India is once again heading into the worst time of year for air pollution, a season where the country’s notoriously poor quality becomes even more toxic.

Soot, dust, ozone, and sulfur oxides are a growing threat for billions of people around the world. The World Health Organization reported this week that 93 percent of all children in the world breathe air with pollution levels that exceed their guidelines.

A whopping nine in 10 people on Earth breathe highly polluted air, and more than 80 percent of urban dwellers have to endure outdoor pollution that exceeds health standards, according to the WHO’s World Global Ambient Air Quality Database.

But even among countries gasping for breath, India stands out for air that is consistently, epically terrible.

In recent years, the first two weeks of November have brought the worst air pollution of the year to #Delhi, #India.



With daily averages already in the "very unhealthy" range, 2018 seems set to repeat the pattern. #AirQuality pic.twitter.com/Irals60HFS — Robert Rohde (@RARohde) October 31, 2018

Drawing on measurements and calculations as of 2016 from air monitoring stations in 4,300 cities, the WHO reported in March that India’s cities suffer the most.

When you look at the database’s ranking of particulate pollution in cities, 11 of the 12 cities with the highest levels are located in India. Kanpur, India, population 3 million, tops the list with a yearly average of 319 micrograms per cubic meter of PM2.5, the most hazardous particle commonly measured. (Bamenda, Cameroon, is the one city outside of India in the top 12.)

Cities with the highest small particulate measurements in the world Country City PM2.5 (µg/m³) Country City PM2.5 (µg/m³) India Kanpur 173 India Faridabad 172 India Varanasi 151 India Gaya 149 India Patna 144 India Delhi 143 India Lucknow 138 Cameroon Bamenda 132 India Agra 131 India Muzaffarpur 120 India Srinagar 113 India Gurgaon 113

Many different air pollutants can impact health — nitrogen oxide, carbon monoxide, and ozone, among them. But the database classifies air pollution in two ways: by PM2.5, particles smaller than 2.5 microns in diameter, and by PM 10, particles that are 10 microns in diameter. The smaller PM2.5 particles from sources like open flames and diesel exhaust can linger in the air longer and penetrate deeper into the lungs than larger particles, which is why they’re the bigger concern for health officials and a high-priority target for reduction.

Technically, the WHO doesn’t treat its data as a ranking but rather a measurement of where risks are. But it’s clear from the report that India is one of the riskiest countries in the world to breathe, up there with Bangladesh and Georgia. When it comes to comparing PM10 measurements of the world’s largest cities, India’s capital Delhi comes in with an annual average of 292, ahead of Cairo (284), Dhaka (147), Mumbai (104), and Beijing (92), the Washington Post noted.

Frequent unhealthy levels of pollution from sources ranging from vehicles to the burning of coal and wood for cooking, dust storms, or forest fires affect most of the country. India’s hills and mountains also act as basins that trap toxic air over vast swaths of the country, sometimes making the air too dangerous to breathe. Just check out this map of air pollution from October:

But how did it get so bad? Let’s take a closer look.

Poverty and air pollution go hand in hand

Though you can find unhealthy air in pretty much any country, the worst pollution these days is accumulating in developing countries with growing populations. The highest-ranking city from an OECD country in the PM2.5 list is Hobart, Australia, coming in at 206.

“Air pollution threatens us all, but the poorest and most marginalized people bear the brunt of the burden,” said Tedros Adhanom Ghebreyesus, the WHO’s director general, in a statement.

Cookstoves, heating fuel, and kerosene lighting are all common sources of pollution in big cities in developing countries. Poor governance is also a major factor in pollution since lax enforcement of standards for car exhausts, crop burning, or dust from construction sites leads to more particulates in the air.

Some countries like China are making progress. Cities in China have moved down in the rankings, and the government is seeking reforms to keep pollution in check as dirty air has become an increasingly volatile public concern.

Governing air pollution has proved to be much trickier in India.

Why India’s air is so bad

Delhi, India’s capital region, home to nearly 19 million people, is notorious for choking air that is now turning the iconic white marble walls of the Taj Mahal green.

The city faced a major air quality crisis late last year as the literally off-the-charts pollution forced flight cancellations, caused traffic accidents, closed schools, and sparked protests. One minister described Delhi as a “gas chamber,” and the city declared a public health emergency.

Dozens of other Indian cities are dealing with severe pollution too, but many of the particulates that blanket the metro regions originate in rural areas, and rural areas are just as badly affected by poor air, if not more so. In 2015, about 75 percent of deaths linked to air pollution in India, some 1.1 million people, occurred in rural areas.

Two-thirds of India’s population still lives outside of cities, and 80 percent of these households rely on biomass like wood and dung for cooking and heating. Agricultural practices like burning crop stubble also remain widespread.

This smoke can then waft over major cities such as Chennai and Mumbai, where it commingles with traffic exhaust, factory emissions, and construction dust. It can also get trapped by inland by features like hills and mountains, leaving few areas in the country where Indians can breathe easy.

“This region is land-locked — pollution cannot dissipate quickly — and does not have the advantage of the coast like Mumbai or Chennai,” Anumita Roy Chowdhury, head of the air pollution and clean transportation program at the Delhi-based advocacy organization Center for Science and Environment, told the Indian Express. “Also, a lot of the smaller cities have poor waste management, there is a lot of burning, solid fuel use, they are moving from non-motorised to motorised transport. Chulhas [cookstoves], we know, contribute to 25 percent outdoor pollution in India.”

These stoves also cause pollution indoors, which is especially dangerous for children, according to the WHO. The latest report found that mothers in India were more likely to deliver underweight babies in households that used wood or dung as cooking fuel. Children were more likely to develop asthma and acquire infections like tuberculosis.

The other major cause of India’s pollution problem is politics. Kirk Smith, a professor of global environmental health at the University of California Berkeley, observed that there are anti-pollution laws in India but they aren’t enforced well, so while major cities can ban pollution sources like brick kilns from within their boundaries, they can’t stop exhaust from blowing over from the perimeter.

Controlling this kind of pollution requires coordinating across city and provincial boundaries over a geographic basin, but rural politicians and urban politicos in India have vastly different constituencies, making it difficult for urbanites to convince farmers to use less polluting practices or for rural people to ask city dwellers to drive less.

“Outdoor air pollution is pretty much a governance problem,” said Smith.

He also noted that the WHO’s list of most polluted cities should be viewed with caution. Pollution can vary drastically over small areas, and many of the cities on the list are reporting data from just one air quality monitor, which may not be representative of the true picture of pollution in the city.

As for India’s sudden rise in the rankings, a big factor is that pollution in many of these cities is being measured and reported for the first time, meaning the problem may have always been there but hadn’t drawn the same level of attention.

With more measurements and reporting, India’s pollution problems will likely appear worse before they get better. But gathering this information is nonetheless vital for the counteroffensive against poor air quality.