manufacturers to measure the diameter of a particle. Fluid filters can have a micron rating which expresses the filter ability to remove contaminants based on the size of the particles. For example, a filter rated for twenty (20) microns means that the filter will capture and contain all particles in the fluid that are 20 microns and larger in diameter. 

1 Micron equals .001 mm or .000039 inches 
2 Microns is the size of BACTERIA 
8 Microns is the size of RED BLOOD CELLS 
25 Microns is the size of WHITE BLOOD CELLS 
40 Microns is VISIBLE WITH THE NAKED EYE 
70 Microns is the size of DIAMETER OF A HUMAN HAIR 

Measuring a micron rating through testing can be done through three methods: Nominal Micron Rating means that the filter can capture a given percentage of particles of the stated size. For example, a nominal micron rating of 10 microns at 95% means that the filter media retains 95% of the particles that are 10 microns and larger.

Absolute Micron Rating means that the filter is tested using a single pass of contaminants through a filter media and any particles that pass through are measured. You could end up with a 96.7% retention of that micron number. 

Multipass Beta Ratio Testing is the most accurate method of rating filters. Measured samples of the fluid are then taken at timed intervals from both the downstream and the upstream of the filter simultaneously; particles are measured and counted by electronic means using automatic particles counters. From these measurements a Beta Ratio (b) is formulated by dividing the number of particles of a particular size in the upstream flow by the number of particles of the same size in the downstream flow, and is an indicator of how well a filter stops a particulate. For example, if one out of every two particles in the fluid passes through the filter, the beta ratio is 2; if one out of every 200 of the particles pass through the filter the beta ratio is 200. Therefore, filters with a higher beta ratio retain more particles and have higher efficiency.