Particulate matter (PM) includes microscopic matter suspended in air or water. Airborne particles are called aerosols. PM10 includes particles less than 10 µm in diameter, PM2.5 those less than 2.5 µm.
The toxicity of suspended particles is mainly due to particles with a diameter of less than 10µm. They can be emitted directly into the air from anthropogenic activities (industry, residential, agriculture, transport) and natural sources (forest fires, volcanic eruptions, etc.). Particles can also be formed directly in the atmosphere by physico-chemical reactions between pollutants already present in the atmosphere.