A micron is the unit of length used to determine the particle’s size. 1 micron = one-millionth of a meter. There are various types and sizes of particles floating in the air at all times. The size of a particle determines how harmful it can be to human health. The relative sizes of some most common air particles are human hair 80 microns, pollen 20 microns, dust 5 microns, pet dander 3 microns, smoke 1 micron, bacteria 0.3 microns.