Other air pollutants

Official monitoring stations may measure gases such as NO₂, O₃, SO₂, and CO in addition to PM2.5 and PM10. Not every station measures every pollutant, so availability depends on its equipment and published data.

NO₂, nitrogen dioxide

NO₂ is mainly linked to fuel combustion. Road traffic is an important source in cities, particularly diesel vehicles, while heating and industry may also contribute.

Levels are often higher beside busy roads than at urban background stations. NO₂ can irritate the airways and aggravate symptoms in people with asthma.

O₃, ground-level ozone

Ground-level ozone is not emitted directly from one source. It forms through chemical reactions involving other pollutants in sunlight.

It is therefore more often a problem on warm, sunny days. Levels can also be high outside city centres, sometimes higher than directly beside a road.

Ground-level ozone is not the same as the ozone layer high in the atmosphere. The first is an air pollutant, while the second protects Earth from part of the Sun's ultraviolet radiation.

SO₂, sulfur dioxide

SO₂ is produced by burning fuels containing sulfur and by some industrial processes. Short-term concentrations may be high near major sources.

It can irritate the respiratory system, especially in people with asthma.

CO, carbon monoxide

CO is produced by incomplete combustion. Sources include traffic, faulty heating appliances, fires, and other combustion processes.

Outdoor concentrations are normally diluted, but CO can be extremely dangerous indoors. An outdoor monitoring-station reading is not a substitute for a working home CO alarm.

Are these pollutants included in the website's overall AQI

No. The overall AQI on AirQuality.city is based only on PM2.5 and PM10.

NO₂, O₃, SO₂, and CO are shown separately when the source station publishes them. Their concentrations may still matter when the website's overall AQI is low.

The European Air Quality Index uses five key pollutants: PM2.5, PM10, O₃, NO₂, and SO₂. Its result may differ from AirQuality.city because the pollutant set and methodology are different.

Details of the website's calculation are available on the Air-quality methodology page.