PM10, PM2.5, PM0.1

PM₁₀, PM_2.5, PM_0.1

Definition:

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.
PM_0.1 is particulate matter with an aerodynamic diameter of up to 0.1 µm, referred to as the ultrafine particle fraction.
PM_2.5 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, PM_2.5, & PM_0.1he PM₁₀ fraction comprises both coarse particles (PM_10-2.5) and fine particles (PM_2.5), while fine particles (PM_2.5) include the ultrafine particles (PM_0.1). Hence, because PM₁₀ comprises PM_2.5 which comprises PM_0.1, these three fractions should never be added together. For example, PM₁₀ should never be added to PM_2.5.