Monitoring and predicting air pollution

Since 2016, London’s air quality has improved dramatically, and the number of Londoners living in areas that exceed the UK’s legal air pollution limits has decreased significantly. 

Nonetheless, London still faces a challenges in improving its air quality, with parts of the city still exceeding UK legal limits for NO₂ and most of the city still exceeding the WHO air quality guidelines for nitrogen dioxide (NO₂) and particulate matter (PM₁₀ and PM_2.5).

These pollutants have well-documented health and economic impacts that fall unequally on those least likely to contribute to the problem.

We are working hard to reduce concentrations of these pollutants, however continued coordinated action from all levels of government and the private sector is required to ensure all Londoners can breathe cleaner air.

In London, air pollution is monitored using different instruments and techniques, which are outlined below.

Reference grade continuous monitors

London has one of the most comprehensive air quality monitoring networks of any global city and air quality is constantly monitored using reference grade continuous monitors at over 150 locations.

All data from the reference air quality monitors in London are publicly available via the following sources:

• London Air website, which is run by Imperial College London records real time and historical monitoring data
• Ricardo Energy and Environment also manages the data sets from some sites within London, more information is on the Air Quality England website
• Defra UK Air website also provides data from some sites across London.

Diffusion tubes

London boroughs also monitor nitrogen dioxide (NO₂) using over 1,900 passive diffusion tubes across London. This data has been collated and uploaded to the London Data Store.

Find out more about diffusion tubes on the Defra website: Diffusion Tubes Overview | LAQM.

Low cost air quality sensors

The Breathe London Programme provides a network of reliable, low cost air quality sensors located at over 350 monitoring sites across the city. Sites include priority locations such as schools and hospitals. The network also includes sensors co-located with reference monitoring sites, enhancing the accuracy of the data. By using low cost sensors the programme enables wider coverage, helping to make air quality data more accessible, delivering hyper-local real time data to Londoners.

Find out more and see the latest data from the network on the Breathe London website.

The Breathe London Community Programme is a Breathe London project providing funded air quality sensors to community groups, find the latest data on the Breathe London Community Programme website.

Personal Air Quality monitoring systems

Personal and relatively low-cost monitoring systems are becoming more accessible to buy. These can be valuable tools, but knowing how best to use them effectively is vital if the results are to provide meaningful information.

A monitoring guidance report was previously published, aimed at assisting Londoners who are interested in monitoring air quality themselves.

Modelling air pollution

We predict London’s air pollution levels using a model that estimates how pollution will spread over time and distance.

At the Greater London Authority (GLA), we focus on reducing nitrogen dioxide (NO₂) and particulate matter (PM₁₀ and PM_2.5) because these pollutants are often linked to health problems. The UK Air Quality Standards set legal limits for these pollutants, and the World Health Organisation (WHO) has established guideline based on the best available health evidence.

Transport for London (TfL) and the GLA have put together the London Atmospheric Emission Inventory (LAEI), which is periodically updated. Visit the London Datastore to access the latest data.

This database has information on emissions from all sources in Greater London that can be identified, and provides estimated ground-level concentrations of NO_x, NO₂, PM₁₀ and PM_2.5. The current LAEI includes emissions data for the ‘base year’ 2019 and forward projections for years 2025 and 2030.

In 2018 the Mayor of London announced that he was improving the way he informs the public, particularly those who are most vulnerable, about episodes of poor air quality in the capital.

On rare occasions, London’s air pollution is high. On these days, the Mayor can trigger alerts across the capital to warn people about pollution episodes, through the Mayor’s Air Quality Alert System. The system sends alerts through social media channels, at bus stops, tube stations and road-side signs across the city to notify Londoners.  Information is also published on the TfL website.

The system also warns stakeholders, such as boroughs, schools and the NHS, through an email alert. These stakeholders can then inform their own networks as appropriate.

The Mayor’s air pollution forecasts are issued by Imperial College London. These are combined forecasts - meaning they are based on a number of public forecasts: airText, Defra and Imperial. As with weather forecasts, there are occasions when the forecast providers will not agree. The Mayor’s forecast will represent the most likely picture drawn from the three forecasts, expected across the whole of London. So, it’s possible that the Mayor’s forecast (which is issued by Imperial) may not be the same as the Imperial's forecast since this is only one of the three inputs. 

All forecasts considered in compiling the Mayor’s combined forecast provide information based on the UK Daily Air Quality Index (DAQI). Particulate forecasts relate to a full day (midnight to midnight). There are however differences in the way that current levels for particulate matter (PM₁₀ & PM_2.5) are shown between other public forecasts.

Healthcare professionals

In February 2024, the Mayor and the London Air Quality and Health Programme Office announced further improvements with the launch of tailored Air Quality Alerts for GP practices and Emergency Departments.

These alerts are issued jointly by NHS England London and the Mayor of London, using the Mayor of London’s existing air pollution alert system.

Schools and nurseries 

All London schools have been auto-enrolled to receive AQ alerts during Moderate, High and Very high episodes of air pollution. These alerts also go out to all London Boroughs. Private nurseries can sign up by contacting the GLA Air Quality Team: [email protected].

Get free air pollution alerts 

Individuals from the general public can get free alerts which are issued by the following providers:

• airTEXT - a free service for the public providing air quality alerts by SMS text message, email and voicemail. airTEXT is an independent service, operated by Cambridge Environmental Research Consultants (CERC) Ltd in partnership with local authorities, the GLA, Public Health England and the Environment Agency. Sign up for free pollution alerts on the airTEXT website 
• Londonair- is part of the London Air Quality Network (LAQN) and provides information about air pollution in London and south-east England, which is accessible to the public, policy makers. Londonair is managed by the Environmental Research Group (ERG) of Imperial College London in partnership with other organisations including local authorities.

The data is used to generate the Londonair pollution map and alerts. See air quality updates on London Air.

Reducing exposure during elevated pollution episodes

You may wish to consider the following to reduce exposure and contribution to air pollution during elevated pollution episodes: 

• Walk, cycle and scoot more, and avoid the busiest roads and times when you can.
• Use public transport where you can, but if you do have to drive, switch your engine off when you’re stationary.
• Avoid unnecessary burning at home (e.g. in a wood burning stove or open fire) unless it is your only source of heat.