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PM₂.₅ consists of very small airborne particles. Sources include fuel burning in vehicles, heating and industry, as well as fires. Some particles form when gases react in the atmosphere. Their small size allows them to reach deep into the lungs.
PM₁₀ includes both fine particles (PM₂.₅) and larger particles up to 10 micrometres across. Sources include road and construction dust, wind-blown soil, fuel burning and particles formed in the atmosphere. PM₁₀ is not the same as coarse particles alone: those are the fraction between 2.5 and 10 micrometres.