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Marcus Holloway, Breath and Body. 18 September 2026 – 12:24 PM
There’s a version of this article that’s just a list. Ten cities, ten numbers, done.
I’ve read plenty of those, and they always leave me with the same nagging question: why these cities? Because once you look at what’s driving the numbers, the list starts making a lot more sense.
The most recent global picture comes from IQAir’s World Air Quality Report, released in March. It’s the eighth edition, and this year’s findings aren’t encouraging. Let’s get into it.
The headline: things got worse, not better
Only 14% of global cities met the World Health Organization annual PM2.5 guideline of 5 µg/m³, down from 17% the previous year.
That’s the number I’d focus on. Not the worst city — the direction of travel.
The report analysed data from monitoring stations across 9,446 cities in 143 countries, regions and territories, so this isn’t a small sample. Comparing against the previous year, 54 countries recorded increases in annual average PM2.5, 75 saw reductions and two were unchanged.
Mixed, then. But the cities meeting the guideline shrank, which tells you the losses were concentrated where it matters.
A quick note on what PM2.5 actually is
Worth pausing here, because the term gets thrown around without explanation.
PM2.5 is fine particulate matter — particles small enough to get deep into your lungs and, from there, into your bloodstream. It refers to particles with a diameter of 2.5 micrometres or smaller, measured in micrograms per cubic metre.
The WHO’s annual guideline is 5 µg/m³. Keep that number in mind, because the figures below will look absurd against it.
The most polluted cities
Loni, Hotan, Byrnihat, Delhi and Faisalabad lead the list, and the full top 25 is dominated by cities in India, Pakistan and China.
At the top: Loni, India, recorded an annual average PM2.5 concentration of 112.5 µg/m³ — a nearly 23% increase from 2024 and more than 22 times the WHO guideline.
Read that increase again. Not high and stable. High and rising, sharply.
The world’s 25 most polluted cities were all located in India, Pakistan and China, with India home to three of the four most polluted.
Why each of the top few is where it is
This is the part I find genuinely interesting, because the causes aren’t the same.
Loni sits near Delhi in Uttar Pradesh. Urban growth, traffic, construction and nearby industrial activity all contribute to its poor air quality. Classic rapid-urbanisation profile.
Hotan is different. Located in Xinjiang in western China, it suffers a combination of natural and human-made pollution — frequent sandstorms from the nearby Taklamakan Desert carry fine dust, while many households rely on coal-fired heating during colder months.
That distinction matters. You can regulate a factory. You cannot regulate a desert.
Byrnihat is the industrial case. It sits on the Meghalaya–Assam border, where cement, steel and manufacturing activity, along with heavy vehicle movement, drive its pollution levels.
Delhi is its own category, which I’ll come to.
Delhi: the world’s most polluted capital, again
Delhi, home to over 30 million people, remains the world’s most polluted capital, with an average PM2.5 of 99.6 µg/m³ — 20 times the WHO guideline.
Here’s the frustrating bit. That’s despite an 8% fall in annual concentration, and it’s Delhi’s seventh time at the top in eight editions of the ranking.
An 8% improvement sounds like progress. At twenty times the guideline, it barely registers. That’s the trap with these numbers — you can be genuinely improving and still be nowhere near acceptable.
Last November brought something new, though. The report notes Delhi saw “rare” public protests in November 2025 when daily average pollution peaked near 460 micrograms of PM2.5.
The poor air corresponded with a rise in hospital patients dealing with asthma, cardiac issues and breathing difficulty. Authorities closed schools, encouraged online work and classes, banned construction and restricted diesel generators — but did not immediately respond to demands for long-term emission cuts.
Emergency measures, in other words. Not structural ones. This is a pattern you see repeatedly in heavily polluted cities, and I understand why — the emergency is visible and the structural fix is expensive.
Countries, not just cities
The five most polluted countries were Pakistan (67.3 µg/m³), Bangladesh (66.1), Tajikistan (57.3), Chad (53.6) and the Democratic Republic of the Congo (50.2).
Overall, 130 out of 143 countries and territories — 91% — exceeded the WHO annual guideline.
Only thirteen met it: French Polynesia, Puerto Rico, the U.S. Virgin Islands, Barbados, New Caledonia, Iceland, Bermuda, Réunion, Andorra, Australia, Grenada, Panama and Estonia.
Look at that list. Islands, small populations, oceanic weather. Clean air is partly a policy achievement and partly a geographic lottery win.
The India counting problem
This is the section I’d most want people to read, because the rankings get misused constantly.
Sixty-six of the 100 most polluted cities are in India, and just 15 are from Pakistan. On the surface, damning.
But there was data from 259 Indian cities and only 18 Pakistani ones. That means one in four monitored Indian cities is in the top 100, while almost all Pakistani cities are included.
IQAir’s CEO Frank Hammes put it neatly to Health Policy Watch — “India is a victim of its success in monitoring.” He noted India is discovering that air pollution is a widely distributed problem, and that this may be part of why India falls in the listing as more countries begin monitoring.
The uncomfortable implication: some places don’t appear on these lists because nobody is measuring. Absence of data isn’t absence of pollution.
Wildfires rewrote the map in 2025
Here’s what changed this year, and it caught out places that usually feel immune.
Wildfires, bolstered by climate change, played a primary role in degrading global air quality in 2025, with record emissions from Europe and Canada contributing to roughly 1,380 megatonnes of carbon released from biomass burning.
Canada was the most polluted country in Northern America for only the second time in the report’s eight-year history, as its second-worst wildfire season on record affected air quality across Canada, the United States and parts of Europe.
The transboundary effect is the striking part. Switzerland and Greece recorded increases exceeding 30% due to wildfire smoke from North America and Saharan dust from Africa.
Smoke from Canadian forests measurably worsening Swiss air. That’s the kind of fact that reframes the whole issue — this isn’t a problem individual cities can solve at their own borders.
As the report puts it, wildfire activity severely impacted regions that historically had relatively low PM2.5 levels, which is why only 14% of cities met the guideline compared with 17% a year earlier — a reminder that maintaining clean air requires active stewardship regardless of past performance.
The United States, briefly
Worth a mention because it upends expectations.
El Paso was the most polluted major city in the US, with historic dust storms triggering a 46% increase in PM2.5 to 11.4 µg/m³ as the city recorded the highest number of major pre-summer dust storms since the 1930s.
Not Los Angeles. Not an industrial city. Dust.
The Southeast Los Angeles region — Cudahy, East Los Angeles, Huntington Park — heavily affected by wildland-urban interface fires, ranked as the most polluted area in the country. Seattle remained the cleanest major US city for a second year at 4.5 µg/m³.
The monitoring problem nobody planned for
This one genuinely bothers me.
The end of the US State Department’s global air quality monitoring programme at embassies and consulates in March 2025 caused millions to lose access to air quality data. The Centre for Research on Energy and Clean Air reported that monitoring in 44 countries was weakened, and six were left without any monitoring at all.
For a lot of cities, an embassy rooftop sensor was the only independent reading available. Now it’s gone.
The report flags vast data gaps, especially in Africa and West Asia. There’s some good news alongside it — contributions from West Asia including Iran, Syria and Jordan drove a 33% increase in cities covered, a step toward closing those gaps — but Burundi, Turkmenistan and Togo, all in the top 30 most polluted countries in 2024, are absent this year for lack of data.
You can’t manage what you don’t measure. Old line, still true.
Reading these rankings sensibly
A few caveats I’d hold in mind before quoting any of this at a dinner party:
- Annual averages aren’t daily conditions. A high annual rank doesn’t mean the worst air every hour — it means the yearly mean was among the highest, and seasonal smog, wildfire smoke, dust events and winter inversions heavily influence that figure.
- Clean days still happen in dirty cities. Rain or wind can clear the air, and a city can still rank high because winter smog or stagnant weather pushes the annual mean up.
- There’s no single official list. No worldwide city ranking covers every city with identical monitoring density, boundaries and methods.
- Monitoring density skews everything. As above.
Some genuine progress, worth noting
I don’t want to end on pure gloom, partly because it isn’t accurate.
In Latin America and the Caribbean, trends were largely positive — 208 cities recorded decreases in annual average PM2.5, 95 increased and nine were unchanged. Oceania remained one of the world’s cleanest regions, with 61% of cities meeting the WHO guideline.
And the issue has finally climbed the political agenda. The report notes that 2025 marked a turning point as international institutions elevated the crisis — the World Economic Forum’s Global Risks Report dedicated a section to air pollutants for the first time, the UN General Assembly formally categorised air pollution as a major risk factor for non-communicable diseases, and the World Health Assembly approved a roadmap to halve deaths from air pollution by 2040.
Roadmaps aren’t clean air. But a decade ago this wasn’t even on the list.
What actually stays with me
Two things.
First, the child health point from the report. Respiratory damage sustained during developmental years is often irreversible, and children — the group with the least agency in any of this — bear the permanent health costs of air they did not choose.
That’s the whole argument, really. Adults can move, buy purifiers, stay indoors. Kids breathe what they’re given.
Second, Greenpeace scientist Dr Aidan Farrow’s framing in the report — that the data reveals “two competing realities: an air pollution crisis and the rise of communities, scientists and data working to meet the challenge.”
Both things are true at once. The air got worse and our ability to see it got better. Which of those wins over the next decade is, I suspect, still genuinely open.
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