Image courtesy of Breathe London

TL;DR — Cities across the globe struggle with the air pollution that often comes hand-in-hand with increasing urbanization. There are a number of major air pollutant sources that tend to be in or near urban areas, and due to population density urban emissions tend to lead to more human exposure. Fortunately, there are a number of ways to reduce air pollution in cities — such as establishing air quality monitoring, setting stringent air quality targets, enacting clean air zones, moving towards renewable energy, and shifting towards greater public transportation use. Innovative programs such as Breathe London and Breathe Cities are spearheading this work by connecting communities and dismantling barriers to clean air around the world.

Global cities need new solutions in their battle for clean air. This blog is the first in a series that will outline a new model for the effective management of air pollution in urban areas. Visit this page to read our white paper on Air Quality Management 2.0

How urbanization affects air quality

Urbanization can be defined as a process in which cities’ size and structure grow in response to population change, which can lead to lasting impacts on the air pollution levels faced in the area.

Urbanization generally means degraded air quality and increased air pollution emissions.

Among all types of ecosystems, urban produce roughly 78% of carbon emissions and substantial airborne pollutants that adversely affect over 50% of the world’s population living in them”

— Nature

However, the air quality landscape can also vary widely among different cities. A survey of 499 global cities found that the most polluted cities had nearly 20 times the PM2.5 levels of the cleanest city.

There is mixed evidence for how different types of urban areas interplay with air pollution levels. Some research shows that compact city layouts mean a greater reliance on public transportation rather than private vehicle usage — and, thus, lower nitrogen dioxide emissions.

In contrast, dispersed urban areas can also mean pollution sources are further apart, there is less traffic congestion, and street canyon effects are reduced to allow pollution to diffuse. 

Evidence also shows that compact cities tend to have stronger urban heat island effects, which can raise air pollution levels.

Thus, there is not a clear-cut answer on just how the layout of an urban area affects its air quality — implying that this relationship may vary both across cities with different urbanization levels and over different periods of time.

Why is air pollution a problem in cities?

Air pollution tends to be a problem in cities for a number of reasons — including the abundance of pollution sources such as vehicle traffic and the high density of residents living in one area.

Air pollution emissions also cause more damage in urban areas as compared to rural ones because they lead to more human exposures.

The graphic above displays the percent of the population in major world regions living in urban areas with both historical and projected future percentages. As an increasing portion of the global population lives in urban areas, addressing air quality in cities becomes even more vital to protecting public health. (Image source: UN, World Urbanization Projects)

Air pollution is poor in many cities, but there is also a stark contrast between air quality in those in low- and middle-income countries compared to higher-income countries.

According to the latest urban air quality database, 98% of cities in low- and middle-income countries with more than 100,000 inhabitants do not meet WHO air quality guidelines. However, in high-income countries, that percentage decreases to 56%”

— World Health Organization

There are a number of major pollution sources in or near urban areas, such as:

  • Vehicles
  • Construction
  • Industrial facilities
  • Generators
  • Heating

The urban heat island effect can also contribute to worse air quality in cities. 

This effect comes about when natural vegetation cover is replaced by pavement, buildings, and other surfaces in cities that absorb and retain heat. In addition to heightened air pollution levels, the urban heat island effect also incurs increased energy costs for cooling as well as heat-related illness and mortality.

The graphics above display air pollution concentrations of PM2.5 and NO2 in major cities across the globe, with pollution levels varying widely across different cities and in different regions.

How to reduce air pollution in cities

While air quality can suffer in urban areas, a significant amount of research on this issue also helps to inform us and drive appropriate action.

C40 Cities, a network of city mayors focused on climate action, suggests a number of actions that can be taken to improve urban air quality, including:

  • Adopting the World Health Organization guidelines on recommended maximum levels of air pollution levels as your city’s air quality target, in addition to monitoring air quality and identifying priorities when it comes to decreasing air pollution
  • Establishing clean air zones to reduce vehicle-related pollution, such as the Ultra-Low Emission Zone (ULEZ) implemented in London
  • Encouraging a shift to zero-emission vehicles, including both private vehicles and city bus fleets
  • Move towards 100% renewable energy city-wide
  • Supporting increased use of public transportation, walking, and biking over private vehicle use, such as through creating pedestrian- and bicycle-only zones

To learn more about the link between air pollution and traffic, and how vehicle-related pollution can be decreased, read our blog here.

Specific actions can also be taken to reduce the urban heat island effect, such as:

  • Planting more trees and vegetation in urban areas and integrating green infrastructure practices, even into small areas, vacant lots, or barren areas in cities
  • Integrating green infrastructure improvements into regular street upgrades and capital improvement projects to ensure that heat-reducing changes become continually invested in as a regular part of city improvement
  • Building green roofs, which both help to cool and also improve air quality by absorbing pollutants

To learn more about innovative approaches to improving air quality in urban settings, read our blog here.

How governments can improve air quality monitoring

Governments play a vital role in taking action to improve urban air quality.

By adopting and enforcing air quality standards that are at least as stringent as the updated WHO air quality guidelines — and especially working to implement them where people suffer disproportionately from poor air quality — governments can ensure a strong base for air quality management.

When thinking about implementing neighborhood-scale air quality monitoring, it is crucial to follow the principles of environmental justice to avoid perpetuating the decades-long impacts of historical discriminatory practices, such as redlining, that took place in cities and whose legacy continues to impact marginalized communities today — including in their air pollution exposure.

Read our blog here to learn more about the essential role that environmental justice plays in bringing clean air for all.

Local governments can take action to address air pollution sources and their impacts within their communities, such as by creating and enforcing local policies, codes, and action plans that promote clean air

While individual action such as choosing public transportation over private vehicle use can help, the issue ultimately demands greater action.

Most sources of urban outdoor air pollution are well beyond the control of individuals and demand action by cities, as well as national and international policymakers to promote cleaner transport, more efficient energy production and waste management”

— World Health Organization

To learn more about the importance of connecting local action with initiatives at the state, federal, and global levels to bring about effective air quality and climate action, read our blog here.

City-scale particulate matter monitoring using IoT-based air quality devices

IoT-based air quality monitoring systems can be an extremely useful tool when it comes to urban air quality monitoring. 

By combining the measurement of a variety of key pollutants — such as PM2.5 and NO2 — with environmental parameters like temperature and humidity, and using IoT technology to upload this data on a cloud, air quality monitoring data can more effectively gain insight into the issues faced by a city.

Read our blog covering why cities need higher-resolution air quality data here to learn more.

High-resolution air quality monitoring networks ensure that you have a complete picture of air quality in a given city in order to take effective action. Let’s take a look at how this type of air quality monitoring network has been successfully employed in the city of London.

How air quality is measured in big cities such as London

The Breathe London initiative, which has brought neighborhood-scale air quality monitoring to the city of London, has acted as a pioneer in urban air quality monitoring that brings together different collaborators and works to involve residents in their community’s air.

The initiative highlights the importance and power of hyperlocal data to provide stakeholders with knowledge of their direct exposure to air pollution in their neighborhood, and the inequalities across this exposure among different areas of a single city.

London is a city that has historically struggled with air pollution. Though its air quality has significantly improved over time, it still exceeds the WHO limits for key air pollutants like NO2 and PM2.5, and 2019 data shows that air pollution contributed to 4,000 premature deaths in the city.

The Ultra-Low Emission Zone (ULEZ) in London has been a monumental part of enacting positive change. The zone benefits four million city residents and has lowered nitrogen dioxide levels by 21% in inner London and 46% in central London.

Since the ULEZ was first introduced four years ago it has led to a reduction of around 800,000 tonnes of CO2 emissions from vehicles across London – showing how air quality and carbon reduction are linked issues”

— C40 Cities

The city’s regulatory-grade ambient air quality monitoring network provided limited spatial coverage of actual air pollution levels around the city, driving the Greater London Authority (GLA) to partner with Imperial College London to use low-cost sensors to supplement the network and engage more citizens in the work for cleaner air.

The Breathe Cities project

This past June, the Breathe Cities project was announced, which will work to connect communities and dismantle barriers to clean air around the world. The project follows the model of Breathe London, spearheaded by London Mayor Sadiq Khan. 

Breathe Cities is a collaborative initiative of Bloomberg Philanthropies, the Clean Air Fund, and C40 Cities.

Building on clean air action in London and other cities, Breathe Cities is a pioneering initiative that will use $30 million to reduce air pollution, cut carbon emissions, and improve public health in cities around the world” 

— Clean Air Fund

The Breathe Cities initiative takes the knowledge of the vast harms that urban air pollution creates — such as the fact that almost no urban area meets the air quality guidelines of the WHO, with 41% of cities having air pollution levels over seven times this level. 

The project will work to implement the technology and support that cities around the globe need to improve their air quality management, reduce air pollution levels, and protect their residents.

Breathe Cities will focus on four key pillars: increasing the use of technology and research to expand local air quality data; engaging a variety of stakeholders to support coordinated air quality action; supporting local governments and policymakers in clean air action; and sharing the lessons learned across cities to improve global clean air gains. (Image source: C40 Cities)

Bringing clean air to cities across the globe

Interested in learning more about how Breathe London works — and how to bring improved air quality management to your city? Check out our page on Breathe London here, featuring our “Air Quality Management 2.0” white paper.