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.

WHO: air pollutants and health

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.

WHO: air pollutants and health

Ground-level ozone (O₃) forms when sunlight drives reactions involving nitrogen oxides and volatile organic compounds. Levels often rise in sunny weather, and pollution can be transported away from its sources. Ground-level ozone is harmful; it is different from the protective ozone layer high in the atmosphere.

WHO: air pollutants and health