Understanding PM2.5 vs PM10

Why fine particulate matter is more dangerous than coarse — health pathways, sources, and where the EU and WHO 2021 numbers come from.

What the names mean

"PM" stands for particulate matter — solid or liquid particles suspended in air. The number after PM is the maximum aerodynamic diameter of the particles in micrometres. PM10 = particles up to 10 µm; PM2.5 = particles up to 2.5 µm. For comparison, a typical human hair is 50–70 µm thick, so PM2.5 particles are roughly 30× smaller than a hair's width.

Why size matters

The size of an airborne particle determines how far into the body it can travel. PM10 particles mostly deposit in the nose, throat, and upper airways, where they can be cleared by mucociliary action. PM2.5 particles are small enough to bypass the body's upper-airway filtration entirely. They reach the alveoli (the gas-exchange sacs of the lung) and a fraction translocates into the bloodstream, from where they can reach every organ — heart, brain, kidneys, placenta. This is why the WHO and EEA both treat PM2.5 as the priority pollutant for population health.

Where each comes from

PM10 sources are dominated by mechanical processes: road wear, brake wear, tyre wear, construction dust, agricultural cultivation, wind-blown soil, sea salt, and natural mineral dust (notably Saharan dust intrusions across southern Europe). PM2.5 sources are dominated by combustion: vehicle exhaust (especially diesel), residential heating (wood and coal stoves), industrial combustion, power generation, and secondary formation in the atmosphere from sulfur dioxide, nitrogen oxides, ammonia, and volatile organic compounds. Wildfire smoke is a major short-term PM2.5 source across the Mediterranean.

The EU and WHO numbers

The EU Ambient Air Quality Directive 2008/50/EC sets the regulatory annual limits at PM2.5 = 10 µg/m³ and PM10 = 40 µg/m³. The WHO 2021 update tightened the guidelines to PM2.5 = 5 µg/m³ and PM10 = 15 µg/m³. The revised EU directive adopted in October 2024 will lower the PM2.5 limit to 10 µg/m³ (already in force) and align more closely with WHO targets by 2030, though the EU did not adopt the full WHO values. Most European cities exceed the WHO PM2.5 guideline; many also sit above the current EU PM2.5 limit, especially in central and eastern Europe.

How readings are taken

The European reference method for both PM2.5 and PM10 is the gravimetric standard EN 12341 — air is drawn at a defined flow rate through a pre-weighed filter for 24 hours, the filter is reweighed in a controlled environment, and the mass gain divided by the air volume yields micrograms per cubic metre. Automated equivalent methods (tapered-element oscillating microbalance with film-dynamic measurement, beta-attenuation monitors, optical particle counters) are permitted when calibrated against the gravimetric reference. Each EEA monitoring station declares its measurement method in the metadata.

Source: European Environment Agency Air Quality e-Reporting Database, Dataflow E1a (validated annual).

Source: World Health Organization Global Air Quality Guidelines (2021 update).

Sources: European Environment Agency Air Quality e-Reporting; WHO Global Air Quality Guidelines 2021; EU Directive 2008/50/EC and the revised Directive (EU) 2024/2881; Copernicus Atmosphere Monitoring Service.