Air Pollution

Introducing SmogStripes: An online tool to visualize and compare global air pollution

Air pollution is a silent killer that virtually every person around the world faces — #SmogStripes helps us to better understand how our own city's air pollution stacks up to others around the world.

Air pollution is a silent killer that virtually every person around the world faces — #SmogStripes helps us to better understand how our own city's air pollution stacks up to others around the world. Image: REUTERS/Mike Hutchings

Angel Hsu
Associate Professor, Environmental Science, University of North Carolina at Chapel Hill
Diego Manya
Research Scientist, Data-Driven EnviroLab
Chester Ling
Visiting Scholar, Data-Driven EnviroLab
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Air Pollution

This article is part of: Centre for Nature and Climate
  • Air pollution is the deadliest environmental threat humans face.
  • In fact, 99% of the global population breathes air deemed unhealthy by the World Health Organization.
  • Now, anyone can compare the air they breathe with other cities around the world with #SmogStripes.

Air pollution is quietly the deadliest environmental threat to humans. Even with more than half of the world’s population living in cities, air pollution remains an often overlooked and under-prioritized issue. Effectively, the entire global population – 99% – breathes air that exceeds safe pollution limits.

Now, a tool is available for anyone to check the air quality in their city, compare it to other cities in their country or around the world and understand whether the air they breathe is getting better or worse.

2,000 cities and counting around the world are now reporting data for particulate matter pollution, both coarse (PM10) and fine (PM2.5), which has severe health consequences including cardiovascular disease and stroke. The World Health organization (WHO) has recently revised its guidelines on what constitutes safe air to breathe, citing concrete evidence of air pollution’s pernicious effects on human health as the major impetus.

That update is the first change in 15 years. To understand what it means for the air we breathe in cities around the world, people can now turn to #SmogStripes.

Have you read?

Introducing #SmogStripes

#SmogStripes is an interactive application where anyone can access air pollution data for nearly 300 cities around the world.

Anyone can explore the data, compare air quality over time and toggle stripes showing relative performances under the WHO 2005 and 2021 guidelines. #SmogStripes evaluates cities’ performance using open-source data on local ambient air quality, measured by annual mean PM2.5 concentration.

The platform includes arrows indicating whether a city has seen its air quality improve (▲), decline (▼) or hold steady (◀▶) over the time period that the data covers (1998-2022).


To access the full app, where you can select your own city, visit:

How #SmogStripes works

Cities’ annual average PM2.5 concentration, measured by both satellite and ground-based monitors that feed a range of publicly-available data sources (including the WHO, State of Global Air, Air Quality Life Index and more) are plotted as #SmogStripes according to the corresponding thresholds in WHO’s index.

Green indicates that air quality complies with all WHO targets, and shades of red indicate an exceedance of the recommended WHO level. The darkest red indicates that all WHO guidelines have been exceeded.

Data-Driven EnviroLab's scientists calculated the stripes according to both the 2005 and 2021 WHO annual thresholds to reflect upgrades in the guidelines and the differences in evaluating a city’s air quality when using both standards.

For example, the recently established 2021 guidelines for “safe” PM2.5 concentrations stand at approximately 5 μg/m3, whereas the 2005 standards for safe PM2.5 concentrations were set at 10 μg/m3. Consequently, cities that had demonstrated commendable performance below 2005’s 10 μg/m3 benchmark might now be categorized as having unsafe air quality.

What #SmogStripes tells us

An overall analysis of all cities in the #SmogStripes database found that 49% of cities in the sample have improved their air quality by an average of 2.28 μg/m3 reduction of PM2.5 per decade. Around 18% of cities have experienced worsening air quality by an average increase of 2.79 μg/m3 of PM2.5 per decade. One-third (32%) of cities have not seen a significant change in air quality.

While some of these numbers are encouraging, particularly for those living in cities who have improved air quality, these advancements are relatively modest in light of the substantial reductions needed to align air quality with WHO’s recommended thresholds.

To illustrate: assuming the average reduction in PM2.5 concentrations for improving cities (2.28 μg/m3 per decade), a city with residents exposed to 35 μg/m3 (the lowest WHO interim threshold for both 2005 and 2021) would need 131 years to reach safe levels of PM2.5 (around 5 μg/m3).

This example makes clear the imperative for urgent and immediate actions to improve air quality.

Harnessing #SmogStripes for action

#SmogStripes show that the majority of cities need to heavily invest in air pollution mitigation and control efforts to meet the new WHO standards and change their stripes from red to green.

This is easier said than done, however, and policymakers who are already struggling with meeting the old WHO thresholds face an uphill climb. In Europe, local policymakers are pushing back on the EU’s efforts to strengthen air quality guidelines, in part due to public controversy over policies to curb transport-related emissions that propose restricting vehicle access or mandating stricter controls. The data, meanwhile, show that more than 40% of PM and NOx emissions come from the transport sector in Europe.

Despite this pushback, there are recent examples of progress. Take Warsaw, Poland — Warsaw has regularly failed to meet WHO pollution limits, ranking 14th worst out of 57 cities in Europe. Warsaw’s #SmogStripes show recent improvements in PM2.5 concentrations, showcasing early results from policies that reduce air pollution from major sources like coal-fired furnaces and passenger vehicles.

In December 2023, city councilors voted in favour of establishing Poland’s second low emission zone (LEZ) that would require vehicles to meet certain emission standards. To garner support for Warsaw’s LEZ, civil society groups worked with the local government to initiate a public awareness campaign and consultation that gathered feedback from 3,000 community members over several months.

Mobilizing action through public awareness

The case of Warsaw’s LEZ and the political challenges it faced to achieve community and decisionmaker support illustrates the importance of public awareness. This buy-in is necessary for air pollution control policies for major pollution sources like vehicles because the solution requires the active involvement of those contributing to the problem.

#SmogStripes illustrates the problem of air quality for all the world to see. Now must come meaningful dialogue between civil society, community members, planners and politicians to take urgent action on air pollution.

Collective efforts are needed to expand the number of cities improving air quality and also to accelerate the pace of improvements.

How does your city’s #SmogStripes fare, and how does it compare with others in your country? Has your city’s #SmogStripes become greener or redder over time?

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