Particles that are 10 micrometers (µm) in diameter or smaller pose the greatest problems. These smaller particles generally pass through the nose and throat and enter the lungs. Once inhaled, these particles can affect the lungs and heart and cause serious health effects. Larger particles (> 10 µm) are generally of less concern because they usually do not enter the lungs, although they can still irritate the eyes, nose, and throat.

Particles of concern can be grouped into two main categories:

Coarse particles (also known as PM10-2.5): particles with diameters generally larger than 2.5 µm and smaller than, or equal to, 10 µm in diameter.

Fine particles (also known as PM2.5): particles generally 2.5 µm in diameter or smaller. This group of particles also encompasses ultrafine and nanoparticles which are generally classified as having diameters less than 0.1 µm.