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 current regulatory annual limits at PM2.5 = 25 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 (EU 2024/2881) will lower the PM2.5 limit to 10 µg/m³ from 2030, a future target, not yet in force, moving closer to WHO without adopting the full guideline values. Most European cities exceed the WHO PM2.5 guideline by several times; a handful of central- and eastern-European hotspots still approach or exceed even the EU's looser 25 µg/m³ limit.
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 Average Exposure Indicator (AEI) 2021-2023 and Burden of Disease 2022 (wired extract).
Source: World Health Organization Global Air Quality Guidelines, 2021 update.
Frequently asked questions
- What's the difference between PM2.5 and PM10?
- "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 covers particles up to 10 µm; PM2.5 covers particles up to 2.5 µm, roughly 30x smaller than a human hair.
- Why is PM2.5 more dangerous than PM10?
- 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 upper-airway filtration, reach the lungs' gas-exchange sacs, and a fraction translocates into the bloodstream, from where it can reach every organ. This is why the WHO and EEA both treat PM2.5 as the priority pollutant for population health.
- Where do PM2.5 and PM10 come 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 including Saharan dust intrusions across southern Europe. PM2.5 sources are dominated by combustion: vehicle exhaust (especially diesel), residential heating, industrial combustion, power generation, and secondary formation from sulfur dioxide, nitrogen oxides, ammonia, and volatile organic compounds. Wildfire smoke is a major short-term PM2.5 source across the Mediterranean.
- What are the EU and WHO limits for PM2.5 and PM10?
- The EU Ambient Air Quality Directive 2008/50/EC sets current regulatory annual limits at PM2.5 = 25 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 (EU 2024/2881) will lower the PM2.5 limit to 10 µg/m³ from 2030, a future target not yet in force.
- How are PM2.5 and PM10 measured?
- The European reference method for both 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 air volume yields micrograms per cubic metre. Automated equivalent methods (tapered-element oscillating microbalance, beta-attenuation monitors, optical particle counters) are permitted when calibrated against the gravimetric reference.