Researchers categorize particles according to size, grouping them as coarse, fine and ultrafine. Coarse particles (shown as blue dots in the illustration) fall between 2.5 microns and 10 microns in diameter and are called PM10-2.5. Fine particles (shown as pink dots) are 2.5 microns in diameter or smaller and are called PM2.5. Ultrafine particles (not shown) are smaller than 0.1 micron in diameter1 and are small enough to pass through the lung tissue into the blood stream, circulating like the oxygen molecules themselves.