Air quality guides

PM2.5 vs PM10 - understanding particles in the air

PM2.5 and PM10 describe particle size. The number next to each name tells you how much particulate matter is in the air. To interpret it, also check the unit, location and averaging period.

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Two particle fractions

PM10 includes particles with an aerodynamic diameter up to 10 micrometres; PM2.5 is the finer fraction, up to 2.5 micrometres. PM2.5 is therefore part of PM10. Do not add the two concentrations to calculate “total particles”: that would count some particles twice.

What does 20 µg/m³ mean?

It means 20 micrograms of particles per cubic metre of air. A concentration is neither an AQI score nor a percentage of a legal limit. PM2.5 at 20 µg/m³ and PM10 at 30 µg/m³ describe the mass of two fractions in the same volume. Their quality categories use different thresholds.

An hour, a day and a year answer different questions

An hourly value describes a short period. A daily average combines values within one day; an annual average helps describe longer-term exposure. Do not treat a single hour and an annual average as interchangeable. Check whether our historical tables show daily or monthly averages. The latest day may include only part of that day.

WHO guidelines as a reference

The 2021 WHO guidelines give PM2.5 annual and daily reference concentrations of 5 and 15 µg/m³ respectively; for PM10, these are 15 and 45 µg/m³. These are health guidelines, not local legal limits. Assessment of the daily guideline considers the distribution of days across a year, rather than just one reading. Our table alone cannot establish compliance without information about daily completeness.

Comparing two cities

Choose the same pollutant, a comparable time and the same data type. A roadside station and a model point representing a city may describe different conditions. Lower PM2.5 does not automatically mean lower ozone or NO₂. Look at the other pollutants and the recent trend too.

Sources and further reading

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PM2.5 · PM10 · NO₂ · O₃

Understanding air quality and pollution data

What do PM2.5 and PM10 mean?

PM2.5 and PM10 are fractions of airborne particulate matter. PM2.5 covers fine particles up to 2.5 micrometres in diameter, while PM10 covers particles up to 10 micrometres. Concentrations in µg/m³ tell you how many micrograms of particles are present in a cubic metre of air. Compare values over the same time period.

Where does the air pollution data come from?

The latest concentrations come from the CAMS atmospheric model via Open-Meteo for each city’s coordinates. Values describe a model area, so conditions on individual streets may differ. Historical monitoring station data is shown separately where available.

How does PM2.5 concentration differ from AQI?

PM2.5 is the concentration of one particle fraction, expressed in µg/m³. AQI is an index calculated using a particular scale and thresholds. The pollutant cards show concentrations. Our city ranking compares PM2.5; PM10, ozone, nitrogen dioxide and other available gases add to the picture of air quality.

Which dates do the values and charts cover?

Each current value has a data date and time. Current concentration cards show the latest available hour, daily charts summarise individual days, and monthly records let you compare longer periods. These values represent different time intervals.

How can I check pollution in my city?

Search for your city and open its air quality page. Start with PM2.5 and PM10, then check the other available pollutants and the data timestamp. Charts and tables help you compare concentrations over time. Weather and historical climate information complement the pollution data.