Nitrogen dioxide (NO₂) is produced when fossil fuels such as coal, oil, gas or diesel are burned at high temperatures. Transportation, power plants, commercial and domestic gas boilers and industrial equipment are major sources. Short term exposure to NO2 irritates our lungs and aggravates respiratory diseases like asthma, leading to symptoms (such as coughing, wheezing or difficulty breathing), hospital admissions and visits to A&E. Chronic exposure to NO2 is associated with decreased lung growth in children, development of new cases of asthma and heart disease, as well as higher risk of premature mortality.

Particulate matter (PM) is made up of small airborne particles like dust, soot, and drops of liquids. Most particulate matter (PM) in urban areas is formed from fossil fuels used in power plants, vehicles, construction equipment, and industrial facilities. PM2.5 (fine particulate matter, or particles with a diameter smaller than 2.5 microns) is linked to lung disease, heart attacks, strokes, asthma, cancer and eventually premature death. New evidence suggests it also leads to impaired brain development in children.

The map shows current and average readings from the Breathe London Pilot stationary monitors. The map also displays other networks of stationary regulatory monitors ("Other monitors") like London Air Quality Network and Air Quality England.

The Breathe London Pilot sensor pods are not regulatory grade monitors, but can complement them and add another, more localized, layer of air pollution information.

The different colours inside the circles and diamonds representing monitor locations indicate the average pollutant concentration measured at that site, if data is available. The colour scale used is based on a heat-palette with the colours getting darker, or warmer, as the pollution levels increase.

Data from the Breathe London Pilot monitors are indicative only and cannot be used to determine health impacts, but the rise and fall of pollution and changes in pollution on different days; at different times of day; and between different locations can be seen and compared. Data from the regulatory network can be used to determine health impacts, but the live data provided here is still provisional. For NO2, the World Health Organisation (WHO) limits for short-term exposure indicate levels exceeding 200 μg/m³ (hourly average) can cause acute health effects. The long-term or annual average guideline is at 40 μg/m³. UK legal limits are in line with WHO guidelines for NO2.

For PM2.5, WHO short-term limits are 25 μg/m³ (24-hour average), and long-term or annual average guideline is 10 μg/m³ annual mean. Current UK legal limits are not in line with WHO guidelines for PM2.5; UK annual limits are 25 μg/m³.

The Ultra Low Emission Zone (ULEZ) was introduced in April 2019 and charges drivers of more polluting vehicles to enter central London. It operates 24 hours a day, every day of the year, in addition to the Congestion Charge (NOT 24/7).

The Low Emission Zone (LEZ) charges older heavy vehicles in most of Greater London and, from October 2020, the tighter ULEZ standards will apply across this area for buses, coaches and lorries. For lighter vehicles, the ULEZ is due to expand to cover an area that reaches to the North and South Circulars from October 2021.

Most monitoring sites are categorised as kerbside, roadside or urban background. Kerbside sampling sites are within 1m of the kerb of a busy road. Roadside sites are within 1 — 5m of a busy road and ideally located at breathing height. Background sites should be at least 50m away from any large pollution sources, such as construction sites, petrol stations, multi-storey car parks and airports. These categories help to explain why levels of pollution can vary so much from pod to pod, even in a small area.

The Breathe London Pilot map shows the contribution of different sources of nitrogen oxides (NOx) pollution at monitoring sites across the city in 2019. London's pollution sources include emissions from transport such as diesel vehicles and lorries, and from the way we heat and power our homes and businesses. We estimated the influence of these sources at each location using the ADMS model to account for factors such as the influence of weather and the distance from the sources to the locations.