Small airborne particles less than 10 micrometres across (about the fifth of the thickness of a human hair).  They are produced by the combustion of wood and fossil fuels, as well as by various industrial and natural processes.  These particles can easily be breathed in. When the largest particles are inhaled they get trapped in the nose and nasal passages and can cause irritation. The smaller particles (PM2.5, less than 2.5 micrometres across, and are included in PM10 measurements), can enter our lungs and also reach the bloodstream, resulting in respiratory diseases, heart attacks, and lung cancer. These concentrations are measured in micrograms per cubic metre (µg/m3) of air.