A micron is short for micrometer, one-millionth (0.000001) of one meter. There are 1,000 microns in a millimeter and 10,000 microns in a centimeter. 40 to 50 microns are the smallest that are still visible to the human eye. 20 to 25 micron is the size of a white blood cell and talcum power is around 10 micron, bacteria is 3 micron. The smaller the micron the smaller the size.