New Delhi’s air was nearly twice as bad as other capitals on this list, with average concentrations of PM2.5 reaching unhealthy or very unhealthy levels most months in 2018. The most populated capital in the world, New Delhi’s air pollution problem is intensifying as the city’s growing middle class buys cars, poor households burn dirty fuels for cooking, and construction sites spring up.

But the crisis isn’t limited to New Delhi: northern cities just outside the capital, such as Gurugram and Ghaziabad, experienced higher levels of pollution. India was home to fifteen of the twenty most polluted cities in the world, according to IQAir’s report. The government has issued a five-year plan to significantly reduce air pollution in more than one hundred cities, but the report indicates a long road ahead.

Brick kilns are to blame for nearly 60 percent of air pollution in Bangladesh. Thousands of people use the wood- and coal-fired furnaces, most in and around Dhaka, to produce twenty-three billion bricks every year, primarily for domestic construction projects. Bangladesh’s Department of Environment is cracking down on illegal kilns [PDF] and encouraging kiln owners to transition to sustainable power sources. Construction and a growing number of vehicles also contribute to high levels of hazardous particles in the air.

In the Afghan capital, air pollution is worse during winter months, when people burn wood, animal fat, and other materials to warm their homes. Making matters worse, the city’s population has tripled in the past decade, which has increased traffic and vehicle emissions. More than seventeen thousand people died from diseases caused by air pollution in Afghanistan in 2016, according to WHO data. Late last year, members of the Afghan parliament called on health officials to address Kabul’s deadly air. The Ministry of Public Health said the government plans to ban inefficient vehicles from roads and upgrade its air quality testing equipment.

Unlike many Asian capitals, where man-made factors account for high levels of PM2.5, the Middle Eastern cities on this list experience high levels of dust and sand in the air. Manama is no exception. With a much smaller population than in other capitals, at around 157,000 people, Manama suffers from poor air quality largely due to environmental factors. A UN report [PDF] found that emissions from industries including aluminum smelting and ship repairing are the primary man-made source of PM2.5 in Bahrain. Many citizens in Manama, where gas is relatively cheap, rely on private cars to get around, a major emissions contributor. A U.S. diplomat described the air quality as “a stinging cocktail that gives many residents red or teary eyes, a constant low-level cough, and post-nasal drip,” in a 2016 blog post.

The coldest capital city in the world, Ulaanbaatar can experience temperatures as low as -40° Fahrenheit and Celsius in the winter, and its air quality is the worst when people burn coal and wood in their homes for warmth. Trapped by surrounding mountains, the smog tends to loom over the city. Children living in Ulaanbaatar have 40 percent lower lung function than children in rural areas, a 2018 UNICEF report [PDF] found.

Kuwait is particularly at risk for high levels of sand in the air—it’s one of the countries most affected by dust storms. The problem will likely get worse, with the United Nations predicting that the number of storms could rise in the next decade due to climate change and land and water mismanagement. Like other particles, dust can get into people’s lungs and cause respiratory problems and heart disease. Refineries, oil fields, and power stations [PDF] just outside the capital city also contribute to air pollution.

Like Ulaanbaatar, Nepal’s capital is surrounded by mountains that prevent pollutants from dispersing for weeks. Even though it has a relatively small population, at around one million people, Kathmandu still suffers from air pollution caused by increasing vehicle traffic [PDF] and road construction. In an effort to curb vehicle emissions, the government reduced taxes on electric cars and more than doubled import taxes on gasoline-fueled vehicles.

Gone are the days when Chinese cities dominated media coverage for their smoggy skies and rising carbon emissions—well, kind of. Thanks to Chinese President Xi Jinping’s emphasis on sustainable development and ambitious emissions-reduction targets, average concentrations of pollutants in China’s cities fell more than 10 percent from 2017 to 2018. But 89 percent of its cities still have PM2.5 levels exceeding the WHO’s target, and China is the twelfth-most-polluted country in the world, according to IQAir’s report. Beijing remains one of the world’s most polluted capitals, with burning of dirty coal as a major contributor.

Like other cities on the Arabian Peninsula, Abu Dhabi experiences dust storms and high levels of sand in the air even on average days. Besides environmental factors, road transport and the production of aluminum, iron, and steel contribute to emissions, according to the city’s environmental agency [PDF]. To help minimize man-made pollutants, the government released a plan in 2016 to introduce electric and low-emissions vehicles and improve public transportation.

Greater Jakarta’s eight coal-fired power plants are major sources of air pollution, according to a 2017 Greenpeace report [PDF], and the government has proposed building more in the coming years. Jakarta’s motorized traffic also accounts for a significant portion of emissions—one report estimated 70 percent of hazardous particles came from vehicles.