PM2.5 and PM10 particles

PM2.5 and PM10 are not names for one specific chemical. They are groups of airborne particles classified by size.

• PM10 includes particles with an aerodynamic diameter of up to 10 micrometres.
• PM2.5 includes the finer part, with a diameter of up to 2.5 micrometres.

In simple terms, PM2.5 is part of the PM10 fraction. A PM10 reading should therefore normally not be lower than the PM2.5 reading from the same instrument, place, and time.

Where the particles come from

Sources depend on the location and weather. Common examples include:

• wood burning and other domestic heating
• traffic exhaust
• brake, tyre, and road wear
• industry and construction
• dust lifted from roads and soil
• fires and open burning
• secondary particles formed by chemical reactions in the atmosphere

Winter pollution episodes in residential areas often contain a large PM2.5 component from combustion. PM10 may also rise sharply because of dust, agricultural work, or dry and windy weather.

Why PM2.5 receives particular attention

Finer particles can travel deeper into the respiratory system, and some very small particles can enter the bloodstream. Short-term and long-term exposure is associated with respiratory and cardiovascular harm.

That does not make PM10 harmless. Larger particles can irritate the airways and aggravate existing conditions.

Why readings can change quickly

A station's concentration may change within a short period because of:

• a nearby source starting or stopping
• a change in wind
• a temperature inversion
• rain
• fog and high humidity
• passing traffic or lifted dust

One high reading is best checked against the trend. A value that falls quickly may reflect a very local event. A level that remains elevated for several hours is more likely to be part of a wider pollution episode.

Sensor limitations

Sensorbox stations use lower-cost optical sensors. They are useful for following local changes, but they are not regulatory instruments.

Humidity, condensation, placement, contamination, and differences between individual devices can affect the readings. Official assessments should rely on validated data from competent monitoring networks.

More information about processing is available on the Air-quality methodology page.