More than 90 percent of the global population breathes dangerously high levels of air pollution1. Due to its ubiquity and severity, air pollution constitutes the world’s biggest environmental health hazard, contributing to as many as 7 million premature deaths globally per year (more than 3 times higher than deaths associated with COVID-19).2 Air pollution also burdens the global economy with more than $5 trillion in welfare losses.3
In 2020, the spread of COVID-19 raised new concerns, as exposure to particle pollution was found to increase vulnerability to the virus and its health impacts. Early reporting suggests that the proportion of COVID-19 deaths attributed to air pollution exposure ranges from 7 to 33 percent.4
In a year defined by dramatic measures taken around the world to reduce the spread of COVID-19, IQAir published its 2020 World Air Quality Report to raise awareness around air pollution – a silent killer.
The 2020 World Air Quality report (download the PDF report) aggregates PM2.5 data from 106 countries, collected from ground-based government monitors and a growing network of validated, non-governmental air quality monitors contributed by organizations and individuals in order to learn from the world’s largest air pollution database.
Note: Only data with high availability has been included. However, since both governmental and non-governmental monitoring stations have a degree of error (even after careful data validation), location (city/country/region) rankings should be treated as indicative of trends rather than an absolute value in the ranking.
Major cities from the 2020 World Air Quality Report were analyzed and mapped:
Singapore (-25%), Beijing (-23%), and Bangkok (-20%) observed the greatest reductions in PM2.5 emission from the sample. São Paulo (+5%), Los Angeles (+1%), and Melbourne (+1%) observed the greatest increases in PM2.5 from 2019 levels – all three were impacted by severe wildfire seasons, which greatly affected annual PM2.5 averages.
86 percent of Chinese cities experienced cleaner air than the year prior, while annual PM2.5 exposure by population fell 11 percent. Despite this progress, China continues to dominate the ranking of top 100 most polluted cities globally, with 42 cities represented.Hotan, a desert oasis in Xinjiang province, ranked as the most polluted city globally, with pollution levels 11 times higher than the WHO target for annual pollution exposure (< 10 μg/m3). The city also had the highest monthly PM2.5 averages worldwide from March to June, when weather typically increases the intensity of sandstorms (with March peaking at an average 264.4 μg/m3).
Beijing experienced improved air quality for its 8th consecutive year, with air pollution levels falling 11 percent from 2019. Air pollution still remains a dire concern in the Chinese capital, with 58 percent of annual days exceeding daily WHO PM2.5 targets (< 25 μg/m3).
No South Korean cities achieved the WHO target for annual PM2.5 exposure (< 10 μg/m3) in 2020. Only 5 (of 60) cities in South Korea met the country’s less stringent standard for annual PM2.5 of < 15 μg/m3.
Despite chronically high pollution levels, South Korea did observe dramatic air quality improvements in 2020, with population-weighted PM2.5 exposure falling 21 percent. These improvements, however, are largely attributed to short-term measures intended to reduce the spread of COVID-19 and limit coal factory emissions during the polluted winter months.
Long-term policy and changes in human behavior are necessary to further reduce South Korea’s PM2.5 levels.
India has observed markedly improved air quality in 2020 despite still experiencing dangerously high pollution levels with severe health consequences. Every city in India observed air quality improvements compared to 2018 and earlier, while 63 percent saw direct improvements against 2019. However, India continues to feature prominently at the top of the most polluted cities ranking with 22 of the top 30 most polluted cities globally. 2020 was a particularly severe year for agricultural burning, an illegal but common practice in which farmers set fire to crop residue after a harvest. Farm fires in Punjab increased 46.5 percent over 2019.Delhi’s most polluted months correlate with the agricultural burning season, which spans October through December. During these months, average pollution exposure often exceeds the WHO annual target (10 μg/m3) by more than 14 times.
Despite social distancing measures that reduced motor vehicle emissions—one of the leading sources of PM2.5 in the U.S. —average air pollution exposure levels across the U.S. were higher in 2020 than in 2018 and 2019.38 percent of U.S. cities failed to meet WHO targets for annual PM2.5 exposure in 2020, compared to 21 percent of U.S. cities in 2019 and 20 percent in 2018. Stagnated and worsening levels of particle pollution in the U.S. correlate with increasingly severe wildfires seasons as well as rollbacks of environmental regulations and lack of enforcement of the Clean Air Act.A record-breaking wildfire season resulted in elevated summer and autumn air pollution levels in most U.S. cities. During September 2020, U.S. cities comprised 77 of the world’s top 100 most polluted cities for PM2.5 by monthly average. 35 of these cities were located in California, 35 in Oregon, and 7 in Washington.
South Asia represents the world’s most polluted region, containing 37 of the 40 most polluted cities globally. The most polluted countries in the region have numerous cities that average US AQI measurements of “Unhealthy” (> 55.5 μg/m3) or worse:
Air pollution remains a severe problem facing the Southeast Asia region: only 10.8 percent of cities here breathe air quality that meets annual PM2.5 exposure targets set by the WHO. Despite the continual health burden, 70 percent of cities in Southeast Asia observed improved air quality in 2020. Cities that did not observe direct improvements over 2019 were predominantly located in northern Thailand, which suffered from huge smoke emissions resulting from the agricultural burning season.The capital cities of Jakarta (39.6 μg/m3) and Hanoi (37.9 μg/m3) again ranked higher than notoriously polluted Beijing (37.5 μg/m3) in 2020.
Lockdown measures and changes in human behavior in response to the spread of the novel coronavirus resulted in healthier air overall in 2020. Air quality improvements over 2019 were observed in 84 percent of countries (weighted by city population) and 65 percent of global cities.5
The most significant air quality improvements were observed during the first lockdown period, when countries around the world mandated relatively strict social distancing measures in an effort to contain the virus.
Cities with higher average PM2.5 levels and denser populations tended to observe the most significant PM2.5 reductions from COVID-19 lockdown measures. Delhi (-60%), Seoul (-54%) and Wuhan (-44%), for example, all observed substantial drops during their respective lockdown periods as compared to the same time frame in 2019. Los Angeles experienced a PM2.5 reduction of -31% during its lockdown period as well as a record-breaking stretch of air quality that met WHO air quality guidelines (< 10 μg/m3).
Often, these initial improvements were short-lived. By the end of 2020, rebounds in industry and transport resulted in smaller average annual reductions:
The 2020 World Air Quality Report shows a correlation between the world’s changing climate and both the frequency and severity of high-pollution episodes.2020 tied with 2016 as the hottest year on record.6 At the same time, climate-linked pollution events, such as wildfires and sandstorms, broke records from California and South America to Serbia and Australia.
These pollution episodes resulted in significant pollution spikes often behind many of the regional “most polluted cities” rankings:
Global air quality monitoring has grown rapidly in recent years. Since 2018, the number of air quality monitoring stations has increased by more than 289 percent, while the number of countries represented by at least one monitor has grown by 33 percent. These gains in air quality monitoring infrastructure are attributable to both expanded or new governmental monitoring networks as well as significant contributions from global citizens and non-governmental organizations. Despite these gains, air quality monitoring is still noticeably lacking in numerous countries and regions, leaving huge populations without the information needed to tackle the pollution problem and empower healthier decisions. Africa, Latin America, and West Asia have the sparsest monitoring networks globally.Carefully calibrated and validated low-cost air quality sensors present an opportunity for quickly bridging the information gap. These sensors, which can be deployed with fewer resources and without government consensus, currently provide the only available air quality data for:
 World Health Organization. (2018). 9 out of 10 people worldwide breathe polluted air, but more countries are taking action. https://www.who.int/news/item/02-05-2018-9-out-of-10-people-worldwide-breathe-polluted-air-but-more-countries-are-taking-action
 Law T. (2021, January 15). 2 million people have died from COVID-19 worldwide. Time. https://time.com/5930111/2-million-covid-19-deaths/
 The World Bank. (2016, September 8). Air pollution deaths cost the global economy us $225 billion. https://www.worldbank.org/en/news/press-release/2016/09/08/air-pollution- deaths-cost-global-economy-225-billion
 European Society of Cardiology. (2020, October 27). Study estimates exposure to air pollution increases COVID-19 deaths by 15% worldwide. https://www.escardio.org/The-ESC/Press-Office/Press-releases/study-estimates-exposure-to-air-pollution-increases-covid-19-deaths-by-15-world
 The United States has the biggest air quality monitoring network, two times larger than the second biggest in China. Locations in the U.S. tended to experience worsened air quality in 2020 as a record-breaking wildfire season overshadowed PM2.5 reductions. As a result, the percentage of global cities that observed air quality improvements was smaller than the population-weighted country average.
 National Aeronautics and Space Administration (NASA). (2021, January 14). 2020 tied for warmest year on record, NASA analysis shows. https://www.nasa.gov/press-release/2020-tied-for-warmest-year-on-record-nasa-analysis-shows
2020 World air quality report