PM stands for particulate matter suspended in air. PM followed by a number refers to all particles with a certain maximum size (aerodynamic diameter). All smaller particles are included. PM0.1 is particulate matter with an aerodynamic diameter of up to 0.1 µm, referred to as the ultrafine particle fraction. PM2.5 is particulate matter with an aerodynamic diameter of up to 2.5 µm, referred to as the fine particle fraction (which per definition includes the ultrafine particles). PM10 is particulate matter with an aerodynamic diameter of up to 10 µm, i.e. the fine and coarse particle fractions combined. PM10, PM2.5, & PM0.1 More: he PM10 fraction comprises both coarse particles (PM10-2.5) and fine particles (PM2.5), while fine particles (PM2.5) include the ultrafine particles (PM0.1). Hence, because PM10 comprises PM2.5 which comprises PM0.1, these three fractions should never be added together. For example, PM10 should never be added to PM2.5.