Nitrogen dioxide (NO2) is produced when fossil fuels such as coal, oil, gas or diesel are burned at high temperatures. Transportation, power plants 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, 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 nasal and upper respiratory problems.

O3 (Ozone) 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 Breathe London's stationary monitors as well as the London Air Quality Network, an existing network of regulatory monitors ("Other networks"). 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 two 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 dates for which it was active.

The different colours indicate the most recent hourly average concentration measured at that site, if data is available. The colour scale used is based on the World Health Organisation's (WHO) short-term guidelines, or recommended limits for hourly exposure to a given pollutant. Red means the level of the pollutant is above the WHO hourly threshold, orange are elevated levels below the WHO hourly threshold, and yellow are safest. Breathe London refers to the WHO guidelines as they are the most protective available.

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. The ULEZ is due to be expanded to cover a much larger area ‐ to the North and South Circulars ‐ from October 2021. The Low Emission Zone (LEZ) also charges older heavy vehicles in most of Greater London and new, tighter standards will apply from October 2020.

The Breathe London monitors measure all of the pollutants shown on the map (NO2, PM2.5, PM10 and O3). Currently the NO2 data is available and public. Breathe London will release the other pollutants later in 2019, and the map will be updated to reflect this data.