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.

Most particulate matter (PM) in urban areas is formed from fossil fuels used in power plants, vehicles, construction equipment, and industrial facilities. PM10 (coarse particulate matter, or particles smaller than 10 microns in diameter) is linked to heart and lung disease.

Ozone (O₃) is a secondary pollutant, formed mainly through reactions of volatile organic compounds, nitrogen oxides, and sunlight. Short-term exposure to ozone can cause chest pain, coughing and aggravated respiratory disease, which can lead to hospital admissions and A&E visits. Long-term exposure can lead to decreased lung function and cause chronic obstructive pulmonary disease (COPD).

The map shows current and average readings from Breathe London's stationary monitors, as well as averages from mobile sensors on specially-equipped Google Street View cars. The map also displays existing networks of regulatory monitors ("Other monitors") including the London Air Quality Network and Air Quality England. Breathe London's sensor pods are not regulatory grade monitors, but can complement the existing monitors and add another, more localized, layer of air pollution information. To learn more about the monitor types, visit our FAQ page.

We are currently unable to see data from some of these pods due to a temporary delay. In these cases, we are working to resolve the underlying issue and restore data collection and transmission as soon as possible. Please note the data from some of the pods is still undergoing quality assurance and control; these will be live on the map soon.

These pods are no longer in place, likely because the location was found to be unsuitable. Clicking on the pod will display the average reading for the dates it was active. .

The different colours indicate the average pollutant concentration measured at that site, if data is available. The colour scale used is based on the World Health Organisation's (WHO) guidelines for both NO2 and PM2.5. WHO recommends both short-term and long-term (annual) limits for exposure to a given pollutant. Breathe London refers to the WHO guidelines as they are the most protective available.

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.

The Breathe London monitors measure all of the pollutants shown on the map (NO2, PM2.5, PM10 and O3). Currently the NO2 stationary and mobile data, and the PM2.5 stationary data, are available and public. The map will be updated to reflect new data as it becomes available.

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.