#SmogStripes Reveal Which Cities are Making Progress on Clean Air

By Angel Hsu, Chester Ling, and Diego Manya

The news about this winter’s air pollution has largely centered on India and South Asia, and yet, as global climate patterns like El Nino contribute to poor air quality throughout the world, data indicates that many cities are struggling to address toxic air pollution, which is quietly the most deadly 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. The World Economic Forum’s Annual Global Risks Report 2024 showed that policymakers are not giving priority to pollution as a significant concern, ranking it 10th, behind other global topics like misinformation (1st) and inflation (7th), in severity of short- and medium-term risks. 

The World Health Organization (WHO) in 2022 indicated that effectively the entire global population – 99 percent – breathes air that exceeds safe pollution limits. This evaluation was based on a sample of over 6,000 cities in 117 countries that now regularly monitor air quality from ground-based stations. It is encouraging that so many cities – 2,000 and counting – 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. Regular monitoring of these harmful pollutants is essential for preventing cardiovascular mortality associated with both indoor and outdoor pollution, which is the primary cause of death attributable to air pollution. WHO cited far-reaching, concrete evidence of air pollution’s pernicious effects on human health as the major impetus to revise its air quality guidelines in 2021 – the first major update in 15 years (Figure 1). WHO hopes with these revised recommendations that governments will acknowledge air pollution’s dire implications for human health and act to save lives. 

Figure 1. 2005 vs. 2021 air quality guidelines for particulate matter pollution (PM2.5 and PM10).


What do the WHO recommendations updates mean for cities’ efforts to curb air pollution? Are cities meeting or falling short of these standards? What would it take for cities to meet these benchmarks, and, in turn, save lives? To answer these questions, we’ve compiled city-level air quality data from five data sources spanning 1998 to 2022 for nearly 300 cities, and we created an interactive visualization called #SmogStripes based on climatologist Ed Hawkins’ #showyourstripes data visualization of temperature anomalies. 

#SmogStripes evaluates cities’ performance using open-source data on local ambient air quality, measured by annual mean PM2.5 concentration. We use a color scheme inspired by the U.S. Air Quality Index (AQI), a benchmark used to communicate the severity of air pollution levels to the public through an easy to understand color scheme (Figure 2). Cities’ annual average PM2.5 concentration, measured by both satellite and ground-based monitors that feed a range of publicly-available data sources (e.g., State of Global Air, Air Quality Life Index, World Health Organisation, IQAir; and the Urban Environment and Social Inclusion Index), were 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 shade of red indicates that all WHO guidelines have been exceeded. 

We 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.

Figure 2. WHO Annual Air Quality Guidelines and Interim targets for fine particulate matter pollution (PM2.5) and the associated color gradings for #SmogStripes.

Figure 3 provides an example of #SmogStripes for six cities. We’ve built an interactive application where you can choose a set of cities to explore and compare, as well as toggle stripes showing relative performances under the WHO 2005 and 2021 guidelines. We also include arrows indicating whether a city has improved (), declined (), or held steady (◀▶) over the time period that the data covers (1998-2022). The results show that 49% of cities in our 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, we must acknowledge that 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.

#SmogStripes is a new tool that anyone can use to understand their city’s air pollution challenge and compare it with others. To access the full app, where you can select your own city, visit: https://datadrivenlab.us.reclaim.cloud/smogstripes/. Image: SmogStripes

Harnessing #SmogStripes for Action

#SmogStripes show that the majority of cities will 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 outdated WHO thresholds have an uphill climb. Even 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 percent of PM and NOx emissions come from the transport sector in Europe. 

Despite this pushback, there are recent examples of progress in unexpected places, like in Warsaw, Poland, a European city that 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 Dec. 2023, city councilors voted in favor 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.

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. 

“Thanks to the activities of non-governmental organizations, public awareness of the impact of air pollution on health has significantly increased in Warsaw. This has given local authorities a mandate to introduce solutions such as the LEZ. Data, if presented well, can be a huge support in this effort,” Magdalena Młochowska, Director Coordinator for Green Warsaw, City of Warsaw, told us. 

We hope that #SmogStripes reproduces the success of the viral #ShowYourStripes movement started by Ed Hawkins’ visualization, which has sparked fashion trends and Summer Solstice events. We aim to catalyze meaningful dialogue between civil society, community members, planners, and politicians to foster a sense of urgency for air pollution action. 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?

Let’s drive awareness about the dangers of air pollution and strive for every city to get greener, cleaner, starting today.

This blog was also published to the World Economic Forum here.
Dr. Angel Hsu is an Associate Professor of Public Policy and the Environment at the University of North Carolina at Chapel Hill and a member of the Global Future Council for Clean Air. She founded and directs the Data-Driven EnviroLab, where Diego Manya is a Research Scientist. Chester Ling is a final year student at the Asian School of Environment at Nanyang Technological University in Singapore.