More than 260 million people worldwide live with asthma, making it one of the most common chronic respiratory diseases. For far too many people, poor air quality is an everyday health risk.
For much of the world's population, clean air remains out of reach. According to the 2025 World Air Quality Report, only 14% of cities surveyed met the WHO's annual PM2.5 air quality guideline of 5 μg/m³. Exposure to air pollution triggers asthma attacks and may also increase the risk of developing asthma, particularly in children.
"Exposure to air pollution triggers asthma attacks and may also increase the risk of developing asthma, particularly in children.”
Uncovering invisible air pollutants at their source is a key component in protecting human health. While air pollution is a global health concern, some cities are affected far more than others. Three major cities stand out for their combination of persistent air pollution, documented respiratory health impacts, and environmental conditions that can intensify asthma risks.
3. Ulaanbaatar, Mongolia
Ulaanbaatar is the coldest capital city on Earth, and staying warm there is exactly what makes the air so dangerous. About 60% of the city’s 1.9 million residents live in ger districts, sprawling residential neighborhoods where a household can burn three to six tons of raw coal over a single winter to survive temperatures below -20°C (1).
Winter temperature inversions then seal that smoke over the city like a lid, trapping it in a layer 500 to 800 meters deep (2). The resulting pollution levels are staggering. Cold-season PM2.5 in Ulaanbaatar averages around 114 µg/m³, several times Mongolia’s own national standard (3).
"A ten-year study in Ulaanbaatar found that cold-season asthma admissions were 2.6 times higher than in the warm months.”
Asthma follows the smoke. A ten-year study counted 8,283 asthma hospitalizations in Ulaanbaatar between 2008 and 2017 and found that cold-season admissions were 2.6 times higher than in the warm months. Every 10 µg/m³ rise in PM2.5 raised the risk of an asthma admission by roughly 3 to 4%. Bronchial asthma is now more common in the capital than in Mongolia’s 21 provinces and rising every year (4).
Children carry the heaviest burden. UNICEF found that children in a heavily polluted district of central Ulaanbaatar had 40% lower lung function than children living in rural Mongolia, and during the winter of 2024–2025 one senior pediatrician reported that admissions for respiratory illness had nearly doubled compared to the year before (5).
2. Delhi, India
Delhi was the most polluted major city in the world in 2025, with an annual average PM2.5 concentration of 99.6 µg/m³. For the tens of millions of people who live there, the worst stretch arrives every winter, when PM2.5 climbs to roughly 200 µg/m³ and monitoring stations across the city log AQI readings above 400, deep into the hazardous range (6).
Vehicles produce roughly two-thirds of Delhi’s pollution (7). Industry, construction dust, and landfill fires also contribute to poor air quality. Then every autumn, rice-stubble burning in neighboring Punjab and Haryana sends a wave of smoke over the city just as cooling air traps it near the ground (8).
The health impacts become most visible during these pollution episodes. Government data presented to India's parliament recorded more than 204,000 respiratory emergencies at six major Delhi hospitals between 2022 and 2024, with over 30,000 patients requiring hospitalization (9). Respiratory admissions rise by 15 to 20% during severe pollution events, with children, older adults, outdoor workers, and people living with asthma or chronic obstructive pulmonary disease (COPD) facing the greatest risks (10).
Despite stricter vehicle standards, construction curbs, and seasonal controls, winter smog remains an annual crisis for India's capital (11).
1. Lahore, Pakistan
Lahore sits about 400 kilometers northwest of Delhi and shares many of the same seasonal air pollution challenges. During winter, the two cities regularly alternate at the top of global pollution rankings.
In 2025, Lahore was the second most polluted major city in the world, with an annual average PM2.5 concentration of 88.9 µg/m³ (12).Traffic congestion, industrial emissions, brick kilns, and crop residue burning in surrounding districts all contribute to the city's smog, while winter temperature inversions trap pollutants close to the ground for days at a time (13).
For people living with asthma, the consequences are severe. A study of 500 Lahore residents found that asthma prevalence more than doubled during peak smog season, rising from 12.5% to 28.7%, while hospital admissions increased from 20 to 46 per 1,000 people. Doctors describe winter as a respiratory health emergency, with children among the hardest hit as severe coughing, wheezing, and asthma flare-ups become increasingly common. The burden falls most heavily on lower-income communities, where access to air purifiers, high-quality masks, and other protective measures is often limited (14).
Where else is asthma a problem?
Extreme pollution is not only a South and Central Asian story. IQAir's 2025 ranking data points to trouble spots for asthma sufferers in every region.
North America
Tlaxcalancingo and Saltillo, Mexico both recorded PM2.5 concentrations more than six times the WHO annual guideline in 2025, driven by Puebla's traditional brick kilns and Saltillo's heavy manufacturing.
In the U.S., the three most polluted cities Cudahy, East Los Angeles, and Huntington Park all sit in southeast Los Angeles County, boxed in by freeway and port freight traffic, with air nearly three times the WHO safe limit. But pollution is only part of the asthma story: the Asthma and Allergy Foundation of America's 2025 Asthma Capitals report names Detroit the most challenging American city to live with asthma, followed by Rochester and Allentown, places where asthma prevalence, emergency visits, and deaths pile up alongside poverty and gaps in care (15).
In Canada, a record wildfire season left the northern towns of Flin Flon and Pinehouse breathing air almost five times the safe limit, for Flin Flon, nearly triple its own levels the year before.
Europe
The continent's worst air in 2025 was in Iğdır, a city in eastern Turkey that has Ulaanbaatar's problem in miniature, a mountain-ringed valley where winter inversions trap smoke from coal heating, pushing pollution to roughly thirteen times the WHO limit.
Similar conditions affect parts of the Balkans, with Sarajevo breathing air nearly six times the limit.
Africa
N'Djamena, Chad and Kinshasa, DRC both exceeded the limit roughly tenfold, though the smoke comes from different sources: Saharan dust and biomass fuels in N'Djamena, charcoal production and slash-and-burn agriculture around Kinshasa. With relatively few air quality monitors across the continent, the true picture is likely even worse.
Oceania
Oceana was the world's cleanest region in 2025, though there are areas still impacted by poor air quality and in which asthma suffers must take note. Nelson and Collie, a New Zealand valley town and an Australian coal town respectively, topped the list at more than double the safe limit, driven largely by winter wood-burner smoke.
What to do to prevent asthma flareups
Although it's not always possible to avoid outdoor air pollution, people with asthma have options for reducing exposure and lowering the risk of flareups.
- Check the air before heading out: Pollution levels swing widely through the day, typically peaking during morning and evening traffic and on cold, still nights. Download a free air quality app to know when to move a workout indoors or reschedule errands.
- Keep outdoor air outside: Close windows and doors during peak pollution hours and run a high-performance air purifier indoors, especially in the bedroom.
- Wear a mask on bad days: A well-fitted KN95 mask filters out the fine particles that trigger attacks.
- Don’t exercise next to traffic: Pollutant concentrations are highest close to busy roads. Choose parks and side streets and skip outdoor workouts entirely when air quality is unhealthy.
- Keep your asthma action plan current: Refill medications before smog season begins, carry an reliever inhaler, and ask the doctor how to adjust treatment when air quality deteriorates.
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The takeaway
Whether the source is coal smoke in Ulaanbaatar, traffic in Delhi, or crop burning around Lahore, the pattern is the same: pollution rises, and people with asthma are the first to feel it. The interventions that work begin with reliable air quality data and the public pressure it creates.
For people with asthma, knowing what is in the air today means fewer surprises, flareups, and trips to the emergency room. Controlling air pollution at the source in the world’s biggest cities and maintaining a personal plan for poor air quality days can help manage poor air quality for asthma sufferers everywhere.
