PM2.5 is particulate matter less than 2.5µ (microns) in diameter. PM2.5 is referred to as fine particle and is roughly 1/30th the average width of a human hair. Because of the fine size, it can lodge deeply into lungs and pose a great health risk. Sources of fine particles include all types of combustion activities (motor vehicles, power plants, wood burning, etc.) and certain industrial processes.