Pakistan had the most polluted air in the world overall in 2025, and Delhi was the most polluted capital for the seventh time in the last eight years of reporting by the Swiss-based IQAir.  A town bordering India’s capital is the world’s most polluted place. Despite covering nearly 9,500 cities, pollution data gaps leave millions of people exposed to unhealthy air out of the count.

Air pollution worsened in 2025, with the share of cities globally that met the World Health Organisation’s guideline of safe air quality falling to 14% from 17% the previous year. Progress on air quality progress stalled as wildfire smoke and climate change intensified air pollution concentrations, accordinig to the latest global ranking report by IQAir, released today. 

The report by the Swiss-based air quality technology firm ranked 143 countries and territories, as well as almost 9,500 cities by annual average PM2.5 levels, drawn from a continuous real-time data base, accessible to users worldwide.  

It also flags vast data gaps, especially in Africa and West Asia, which saw setbacks in air quality monitoring coverage last year. One particular blow was the Trump administration’s decision to halt the public reporting on air quality from US Embassy and Consulate locations worlwide in March 2025, depriving many cities of their trusted primary data source. As a result, monitoring efforts in 44 countries were weakened, and six were left without any monitoring, according to the new IQAir report.

South Asia remains world’s most polluted region 

The five most polluted countries in 2025 were Pakistan with an annual average PM2.5 of 67.3 micrograms per cubic metre (µg/m³), Bangladesh with 66.1, Tajikistan at 57.3, Chad at 53.6 and the Democratic Republic of the Congo at 50.2. 

India is sixth, having dropped out of the top five for the first time since the rankings began eight years ago. Its average PM2.5 level dipped by 3% to below 50 μg/m³ as an annual average across the country still nearly 10 times the WHO’s recommended annual average for PM2.5 of 5 μg/m³. 

South Asia remains the world’s most polluted region. Not only are Pakistan, Bangladesh and India ranked 1st, 2nd and 6th as countries, but 83 cities from these three countries as well as Nepal are among the 100 most polluted cities in the world. 

Given how transboundary pollution from neighbouring jurisdictions contributes significantly to the air quality of any country, province or city, the report underscores the need for regional cooperation to tackle this public health crisis. As the report also points out, for children, the impact of air pollution exposure can last a lifetime; the respiratory damage sustained during developmental years is often irreversible.  

Wildfires severely impacted North American regions with historically low air pollution

“Globally, just 13 countries, regions, and territories saw annual average PM2.5 concentrations meeting the WHO annual PM2.5 guideline of 5 μg/m³, with the majority located in the Latin
America and Caribbean region,” the report notes.

The 2025 data serves as a “critical reminder that air quality is not a static achievement, but a fragile asset,” it adds, noting that in 2025, “wildfire activity severely impacted regions that have historically experienced relatively low PM2.5 levels. As a result, only 14% of global cities met the WHO annual PM2.5 guideline in 2025, compared to 17% in 2024. ”

India: ‘victim of its own success in monitoring’

Sixty-six of the most polluted 100 cities are from India, and just 15 are from Pakistan. But the comparison is not as straightforward as it seems. The more monitoring there is, the more air pollution is revealed. There was data from 259 Indian cities and 18 Pakistani cities. That means one in four Indian cities monitored is in the top 100, while almost all Pakistani cities are included. 

“India is a victim of its success in monitoring,” Frank Hammes, IQAir’s CEO, told Health Policy Watch. It’s one of the better monitored countries. “India is now discovering that air pollution is a very widely distributed problem. This is maybe one of the reasons too that India may be falling in the listing as more and more countries (monitor).” IQAir is a Swiss air quality technology firm. 

In fact, the list of the top 20 most polluted places is almost equally divided between India and Pakistan, nine from India and eight from Pakistan, the remaining three being in China. 

The world’s most polluted place is Loni, India in Uttar Pradesh, which borders north-east Delhi. Altogether, the Indian cities in the most polluted top 20 span about 2,000 km across the northern part of the country, from Punjab to Assam, which suffers sharp seasonal pollution peaks due to weather conditions and crop waste burning along with chronic problems with traffic and power production. 

World’s most polluted capital

Delhi, home to over 30 million people, remains the world’s most polluted capital. Its average PM2.5 was 99.6 micrograms, 20 times the WHO’s safe guideline of 5. This is despite an 8% fall in the annual concentration of the pollutant. This is Delhi’s seventh time at the top in eight editions of the IQAir world ranking report. 

The report also notes that India’s capital saw “rare” public protests in November 2025 when the daily average pollution peaked near 460 micrograms of PM2.5. Fueled by seasonal crop burning, vehicular emissions, and stagnant winter air, the poor air quality corresponded to a rise in hospital patients dealing with asthma, cardiac issues, and difficulty breathing. Authorities responded by closing schools, encouraging online work and classes, banning construction, and restricting diesel generators, but did not immediately respond to demands for long-term emission cuts.

Gulf war and air pollution

The war launched by the US and Israel against Iran also has affected air pollution, but data is patchy.

In Israel, IQAir’s researchers noticed a “very quick spike” on government sensors there. But in Tehran, where the city was covered by billowing clouds of smoke after a missile attack on the city’s oil storage facilities, “we currently don’t have any data, but we usually have data embassies putting up sensors out there. Currently that data is down. We hope it’ll come up very soon again,” Hammes says.

The impact of war on air pollution was first observed by an IQAir team tracking the Russia-Ukraine war. Many civilians have used the firm’s app to track the attacks by monitoring the plumes and spikes of air pollution, and then report on real time levels. Formal analysis of the data and its implications is managed in-house.    

Air pollution data gaps

Hammes responded to the pushback received from some governments on the global rankings noting that IQAir uses a dataset larger than most governments. 

IQAir bases its 2025 report on data from about 50,000 reference-grade air quality monitors and low-cost sensors. The online air quality platform aggregates, validates and calibrates air quality data from a wide variety of sources, including governments, private citizens and organizations using a range of lower-cost monitoring devices. 

Data that is not deemed reliable is weeded out; thanks to a “data cleaning”. For instance, this year’s report is based on analysis of about 33,000 data points out of the 50,000. 

The more data you have, the more that less sensitive low-cost monitoring systems can be included because they validate each other, Hammes explained, saying: “The more data you have, the less relevant it is if one or two sensors with questionable accuracy are in one place.”

Even so, expanding pollution data coverage remains a challenge. Despite some regional improvements, major data gaps remain, with only a fraction of the global population having access to hyper-local, real-time air quality information. 

Africa’s representation in the data has expanded significantly this year with the inclusion of seven countries and territories not present in last year’s report: Guinea, Eswatini, Tanzania, Benin, Morocco, the Canary Islands, and Réunion. 

But Africa and West Asia remain significantly underrepresented in terms of station density, despite being home to some of the world’s most polluted cities. 

French Polynesia and Nieuvoudtville South Africa –  the least polluted places

Globally, just 13 countries and territories saw annual average PM2.5 concentrations meeting the WHO annual PM2.5 guideline of 5 μg/m³, with the majority located in the Latin America and Caribbean region. 

The countries where pollution rose include the United States, due to smoke generated by wildfires across North America, as well as Switzerland and Greece which experienced smog days due to the drift of Saharan dust from Africa.

French Polynesia – in the middle of the Pacific Ocean – was the cleanest territory in 2025 with a PM2.5 concentration of just 1.8 micrograms. The single least polluted  place identified by the available data was Nieuwoudtville, South Africa, a town of 2000 people in the country’s northern Cape Province, with average annual PM2.5 of just 1 microgram/cubic meter of air.

Image Credits: IQ Air , Punjab Enviornment Department, IQAir, Mike Newbry/ Unsplash, X/Mohamed Safa@mhdksafa, IQ Air.