The term fine particles, or particulate matter PM 2.5 refers to tiny particles or droplets in the air that are two and one half microns or less in width. Like inches, meters and miles, a micron is a unit of measurement for distance. There are about 25,000 microns in an inch. The widths of the larger particles in the PM 2.5 size range would be about thirty times smaller than that of a human hair. The smaller PM 2.5 particles are so small that several thousand of them could fit on the period at the end of this sentence. All fine particles that are two an a half micrometers in diameter or smaller are also referred to as PM 2.5.