This 2021 World Air Quality Report presents an overview of the state of global air quality in 2021. The report is based on PM2.5 air quality data from 6,475 cities in 117 countries, regions and territories around the world. The data used to create this report was generated by tens of thousands of regulatory and low-cost air quality monitoring stations operated by governments, non-profit organizations, research institutions, educational facilities, companies, and citizen scientists around the world.
The PM2.5 data presented here is reported in units of micrograms per cubic meter (µg/m³) and utilizes the latest World Health Organization (WHO) annual PM2.5 air quality guideline and interim targets as a framework for data visualization.
The dataset used to generate this report was derived from the IQAir real-time online air quality monitoring platform, which aggregates, validates, calibrates, and harmonizes data from air quality monitoring stations around the globe.
Additional information on real-time and historic air quality conditions in cities, countries, and regions around the world is available on the IQAir website, with a ranking of annual air quality data for 6,475 cities and an interactive global map of annual city concentrations.
With these resources, IQAir endeavors to engage, educate, and inspire governments, researchers, NGOs, companies, and citizens to work together to improve air quality and create healthier communities and cities.
Air pollution is now considered to be the world's largest environmental health threat, accounting for seven million deaths around the world every year. Air pollution causes and aggravates many diseases, ranging from asthma to cancer, lung illnesses and heart disease . The estimated daily economic cost of air pollution has been figured at $8 billion (USD), or 3 to 4% of the gross world product .
Air pollution affects those that are most vulnerable the most. It is estimated that in 2021, the deaths of 40,000 children under the age of five were directly linked to PM2.5 air pollution. And in this age of COVID-19, researchers have found that exposure to PM2.5 increases both the risk of contracting the virus and of suffering more severe symptoms when infected, including death .
In September 2021, the World Health Organization (WHO) released a timely and ambitious update to its global air quality guidelines, 15 years after the last update released in 2006. Acknowledging the significant impact of air pollution on global health, the WHO cut the recommended annual PM2.5 concentration by half, from 10 µg/m³ down to 5 µg/m³, with the ultimate goal of preventing millions of deaths .
IQAir's annual World Air Quality Report aggregates and compares millions of PM2.5 measurements taken in thousands of locations around the world. The data is gathered over the course of the year from a combination of regulatory and non-regulatory ground-based air quality monitors. Although many areas around the world still lack access to publicly available air quality information, global air quality data continued to increase in 2021.
While the 2020 World Air Quality Report included data from 4,745 locations in 106 countries and regions, the 2021 report coverage expanded to 6,475 locations in 117 countries, territories, and regions. This is in part due to the increasing number of low-cost air quality sensors mostly operated by non-profit organizations, governments, and citizen scientists. The increased number of PM2.5 stations helps to create a more accurate picture of hyper-local air quality, and global air quality.
While many other air quality reports and apps report air quality information based on satellite data, this report is generated exclusively from PM2.5 measurements obtained by ground-level monitoring stations. Air quality data was aggregated from regulatory monitoring stations operated by governments as well as privately-owned, non-regulatory stations operated by individuals, educational institutions, and non-profit organizations. Most data employed in the report was collected in real time. When available, supplementary year-end historical data sets were also included to provide the most timely and comprehensive global data analysis possible.
The data from individual stations were combined into “settlements,” which can represent a city, town, village, county, or municipality depending on local population patterns and administrative structures. In this report, “settlements” will be hereafter referred to as cities. The data from cities are subsequently population weighted and aggregated to create a regional annual average and ranking.
PM2.5 concentrations, in micrograms per cubic meter (µg/m³) were selected as the standard metric for this report. PM2.5, particulate matter consisting of fine aerosol particles measuring 2.5 microns or smaller in diameter, is one of six routinely measured criteria air pollutants and is commonly accepted as the most harmful to human health due to its prevalence in the environment and broad range of health effects.
PM2.5 is generated from many sources and can vary in chemical composition and physical characteristics. Common chemical constituents of PM2.5 include sulfates, nitrates, black carbon, and ammonium. The most common human-made sources include internal combustion engines, power generation, industrial processes, agricultural processes, construction, and residential wood and coal burning. The most common natural sources for PM2.5 are dust storms, sandstorms, and wildfires.
There has long been a consensus in the public health and scientific communities of the causal relationship between air pollution exposure and adverse health effects. However, connecting the dots between physically measured air pollution levels, represented in this report as PM2.5 concentrations in micrograms per cubic meter (µg/m³), and the resulting exposure-related health risks requires additional context. In 1987, the World Health Organization (WHO) published its first report on global air quality guidelines . The report documented findings of a comprehensive review and analysis of the available information from scientific studies linking air pollution levels and quantified risks of adverse health outcomes. The guidelines were crafted as a tool providing quantitative, evidence-based health information on the burden of disease caused by air pollution. The goal of the guidelines is to assist policymakers around the world in crafting air quality legislation and emissions standards to levels that meaningfully reduce the public health risks posed by air pollution. Since 1987, the guidelines have been periodically updated as new research has become available, and in September 2021,15 years after the last update was released in 2006, the WHO released the latest edition of the global air quality guidelines .
Work to produce the 2021 WHO global air quality guidelines began in 2016. WHO organizers administered the collaborative process by forming a global team of subject matter experts in fields including health effects of air pollution; air pollution emissions and atmospheric chemistry; health economics; vulnerable groups, equity, and human rights; policy implications; and methodology and guideline development. These experts worked together to review the available scientific information linking exposure to specific components of indoor and outdoor air pollution and adverse health effects. Once the information was compiled and assessed, the recommended guidelines were determined as the threshold pollutant concentration above which there is an increased potential for adverse health effects, independent of the microenvironment.
Evidence compiled from the last 20 years of research has more clearly quantified the impact severity of air pollution on human health, a factor now known to have been previously significantly underestimated. A systematic study analyzing the global burden of disease conducted in 2019 found that air pollution, from both indoor and outdoor sources, accounted for approximately 12% of deaths that year, ranking it as the fourth major risk factor for global disease and mortality . Results from the most recent studies quantifying the correlation between air pollution levels and adverse health risks have indicated that exposure to even low levels of air pollution can result in harmful health effects, concluding that there is no level at which exposure to PM2.5 could be deemed safe [7,8,9].
Acknowledging the severity of the impact of air pollution on global health and the non-trivial risk of adverse effects present at low pollution concentrations, the recommended level for the 2021 PM2.5 annual guideline was cut by half, dropping the 2005 guideline level of 10 µg/m³ down to 5 µg/m³. In many areas of the world with exceedingly high air pollution concentrations, achieving this guideline level is not immediately feasible. For these cases, incremental step goals of 10, 15, 25, and 35 µg/m³ were recommended for use in guiding the creation of air pollution reduction plans in highly polluted regions capable of being accomplished in realistic timelines.
Data for this report is presented within the context of the WHO recommended annual air quality guideline levels and interim targets for PM2.5. These guidelines help to determine which cities and regions are most in danger from the health risks of PM2.5 in the hopes that they implement stricter policies to help lower those risks.
This report uses seven color bands to provide an easy identification of multiples of the 5 µg/m³ WHO PM2.5 guideline, and the interim target levels.
While World Air Quality Reports from previous years utilized the U.S. AQI as a tool to communicate health risks associated with PM2.5 exposure, data visualization for this year's report is based on the WHO annual guideline value and interim targets. The U.S. AQI was calibrated to convey information about health risks associated with exposure to multiple pollutants over the period of a day [10,11]. The PM2.5 concentration values reported here are based on an averaging period of a year, rather than a single day. As such, it is deemed more appropriate to use the WHO annual guideline and interim target levels as a framework for data visualization.
2021 global map color coded by annual average PM2.5 concentration
Countries and regions in East Asia, Southeast Asia, and South Asia suffered from the highest annual average PM2.5 concentration weighted by population. Notably, the African continent had only 13 out of 54 countries with sufficient public air quality monitoring data, resulting in a majority of countries showing in gray. Additionally, the Latin America and Caribbean region also lacked sufficient monitoring.
Population weighted, 2021 average PM2.5 concentration (μg/m³) for countries, regions, and territories in descending order
Average annual PM2.5 concentration (µg/m³) for capital cities in descending order
Global distribution of PM2.5 monitoring stations
Distribution of global PM2.5 air quality monitoring stations that contributed data to this report. Government stations are shown in red. Independently operated monitoring stations are shown in blue.
Publicly available air quality monitoring station data continued to increase in 2021. India, New Zealand, and Canada, in particular, saw significant increases.
Government air quality monitoring networks continue to be most commonly found in China, the United States, Japan, and in Europe.
It should be noted that developed countries have a higher density of air quality monitoring stations than developing countries and regions. Latin America, Africa, and Central Asia remain sparsely monitored, despite the high population density in many of these regional areas.
Given the low cost point, ease of deployment, and significant reduction in operational costs, low- cost air quality monitors are increasingly becoming a viable public alternative to those countries, regions, and territories that lack government-operated air quality monitoring stations. In 2021, non-governmental low-cost air quality monitoring provided the only real-time air quality data for Albania, Angola, Azerbaijan, the Bahamas, Belize*, Benin*, Bermuda*, Cambodia, Cameroon*, Cape Verde, Democratic Republic of the Congo*, Dominican Republic*, Egypt, Ethiopia, French Polynesia*, Gambia*, Grenada, Guam, Guyana*, Honduras, Jamaica*, Jersey*, Lebanon, the Maldives*, Moldova*, Morocco*, Nicaragua*, Niger*, Panama*, Qatar, the Republic of the Congo*, Rwanda*, Saba, Senegal*, Sierra Leone*, South Sudan*, Suriname*, Timor-Leste*, Togo*, Uruguay, U.S. Virgin Islands, Venezuela*, Zambia*, and Zimbabwe*.
*Cities in these countries did not meet the required limit of 60% annual data availability and were therefore excluded from the report.
China Mainland | Hong Kong SAR | Japan | Macau SAR | Mongolia | South Korea | Taiwan
The region of East Asia included data from 1,347 cities. Of these cities, 143, or about 11%, had annual average PM2.5 concentrations greater than seven times WHO's PM2.5 guideline of 5 µg/m³. All 143 cities were located in China. The Chinese city of Hotan was the region's most polluted city, with an annual average PM2.5 concentration of 101.5 µg/m³, over 20 times the WHO guideline value. The city of Handan, China showed the region's greatest reduction in absolute PM2.5 concentration, dropping from a 2020 average of 58.9 to 45.6 µg/m³ in 2021. Japan continues to be the regional country with the best air quality in terms of PM2.5 concentrations, with a 7% reduction in 2021 compared to 2020. Of the 529 Japanese cities reporting data in both 2020 and 2021, 81% have seen air quality improvements. Ogasawara and Minami Ward had Japan's best air quality with an annual PM2.5 concentration of 5.3 µg/m³. Tanabe, Japan improved its last year average by over 35%, dropping from a 2020 concentration of 9.0 µg/m³ to a 2021 concentration of 5.8 µg/m³. Besides Japan, the Mongolian towns of Sumber and Chonogol were the least polluted in the region.
In South Korea, air quality improved in 60% of cities reporting data in both 2020 and 2021. Some of the primary contributors of PM2.5 in the region include coal-based energy production, industrial activity, gas and diesel-powered transportation, and household heating.
Government-operated air quality monitoring stations in China, Japan, and South Korea provide East Asia with an extensive network of air quality monitors, resulting in some of the most comprehensive air quality monitoring coverage in the world. Regionally, China has the largest air quality monitoring network in terms of land mass monitored. However, in terms of station density, Japan's network provides the highest monitoring resolution.
In 2021, regional air quality monitoring was expanded to include 114 new cities, primarily located in China (86) and Japan (24).
Air quality in China continued to improve in 2021, with data from 66% of Chinese cities showing decreased PM2.5 concentrations compared to 2020. In 2021, air pollution in the capital city of Beijing continued a five-year trend in PM2.5 reductions and marked the first time the city achieved its own air pollution targets . Since 2018, China overall has seen a 21% reduction in annual PM2.5 concentrations. However, no Chinese city included in this report met the revised WHO PM2.5 guidelines of 5 µg/m³. Hotan was the country's most polluted city for the second consecutive year and exceeded WHO guidelines by over 20 times.
China is the world's largest producer and consumer of coal, and coal remains one of the most significant sources of PM2.5 pollution in China along with industrial sources and motor vehicle emission . Coal is used to produce as much as 60% of the country's electricity. The government has pledged to reduce coal usage for electricity generation by 1.8% before the year 2025, but coal continues to be a significant environmental and health hazard in China .
Seasonal sandstorms due in part to desertification fueled by climate change and other factors continue to raise PM2.5 levels in Northern China. In March of 2021, Northern China was hit with the strongest sandstorms in a decade, causing the city of Beijing's sky to turn yellow with dust. The sky discoloration was an indicator of PM2.5 concentrations at ten times the WHO's recommended daily limit.
Primarily affecting cities close to the Gobi Desert, sandstorms have raised PM2.5 pollution as high as 20 times the levels deemed to be healthy, causing significant loss of property and even the loss of life . Every year, sandstorms aerosolize 800 million tons of sand in this region.
South Korea continues the trend of reducing annual average PM2.5 concentrations, dropping from 19.5 μg/m³ in 2020 to 19.1 μg/m³ in 2021. None of the South Korean cities in the 2021 report met the WHO PM2.5 annual guideline. The city of Gangneung has shown significant improvement over the past 4 years, with over a 50% reduction in PM2.5 concentration between 2017 and 2021 from 24.6 to 11.0 μg/m³. Cheonan had the highest PM2.5 concentration at 28.1 μg/m³ and was one of the top 20 most populated cities in the country.
South Korea has many green incentives and helps finance sustainability research and development . As of 2015, South Korea's energy supply was 65% coal and oil, 31% nuclear, and 2% hydro, but recent policies created the National Strategy for Green Growth, which incentivizes renewable energy. South Korea also imposed taxes to disincentivize nonrenewable energy use, such as a 25% coal import tax.
South Korea experiences increased air pollution around the capital city of Seoul and areas with high concentrations of manufacturing and industrial sites, as well as sand and dust storms from the Gobi Desert. These dust storms have become more frequent and extreme, because of drought and increasing temperatures due to climate change .
The Yellow Sea plays a significant role in atmospheric and environmental conditions across the Korean Peninsula. Sea fog forms due to temperature inversions over the Yellow Sea. Studies have shown that sea fog facilitates the transport of air pollutants from East Asia to the Korean Peninsula, and that chemical reactions occurring in the sea fog create new aerosols. These factors have been shown to result in increased PM2.5 concentrations not only along South Korea's western coast, but even further inland in the Seoul Metropolitan Area [18,19].
Cambodia | Indonesia | Laos | Malaysia | Myanmar | Philippines | Singapore | Thailand | Vietnam
Rapid population growth and the accompanying economic development have been significant factors contributing to increased air pollution in Southeast Asia. Oil and coal are the biggest sources of fuel in the power sector, and with the demand for electricity expanding at a rate of approximately 6% per year, the combustion of fossil fuels is a primary contributor to PM2.5. Other PM2.5 sources in urban areas include construction, industrial emissions, and transportation. Rural PM2.5 emission sources include open burning practices used to manage farmland and clear forests.
Most countries in the region have laws against open burning. However, due to a lack of enforcement, the practice continues to remain widespread. Open burning is estimated to contribute as much as 5 - 30% of the total human-made emission inventory in Southeast Asia . The practice adds to the transboundary movement of air pollutants, creating seasonal air pollution cycles in the countries of Singapore and Malaysia. Human-made changes in land use through deforestation and agriculture have intensified wildfires, another key source of regional pollution .
Regional PM2.5 levels dropped by about 5% from 2020 to 2021. This reduction was driven by six countries showing improved air quality in 2021 and led by Indonesia with a 16% reduction in annual PM2.5 concentrations. Despite the improvements, Indonesia continues to be the region's most polluted country (ranked 17th globally) with a 2021 PM2.5 annual concentration over 30% greater than the region's second ranked country of Myanmar (ranked 31st globally). Interestingly, the three least polluted regional countries, Malaysia, Philippines, and Singapore showed the most significant increases in PM2.5 concentrations in 2021, with relative increases of 24%, 22%, and 17% respectively compared to 2020.
Non-governmental organizations and individuals operate about two-thirds of the region's air quality monitoring stations. Thailand's government operates the region's largest air quality monitoring network encompassing 113 cities, none of which were able to meet the WHO PM2.5 guideline in 2021. Only one city of the 227 Southeast Asian regional cities met the 2021 annual average PM2.5 WHO air quality guideline. Two of the top 10 populated cities in the region, Jakarta and Hanoi, had annual average PM2.5 concentrations that exceed the PM2.5 WHO air quality guideline by more than seven times.
Overall air quality in Indonesia has improved from an average PM2.5 concentration of 40.7 µg/m³ in 2020 to 34.3 µg/m³ in 2021. However, persistently poor air quality in the country's most populous cities continues to endanger the health of residents. The worst air quality in 2021 was measured in the capital city of Jakarta. Jakarta's 2021 annual average PM2.5 concentration was 39.2 µg/m³, a level exceeding the WHO guideline by more than seven times, as well as exceeding all WHO interim targets. Makassar, Indonesia's fifth largest urban center, saw an 8% reduction in PM2.5 concentrations. Of the six most populated cities in Indonesia reported here, only Makassar recorded significant PM2.5 reductions in 2021. The countrywide PM2.5 concentration reductions resulted from significant reductions in the lesser populated cities of Bogor, Pekanbaru, and Denpasar. Only one of the 26 Indonesian cities reported here met the WHO recommended PM2.5 guideline of 5 µg/m³. There was little to no improvement in air quality between 2020 and 2021 for Indonesia's most populated cities like Jakarta, Bandung, and Semarang.
Poor air quality in Indonesia is a relatively recent phenomenon. Prior to 2013, estimated PM2.5 annual average concentrations remained below 15 µg/m³ . Increases in PM2.5 concentrations accelerated dramatically in 2016, with levels peaking in 2019 at 51.7 µg/m³. In 1999, the Indonesian government enacted air quality regulation policies establishing the initial air quality standards and emissions limits for motor vehicles and industrial sources . However, existing regulatory policies are now both outdated and inadequate to protect the public health in Indonesia's key cities. Examples of ineffective policy include those around burning practices for waste management and forest clearing. Though open burning was prohibited by the 1999 Forestry Law, both practices remain commonplace. Activist lawsuits culminated in a September 2021 decision by the Central Jakarta District Court that President Joko Widodo, members of his cabinet, and three provincial governors were guilty of negligence by failing to combat Jakarta's air pollution . In 2021, the Indonesian government revised the national air quality standards (Govt Reg No. 22/2021) lowering the annual PM2.5 concentration limit to 15 µg/m³ .
Open burning is still very common in Indonesia despite policies countering the practice. Much of the land in Indonesia has been cleared by open burning to replace forestland with cash crops . Continued forest logging causing flammable dry land in the province of Riau has resulted in the region containing the most vegetation fires in the country, with 60% of the country's total . Wildfires in the country are increasing in frequency and extremity, and these short-term exposures to extreme pollution levels are associated with both short- and long-term health problems.
In 2021, the country of Malaysia saw a 25% increase in PM2.5 concentrations relative to 2020. Malaysia's 2021 annual average of 19.4 μg/m³ marked an absolute increase of 3.8 μg/m³ over 2020 levels and a return to 2019 levels, erasing the 19% improvement in PM2.5 concentrations recorded in 2020. This change was largely driven by the increased PM2.5 concentrations in all of the country's ten most populous cities reporting data in 2021, with an average increase of 4.5 μg/m³. Nationwide, a total of 49 cities reported escalating PM2.5 concentrations, with average increases of 2.7 μg/m³ in 2021 compared to 2020. Malaysia's annual PM2.5 average was tempered by the 11 Malaysian cities that reported reductions in 2021 compared to 2020.
Major sources of air pollution in Malaysia are combustion of fossil fuels by vehicles, industry and households, as well as forest fires and dust . According to the WHO, air pollution is responsible for one out of every nine deaths in Malaysia, making it one of the top causes of death.
Agricultural burning to clear fields for new crops in Indonesia has led to ongoing transboundary haze issues in Malaysia . The most notable and consequential incident of transboundary haze in Malaysia occurred in 2015. Agricultural burning haze drifted to Malaysia and other countries in the region from late June until October. Researchers from Harvard and Columbia have since concluded that 6,500 Malaysians died from the 2015 haze incident alone, and as many as 100,000 people died in the region . The ASEAN Agreement on Transboundary Haze was established in 2003 by the nations of Malaysia, Indonesia, Thailand, Singapore, Brunei, Myanmar, Vietnam, Laos, Cambodia and the Philippines . However, there are concerns about Indonesia's enforcement of deforestation laws.
Thailand continued the trend of gradual reductions in country-level PM2.5 concentrations dating back to 2018, marking a 5% overall reduction in 2021. Only one city of the top 10 most populous cities, Surat Thani, recorded an increase in PM2.5 concentrations in 2021, measuring a 30% increase compared to 2020. Significant improvements in air quality occurred in the cities of Chiang Mai and Khon Kaen in 2021, with both cities dropping from WHO Interim Target-1 to Target-2 levels following 18% and 13% PM2.5 concentration reductions. However, not one Thai city met the WHO annual average PM2.5 concentration guideline of 5 µg/m³ in 2021, and only two cities, Ko Kut and Nong Khai, achieved the WHO Interim Target-4 with annual averages of 7.1 and 7.8 µg/m³. Air pollution was highest in the city of Chiang Rai with a PM2.5 concentration of 27.1 µg/m³ in 2021, which is more than five times the WHO air quality guideline of 5 µg/m³. In 2021, Bangkok, the nation's capital and most populous city, continued the trend of incremental PM2.5 improvements with an annual average concentration of 20.0 µg/m³, down from 20.6 µg/m³ in 2020.
Thailand's major sources of air pollution include vehicle exhaust, industrial emissions, crop burning, transboundary haze, and electricity generation . Hindrances to combating air pollution include a lack of air quality monitoring, a lack of public awareness about the health impacts of crop burning, and a lack of enforcement of anti-crop burning laws .
Farming and agricultural burning to prepare land for the next crop is common in Thailand . Agriculture burning can raise PM2.5 concentrations to two to three times the WHO Air Quality Guidelines. Crop burning is an issue in Chiang Mai in the north of Thailand. In addition to the air pollution from crop burning, poor air quality can be exacerbated by geographical features and weather anomalies, causing pollutants to linger in the region during the winter months .
In 2021, Vietnam continued to exhibit a declining trend in annual average PM2.5 concentrations. Peaking in 2019 at a concentration of 34.1 µg/m³, annual PM2.5 concentrations fell in 2021 to 24.7 µg/m³. Despite declining concentrations, none of the 15 Vietnamese cities included in this report met the annual average WHO PM2.5 air quality guideline concentration of 5 µg/m³. Annual average PM2.5 concentrations for the capital city of Hanoi decreased by 4.5% in 2021 compared to 2020, dropping from 14.8 to 14.1 µg/m³ and achieving the WHO's annual PM2.5 Interim Target-3. Air quality in Ho Chi Minh City, Vietnam's most populous city, also improved in 2021 with a decrease in PM2.5 concentration from 22 µg/m³ in 2020 to 19.4 µg/m³ in 2021.
As manufacturing activity increased in Vietnam, the country's economy has grown over the past decade, increasing the demand for electricity. In 2020, half of the electricity generated in Vietnam came from coal-fired power plants . Vietnam's usage of fossil fuels for power generation is a primary cause of air pollution, along with exhaust from cars and motorbikes, and factory emissions .
At the UN COP26 conference in 2021, Vietnam vowed to reach carbon neutrality by 2050. With the largest solar power infrastructure in Southeast Asia and the addition of offshore and onshore wind farms, Vietnam is poised for a transition away from the use of fossil fuels to power its growing economy . Actions to achieve carbon neutrality will also have the co-benefit of improving air quality by reducing the use of fossil fuels, the country's major source of air pollution .
Afghanistan | Bangladesh | India | Iran | Kazakhstan | Kyrgyzstan | Nepal | Pakistan | Sri Lanka | Tajikistan | Turkmenistan
In 2021, Central and South Asia had some of the world's worst air quality and was home to 46 of the world's 50 most polluted cities. According to the United Nations Environment Programme, 70% of global air quality related deaths occur in this region . Only two cities in this region met the WHO air quality guideline for annual average concentration of 5 µg/m³, Chu and Zhezqazghan, both in Kazakhstan.
Industrialization and urbanization bring added pressure to air pollution burdens in South Asia. Burning biomass, a popular fuel for cooking in the rural areas, is a frequent source of pollutants in the air. Other sources include dust from construction and vehicles, fossil fuel combustion, and agricultural burning .
India and Pakistan generally experience the worst air quality in this region, with 48% and 67% respectively of cities with PM2.5 concentrations greater than ten times the 2021 WHO air quality guideline level. With the exception of Afghanistan, Bangladesh, and Sri Lanka, PM2.5 concentrations increased in Central and South Asian countries in 2021, wiping out nearly all quarantine- correlated air quality improvements.
In 2021, nearly all Central and South Asia countries have expanded their air quality network by increasing the number of monitors publicly reporting PM2.5 concentration data. Region-wide, the number of air quality monitoring stations increased by 50% during the course of 2021. Government-operated monitoring stations continue to provide most of the public air quality data in the region, with nearly three times as many government-operated stations than independently-operated ones.
India's annual average PM2.5 levels reached 58.1 µg/m³ in 2021, ending a three-year trend of improving air quality. India's annual PM2.5 averages have now returned to pre-quarantine concentrations measured in 2019.
India was home to 11 of the 15 most polluted cities in Central and South Asia in 2021. Delhi saw a 14.6% increase in PM2.5 concentrations in 2021 with levels rising to 96.4 µg/m³ from 84 µg/m³ in 2020. No cities in India met the WHO air quality guideline of 5 µg/m³. In 2021, 48% of India's cities exceeded 50 µg/m³, or more than 10 times the WHO guideline.
Air pollution has a massive impact on human health in India. It is the second biggest risk factor for disease, and the economic cost of air pollution is estimated to exceed $150 billion dollars annually . Major sources of air pollution in India include vehicular emissions, power generation, industrial waste, biomass combustion for cooking, the construction sector, and episodic events like crop burning .
In 2019, India's Ministry of Environment, Forest and Climate Change (MoEF&CC) enacted the National Clean Air Program (NCAP). The plan seeks to reduce PM concentrations by 20% to 30% by 2024 in all identified non-attainment cities, increase air quality monitoring, and implement a city, regional, and state-specific clean air action plan as well as conduct source apportionment studies.
However, the lockdowns, restrictions, and resulting economic downturn due to the COVID-19 pandemic have made it difficult to determine the plan's impact based on air pollution levels alone. A recent report found that apart from city-specific action plans, no other plans have been formulated under NCAP prescribed timelines . Additionally, there is little information about the activities related to the NCAP, making it difficult to dispel the public's dissatisfaction with the slow progress under the program.
It is estimated that 20% to 35% of total urban PM2.5 concentrations is directly or indirectly due to internal combustion engines in motor vehicles . Annual vehicle sales in India are expected to increase, with an estimated fleet number reaching 10.5 million in 2030 . In an effort to curtail the contribution to air pollution from motor vehicles, India has adopted rigorous vehicle emission standards for new vehicles. The BS-VI standard is currently equivalent to the Euro 6-1 standard and will be equivalent to the Euro 6-2 standard beginning in April 2023 . Emissions testing methodologies capable of measuring emissions under real-world driving conditions, rather than more simple laboratory drive cycles, are in development at the International Centre for Automotive Technology in India with an anticipated release of 2023 .
In 2021, Pakistan ranked as the third most polluted global country. Every Pakistani city included in this report reported annual average PM2.5 concentrations at least eight times higher than the recommended WHO guideline. The city of Muridke was the only Pakistani city in which there was a decrease in annual PM2.5 concentrations to 47.6 µg/m³ in 2021 from 61.6 µg/m³ in 2020. The national average PM2.5 concentration increased in 2021 by 8%.
The average life expectancy in Pakistan would increase by 3.87 years if the country reached the WHO Interim Target-4 value of 10 µg/m³ .
Pakistan continues to see poor air quality resulting from transportation, its biggest source of air pollution. In Punjab, the second largest province of Pakistan, 43% of air pollution emissions are from transportation, 25% from industrial sites, and 20% from agriculture . Seasonal air quality issues also affect Pakistan. Crop burning and winter weather patterns result in temperature inversions that promote air stagnation and help to keep air pollution trapped close to the ground.
Environmental health researchers at the University of Punjab have found a 40% decrease in lung capacity during Pakistan's smog events, primarily affecting individuals with tuberculosis, asthma, and cardiovascular disease . In addition, lower literacy study participants were almost 80% more likely to experience multiple symptoms of adverse health effects resulting in wage loss and diminished quality of life compared to those with higher literacy levels.
Pakistan announced a revised Pakistan Clean Air Plan (PCAP) in June of 2021, which assesses air pollution reduction at the national and local level. The PCAP was created to monitor air quality and implement policies and technologies based on measurements in different provinces . Pakistan's Environmental Protection Agency has already implemented federal policy to limit annual average PM2.5 concentrations in ambient air with the current National Environmental Quality Standard at 15 µg/m³. Despite the federal limit in place, annual average PM2.5 concentrations in Pakistan remain some of the highest in the world. Without enacting source-specific PM2.5 emission standards and emission load reduction targets, it remains unclear how effective additional policy changes will be .
Armenia | Azerbaijan | Bahrain | Georgia | Israel | Kuwait | Lebanon | Oman | Qatar | Saudi Arabia | Syria | United Arab Emirates
The capital cities of Muscat (Oman), Manama (Bahrain), Baghdad (Iraq), and Doha (Qatar) experienced the region's highest PM2.5 concentrations in 2021. The region's most polluted city, Muscat, ranks as the fifth most polluted capital city, with an annual average PM2.5 concentration of 53.9 µg/m³, a level exceeding the WHO guideline by more than ten times. While all other regional countries have seen PM2.5 concentrations increase by a median rate of 23%, Qatar and Kuwait achieved a nearly 15% reduction in PM2.5 emissions during 2021.
Major anthropogenic sources of air pollution in the West Asia region include fossil fuel-based energy production, emissions from industrial processes, waste burning, construction, and motor vehicles .
Dust storms are also a common source of natural air pollution in this region, especially from May to August as the temperatures rise and lead to large amounts of airborne dust. In early June 2021, strong winds carried dust from the Tigris-Euphrates River Valley to the western coast of the Persian Gulf and created a dust plume spanning over 500 kilometers or 310 miles .
West Asia is a region with generally scarce air quality monitoring. However, the number of monitors in this region increased by 86% in 2021, primarily due to the large increase in the number of monitors both in Israel and Saudi Arabia whose monitors accounted for 73% of the total number of regional monitors. Government-monitored air quality data was not available for Qatar and Azerbaijan. Both countries are represented here by U.S. State Department monitors or low-cost sensors from organizations or private individuals. Bahrain and Kuwait each have a single regulatory-grade monitor managed by the U.S. State Department.
Albania | Andorra | Austria | Belgium | Bosnia and Herzegovina | Bulgaria | Croatia | Cyprus | Czech Republic | Denmark | Estonia | Finland | France | Germany | Greece | Hungary | Iceland | Ireland | Italy | Kosovo | Latvia | Lithuania | Luxembourg | Malta | Montenegro | Netherlands | North Macedonia | Norway | Poland | Portugal | Romania | Russia | Serbia | Slovakia | Spain | Sweden | Switzerland | Turkey | Ukraine | United Kingdom
The region of Europe is represented by 42 countries in 2021 and a total of 1,588 cities. Average PM2.5 concentrations in this region span from 5.5 µg/m³ in Finland (ranked globally at 113) to 35.2 µg/m³ in Montenegro (ranked globally at 16). PM2.5 concentration data for both 2020 and 2021 was available for 39 European countries. In 2021, air quality improved in 14 countries and declined in 25 countries compared to 2020. Region-wide, only 55 cities were able to meet the recommended 5 µg/m³ WHO PM2.5 annual air quality guideline. The United Kingdom was home to ten cities achieving the guideline, more than any other country in the region, followed by Finland which had eight cities achieve the guideline.
Increased levels of PM2.5 have been reported to be the most serious environmental health risk in Europe, even though emissions have greatly decreased this century . The northern and western parts of Europe have higher PM2.5 levels than southern and eastern Europe due to the burning of coal and biomass for heating in countries that experience long, cold winter months.
A summer of record-breaking heat in southern Europe and central Russia generated immense wildfires in 2021. European countries affected spanned from Spain to Turkey. At one point, the citizens of Athens, Greece, were told to remain indoors due to high levels of air pollutants . Russia suffered through its biggest wildfire season ever recorded, contributing greatly to the unparalleled global levels of fire-produced CO2 emissions [53,54].
Air quality monitoring networks are robust in most European countries. The north and west boast the most concentrated networks, but the southern and eastern areas have impressive granularity, too. Complementing government monitoring are a large number of low-cost sensors operated by numerous individuals and organizations, especially from Bosnia Herzegovina, Greece, Kosovo, and Russia.
United States | Canada
Air quality monitoring networks in both the United States and Canada grew in 2021, as did emissions in the two nations. In 2021, 96% of U.S. and Canadian cities were not able to get below the WHO annual 5 μg/m³ guideline for PM2.5 levels, with those cities accounting for more than 98% of the region's population. The main sources of pollution were gas and diesel-powered transportation, coal-based energy production, industrial emissions, and wildfires .
Annual premature deaths blamed on human-made PM2.5 particulates are estimated at 200,000 in the United States and approximately 15,000 in Canada [56,57]. The 2021 wildfire season in North America was a large factor in the record-setting levels of wildfire-produced CO2 levels in earth's atmosphere . Wildfires were estimated to have generated as much as 70% of ambient PM2.5 on the days that national PM2.5 standards were surpassed.
Last year, the Canadian national heat record was broken, topped the next day, and then topped once more the following day. These unprecedented temperatures greatly contributed to a severe wildfire season. By September 2021, 6,317 outbreaks had scorched 10.34 million acres of land .
Northern America has the largest number of air quality monitors of all regions, covering 2,632 cities.
The vast majority of air quality monitoring stations in this region are in the United States with an approximate monitoring station to population ratio of 1 to 23,000, compared to Canada's 1 to 51,000. This high monitoring ratio is largely due to the increased number of low-cost sensors operated by local organizations and individuals in the U.S., now comprising 87% of the station total.
The monitoring network in the U.S. comprises 2,406 cities. Canada has a monitoring network across 226 cities.
The Clean Air Act has helped reduce PM2.5 air pollution levels while the U.S. economy, population, and energy demands have grown in the last 50 years. While policies continue to reduce air pollution and greenhouse gas (GHG) emissions, in 2021 PM2.5 concentrations increased by 7% compared to 2020. The country's reliance on fossil fuels, increasing severity of wildfires as well as varying enforcement of the Clean Air Act from administration to administration have all added to U.S. air pollution [60,61]. Nationwide, 264 million people lived in areas where pollution levels were above the WHO annual guideline for PM2.5 concentrations in 2021. The U.S. contains the largest number of air quality monitors in the world. In 2021, approximately 87% of data came from low-cost sensors and 13% from regulatory monitoring stations operated by different levels of government.
While improvements in air quality have saved lives, air pollution from fossil fuel combustion and vehicular emissions are estimated to have been responsible for 230,000 premature U.S. deaths in 2020 . In 2021, 97% of U.S. cities reporting data did not meet the WHO target of 5 μg/m³. The West Coast of the United States along with the nation's urban centers report the country's highest PM2.5 concentrations. A 2020 study investigating historic PM2.5 concentrations found that disadvantaged communities disproportionately bear the burden of air pollution for the past 40 years [63,64].
California continues to experience extreme pollution events during an increasingly long and extreme fire season. In 2021, the Dixie fire became the second largest fire in acres burned in California history. The Dixie fire burned across multiple counties, totaling 963,309 acres, and resulted in peak hourly PM2.5 concentrations of 212 µg/m³. The Caldor fire also burned at the same time, just south of the Dixie fire, and reached concentrations of 587 µg/m³. Smoke from the Bootleg fire in Oregon spread across the U.S. and north to Canada . During this time, elevated levels of PM2.5 were found across the United States and reaching into Canada .
Argentina | Bahamas | Brazil | Chile | Colombia | Costa Rica | Ecuador | Grenada | Guatemala | Honduras | México | Perú | Puerto Rico | Saba | Trinidad and Tobago | U.S. Virgin Islands | Uruguay
Latin America and the Caribbean have faced several air quality challenges as regional cities grow and urban populations expand. Urban population growth is a multifaceted air quality issue; as demand for energy and transportation increases, so do PM2.5 emissions. Increases in fossil fuel-generated energy production, vehicle exhaust from outdated vehicle fleets, the rising use of solid-state fuels as a heat source in lower income cities, and the absence of governmental air quality regulation all contribute to poor air quality. Despite all this, the region of Latin America and the Caribbean has made some progress in 2021. Country level PM2.5 concentrations fell in four regional countries; Argentina, Brazil, Colombia, and Costa Rica.
While some progress has been made, the air quality improvements correlating with pandemic mandatory quarantines have shown signs of erosion in 2021 with Peru, Chile, and Ecuador now close to reaching or even exceeding 2019 PM2.5 levels. Steady increases in wood burning use for heating and cooking in lower income regional cities, combined with lower precipitation rates in addition to the post-quarantine economic mobilization are factors that have contributed to this year's increase. Out of the regional 174 cities contributing data to this report, only 12 cities, accounting for only 2% of the regional population, met the 2021 WHO air quality guidelines for PM2.5.
In 2021, the number of air quality monitoring stations in this region increased by over 50% compared to 2020, with real-time air quality data now available for an additional 24 cities. The countries of Chile, Colombia, and Mexico had the greatest gains in air quality monitoring with the number of monitoring stations increasing by 51%, 38%, and 28% respectively. Citizens in this region play an active role in air quality monitoring. Local organizations and individuals now operate 55% of the region's monitoring stations, highlighting the effectiveness of grassroots efforts by citizen scientists in taking the lead to expand air quality networks. One such group is the Aires Nuevos Citizen Air Quality Network for Early Childhood in Latin America which operates numerous air quality monitoring stations in eight Latin American countries; Argentina, Brazil, Chile, Colombia, Ecuador, Mexico, Peru, and Uruguay . Despite these gains, much work remains to be done before ambient air quality monitoring in this region catches up to other regions in the world.
Brazil continues to make incremental progress in lowering rates of air pollution. Overall, PM2.5 concentrations dropped from 14.2 µg/m³ in 2020 to 13.6 µg/m³ in 2021. Of the 33 Brazilian cities included in this report, only Fortaleza and Piancó met the revised WHO air quality guidelines for PM2.5. Contributing sources to air pollution in Brazil include the cement industry, petrochemical production, steel production, mining, forest and agricultural burning, and vehicle production .
Recent economic challenges in Brazil have led to an increased number of households using firewood as the energy source for cooking. Unemployment, rising prices, and inflation are some of the reasons that wood is now the second most used energy source in the nation, preceded by electricity . Wood burning contributes to PM2.5 pollution, resulting in asthma attacks, heart attacks, and heart failure amongst other health effects .
Deforestation and wildfires are a continuing threat to the health and welfare of Brazilian citizens. Despite a decrease in the overall number of fires and total area burned in 2021 compared to 2019 and 2020, over 140 thousand square kilometers burned in Brazil during the period of January 1 through September 5, 2021 . A report released in August 2021 from Brazil's National Institute for Space Research (INPE) showed that deforestation in Brazil's Amazon rainforest was at a 15-year high . Due to deforestation for agricultural and industrial purposes, wildfires in the Amazon have resulted in the world's largest rainforest now emitting more CO2 than it absorbs .
Significant efforts continue in Chile to expand air quality monitoring networks. In 2021, the number of air quality monitoring stations increased by nearly 50%. Most air quality stations, about 60%, are operated by a government entity in Chile. The expansion of air quality monitoring, however, continues to highlight the urgent need for action, as the air quality in Chile is the worst in the region. Chile hosts 11 of the top 15 most polluted cities in Latin America and the Caribbean, where nine out of 10 people live in cities. In 2021, only one Chilean city met the 2021 WHO air quality guideline leaving 99% of the regional population in areas that exceed the guideline by a factor of five. Efforts are being made to reduce emissions in the region, but more than 90 million people still rely on polluting fuels for cooking and heating. Each year, air pollution costs the Chilean health sector approximately $670 million and leads to 4,000 premature deaths.
Chile's geographical and topographical features are significant contributing factors to the country's poor air quality. Many Chilean cities sit in valleys that can trap air pollution, particularly during winter months. This problem is compounded by the common practice of using wood burning stoves. The combustion of firewood is a very significant contributor to PM2.5 concentrations. This trend is quite apparent in the cold weather months in Chile (June to August), when PM2.5 levels rise sharply. The region's top three most polluted cities are in Chile, with a peak PM2.5 concentration in June 2021 of 125 µg/m³ in the city of Angol, 25 times the WHO annual air quality guideline level of 5 µg/m³.
Chile is working on taking action to tackle the problem of air pollution. Firewood makes up 94% of fine particulate matter (PM2.5) emissions in some Chilean cities. However, replacing firewood with alternative, clean energy sources has been challenging. Chile remains committed to the task of improving air quality and in 2014 initiated a program to replace 200,000 firewood heaters with more energy-efficient heating devices such as new gas, paraffin, or wood-pellet based heaters.
As a country, Colombia has seen a 10% reduction in PM2.5 concentrations from 2020 to 2021. This reduction is primarily due to quarantine-related mobility restrictions along with public transportation fleet upgrades to low-carbon, hybrid, and electric buses. This reduction marked the first decrease in annual PM2.5 concentrations in the past three years. The municipality of Guarne saw the greatest concentration reduction year-over- year with a 22% drop in PM2.5 and an annual average of 9.4 µg/m³. This resulted in the city achieving the 2021 WHO Interim Target-4 goal of 10 µg/m³.
The number of Colombian air quality monitoring stations more than doubled in 2021 in comparison to 2020. This expansion is largely due to the public's growing awareness and interest in knowing the quality of the air they are breathing in their cities. In 2017, Colombia created a $2.5 million “Biodivercities” project that funds low-cost measurement of air quality in the country's capital city .
Mobile sources of air pollution, principally road transportation and private vehicles, make a disproportionately large contribution to air pollution in Colombia. In addition to common sources of air pollution, Colombia is also challenged by most of its population living in the Andes Mountains, which are subject to temperature inversions, heavy congestions, enclosed streets, and hilly roads that can affect air quality and contribute to some of the highest air pollution levels in Latin America .
In 2020, Bogotá became the first Latin American city to declare a climate emergency, pioneering real climate action strategies. The strategies are binding and have the necessary budget to promote citizen health and environmental protection.
In 2021, Bogota ordered 1,485 electric buses to replace diesel buses being used, a significant change as 28% of emissions for the country come from transportation [73,74]. The country has also pledged to lower black carbon emissions by 40% from 2014 levels by no later than 2030 according to the country's Nationally Determined Contribution, a non-binding plan to address climate change .
Algeria | Angola | Cape Verde | Chad | Egypt | Ethiopia | Ghana | Kenya | Madagascar | Nigeria | South Africa | Sudan | Uganda
Sparse air quality data makes gauging the exact extent of Africa's air pollution problem difficult. Additionally, the task of measuring the health impacts from dirty air also remains difficult. It is calculated that air pollution is the cause of an estimated 780,000 African deaths a year .
Windblown dust from deserts is a common source of PM2.5 pollution in Africa. So too are fossil fuel combustion, waste, and agricultural burning. With the African population predicted to double in the next 30 years, rapid transportation growth, industrialization, and urbanization is likely to negatively impact regional air quality .
Based on NASA satellite data, 70% of the world's wildfires take place in Africa. Most of these fires burn through grasslands and generate a large amount of ambient PM2.5. To make matters worse, the fires can occur year after year because the frequently burned savanna can regrow burnt grass within a year's time.
Of the cities reporting air quality data in Africa, N'Djamena, Chad ranked number one as the most polluted regional city with an annual average PM2.5 concentration of 77.6 µg/m³ - a value greater than 10 times the WHO air quality guideline.
Air quality monitoring in Africa remains sparse. Most necessary to make important health decisions.
Limited air quality data in Africa contributes to low public air pollution awareness. South Africa has the largest public, real-time governmental air quality monitoring network on the continent. Most African countries in this report relied on monitoring stations operated by the U.S. State Department, individuals and non-governmental organizations.
South Africa's constitution guarantees a clean environment free of pollution: a right enshrined within the country's Bill of Rights . However, poor air quality in many cities means that right remains unrealized for far too many South Africans.
Air quality in a few South African cities did show slight, continued improvement in 2021, building on previous improvements in air quality measured in 2020. However, the country's overall PM2.5 concentrations increased from 18 µg/m³ in 2020 to 22 µg/m³ in 2021.
In 2021, Nieuwoudtville in the Northern Cape province recorded an average PM2.5 concentration of 3.9 µg/m³. It was the only city in South Africa - and in Africa - that met the WHO recommended air quality guidelines that year. Air quality continued to improve in Pretoria over a three-year period, from an annual average PM2.5 concentration of 25.5 µg/m³ in 2019 to 24.4 µg/m³ in 2020 and finally 21.6 µg/m³ in 2021.
Residents along South Africa's western and southern coastal regions generally experience better air quality than other parts of the country. The western coastal cities of Cape Town and Worcester recorded a relatively low PM2.5 concentration of 7.6 µg/m³ and 8.3 µg/m³ respectively for 2021. But inland regions, particularly in the eastern coal belt region of the Highveld, home to 12 coal-fired power stations, struggle with poor air quality .
Major cities in the Highveld measured some of the highest average concentrations of PM2.5 in 2021. Bloemfontein had the highest annual average at 42.2 µg/m³, more than eight times the WHO recommended guidelines. Vereeniging's average annual PM2.5 was 26.6 µg/m³ - roughly as poor as the year prior - while Johannesburg's annual average measurement of 27.3 µg/m³ represented an increase of 22% over 2020.
Coal is a major industry in South Africa, and the country's biggest energy provider for electricity, fulfilling over 80% of the country's energy needs. Coal is also a feedstock for vehicle liquid fuel. Government subsidies play a key role in the coal-to-liquid (CTL) fuel sector, with as much as 5% of a South African driver's fuel bill going to a single South African firm . The company owns a CTL plant in Secunda that has been identified as the world's largest single site source of greenhouse gas emissions .
Australia | New Caledonia | New Zealand
Oceania has the cleanest overall air quality in the world. Yet, typical challenges to air quality in the region can include brushfires and dust storms originating from Australia's desert interior. The 2020/2021 Australian bushfire season was less intense than in previous years, in part due to a cool, wet summer . However, the 2019/2020 fire season was one of the most destructive fire seasons in the country's history. PM2.5 emissions impacted the region as smoke drifted as far away as South America .
Consistent with the previous year, there were higher concentrations of recorded PM2.5 in some cities in Australia and in New Zealand's Canterbury region between May and August. This may be because of increased wood burning in winter. Heaters cause about 40% of winter air pollution in Australia's capital and the region's largest city, Sydney .
Forty-six cities in Australia, one in New Zealand, and one in New Caledonia all met the WHO PM2.5 air quality guideline of 5 µg/m³. However, a further 63 cities in Australia and 20 in New Zealand failed to meet this standard, impacting the health of over 21 million people.
As the population in the region grows, there will be increasing pressure for potentially fossil-burning transportation and energy consumption. Coupled with climate change, the region will become hotter and more arid, creating more intense dust storms and brushfires.
Generally, higher-income countries tend to have more air quality monitoring sensors within their networks. However, Oceania doesn't follow that trend. This is probably due to the fact that Australia and New Zealand have less dense populations than most wealthier nations and lower air pollution levels. In 2021, the number of regional air quality monitoring stations grew by 40%, largely driven by gains in Australia and New Zealand, home to 99% of the monitoring stations in the region.
Expanding awareness of, and access to, air quality information is an important action in the fight against air pollution and its harmful health consequences. Once people know where to find timely local air quality information, there is a much better chance they will become clean air advocates.
Even though the number of air quality monitoring stations, and their global distribution, continues to grow, there are still many areas around the world that do not have access to local air quality information.
Local air quality monitoring by governmental agencies and non-governmental entities is essential to overcome the lack of air quality information and awareness.
Technological advancements over the last few years have allowed the easy deployment of low-cost air quality monitors that can be turned into publicly reporting stations. The more stations that come online, the more air quality data is available for researchers, policymakers, and communities to help create knowledge, awareness, and action for cleaner air and healthier communities.
The PM2.5 air quality data used to create this report was generated exclusively by ground-level monitoring stations. Of those stations, 44% were operated by governmental agencies. The remaining 56% were privately-owned stations operated mainly by individuals, educational institutions, and non-profit organizations.
Most data employed in the report was collected in real time, aggregated on an hourly basis as the measurements were made publicly available. However, there are instances where data collected from ground-based monitors was not published in real time but rather published as year-end historical data sets. These cases primarily occurred for data from European cities. Historical data was obtained from the European Environment Agency (EEA) and integrated with the data collected in real time to provide the most comprehensive global data analysis possible.
Both regulatory-grade air quality monitors and low-cost monitors are subject to data anomalies and inaccuracies because of sensor defects or temporary hyperlocal emissions near the sensor. Post-data acquisition quality control measures are implemented in IQAir's cloud-based data platform to mitigate potential bias introduced by such events. Data points from individual sensors deemed to be outliers are flagged and quarantined prior to additional data processing and integration into the platform. Flagged data points are checked against subsequent data points collected from the same sensor, as well as cross-referenced to nearby sensors. Data points that do not meet the quality control criteria for these two standard reference checks are discarded.
Low-cost PM2.5 sensors included in this report quantify PM2.5 concentrations using laser scattering technology. Since environmental conditions (including humidity and particle composition) can affect particle size, shape, density, and refractive index (how light is reflected), IQAir calibrates these measurements against government reference monitors where available.
The data presented in this report, PM2.5 annual average concentrations for cities, countries, regions, and territories, is a function of data collected from the individual stations located within the geographical borders of the given location. Each individual sensor periodically samples the ambient air to measure the real-time PM2.5 concentration, recording the data point and the time it was measured. Every hour the data is aggregated by averaging all data points collected during the prior 60 minutes, creating a calculated value defined as the hourly average PM2.5 concentration. Throughout the course of the year, this procedure creates a series of hourly average PM2.5 concentrations for each individual sensor. These values are then used to calculate the annual average PM2.5 concentration values for cities, countries, regions, and territories.
City level data included here is reported as both annual and monthly average PM2.5 concentration values. Monthly average PM2.5 concentration values are determined by averaging together all hourly average PM2.5 concentration values for all stations within that city for that month. This ensures the data for each hour of the day and each individual station is weighted the same in the city-level average calculation. City level annual average PM2.5 concentration values are calculated in the same manner, with the averaging period extended to include all data points collected within the year.
Country, region, and territory annual average PM2.5 concentrations are calculated as a function of the city-level annual average PM2.5 concentration and the population for all cities within the geographical boundaries of the country, region, or territory being calculated. In this section, for the sake of clarity and brevity, the term "area" will be used in place of "country, region, and territory".
A primary purpose of this report is to provide an overview of the state of air quality around the world and to report the data in such a way that allows for meaningful comparisons of the ambient air quality experienced by individuals in different areas. A simple metric for comparing air quality would be to average all individual city-level PM2.5 concentrations to obtain a composite PM2.5 average concentration for an area. However, this simple composite calculation could very easily fail to convey any meaningful information about relative air quality experienced if the population density varied among cities.
In order to reflect a more accurate representation of the average air quality experienced in an area, city population data must also be considered. Using population weighting as a normalization factor allows for the air quality conditions experienced in more densely populated urban environments to be proportionately reflected in annual area-level averages. In this way, cities that have larger populations influence the overall area-level average concentration more heavily than those cities with smaller populations. This ensures the composite area concentration is more reflective of the average air quality experienced within that area, providing additional context necessary to directly compare data across global areas.
The following calculation is used to determine area-level, annual average PM2.5 concentration based on the city-level data contained in this report, population weighted to provide global context.
Annual availability was the primary metric used to determine if the average annual PM2.5 concentration value calculated for a particular city was sufficiently representative of that city's actual air quality. This value is defined as the fraction of total hours in a year (8,760 total) for which there is hourly average PM2.5 data available. In order to meet the threshold for report inclusion, a city was required to have an annual data availability of at least 60%, or at least 5,256 recorded values for hourly average PM2.5 concentrations.
A summary of the 2021 data availability is provided below.
The pie chart shows the distribution of the total number of reported cities (6,475) by annual data availability.
This report is limited to PM2.5 data collected in 2021 from ground-level air quality monitoring stations comprised of regulatory-grade monitors and low-cost sensors.
IQAir is politically independent. Graphs, maps, and content included in this report are intended to expand on the dataset and do not indicate any political stance. Regional maps have been created using a data visualization tool.
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Much of the data for this report has been aggregated from governmental agencies, whose work in publishing live air quality information provides a strong basis for global air pollution research and analysis, with the ultimate goal of empowering the world's citizens to join the effort to improve air quality.
IQAir would like to thank the United Nations Environmental Programme (UNEP) and the United Nations Human Settlements Programme (UN-HABITAT) for their support and collaboration.
A special thanks to Greenpeace India, Indonesia, Thailand, Philippines, Turkey, Romania, Hungary, Argentina, Chile, South Africa, and the U.S. for their constructive support and valuable insights.
We would also like to thank CREA for their valuable feedback.
IQAir is a Swiss-based air quality technology company that seeks to empower individuals, organizations and communities to breathe cleaner air through information, collaboration and technology solutions.
IQAir's AirVisual global air quality information platform aggregates, validates and calibrates air quality data from a wide variety of sources, including governments, private citizens and organizations. The AirVisual platform supports the free integration of air quality data from a wide variety of data sources and monitoring devices.