A brown cloud of pollution now frequently shrouds much of northern India. It’s a growing health and environmental problem, and scientists are working to understand its many causes, which range from burning agricultural waste to auto emissions.

On a recent winter morning, the road out of Varanasi, India’s holiest city, was a swirl of exhaust fumes and construction dust. Fog that had disrupted travel for a couple of days had lifted, and everyone was on the move again. Yet a haze remained, with fields of wheat stretching away into a dim miasma. Approaching the industrial town of Kanpur, 200 miles to the northwest, the air was thick with pollution. Brick kilns flashed by, along with heaps of garbage. Smokestacks loomed in the distance. Roadside vegetation was coated with dust and ash.

For years, air pollution in India was seen as a problem largely for New Delhi, especially after the World Health Organization put India’s capital atop its list of most polluted global cities in 2014. But, as new data show, Delhi residents are hardly alone. A December report by IndiaSpend, an independent data journalism initiative, found that Varanasi had the country’s most toxic air, closely followed by other North Indian cities like Allahabad, Patna, and Kanpur. The air is so bad in Varanasi that of the 227 days when air quality was measured in 2015, not a single one met national air quality standards.

A combination of unique geography and rising emissions has turned the air foul in most cities throughout North India and, indeed, across the entire Indo-Gangetic plain, which stretches from Pakistan through India to Bangladesh. This pall of pollution is not a new discovery — scientists first identified a haze blanketing this region in the 1990s. But studies have shown that the pollution has dramatically increased over the past decade, with alarming effects on human health, including significantly shortened life expectancies. New research also suggests that the problem is more complex than previously believed. The toxic brew emanates not just from major polluters such as vehicles and coal-fired power plants, but from a surprising variety of sources, including the burning of agricultural waste and garbage, as well as the combustion of dung and other fuels for cooking and heating. It all adds up to “a regional soup of air pollution,” says Chandra Venkataraman, an aerosol specialist at the Indian Institute of Technology in Mumbai. This vast, brown cloud of pollution travels high into the atmosphere, spreading to Nepal and even across the great Himalayan barrier to Tibet. And it is affecting everything from local wheat yields to monsoon rainfall. New research also shows that pollutants from this regional haze could be lofted as high as the lower stratosphere — with yet-unknown effects on the regional climate.

The lives of Delhi residents are shortened an average of 6.3 years by air pollution.

The cost of this toxic cloud has now come into focus. Air pollution shortens the average Indian life by roughly 3.4 years, according to a study last year by the Indian Institute of Tropical Meteorology and the National Center for Atmospheric Research in Colorado. That number was higher for North India, including Delhi residents, whose lives were shortened an average of 6.3 years by air pollution. Exposure to particulate matter sized 2.5 microns or less — fine particles that slip into the lungs easily — led to 570,000 deaths in India in 2011, the study found, with the Indo-Gangetic region accounting for 42 percent of those mortalities. Another study in Delhi found that air pollution may be having irreversible effects on children’s lung function. Studies show that increased ozone pollution is also destroying millions of tons of crops in North India, as ground-level ozone enters leaves and damages plant metabolism. And there are signs that air pollution may be affecting local weather: Rising emissions have been linked with increased fog, the cause of widespread travel disruptions in the past few winters. Varanasi lies in the state of Uttar Pradesh at the center of the densely populated Indo-Gangetic basin. Rapid economic development and a soaring population have driven up pollution levels here in the past decade. Much of the rural population in the basin, which holds more than 40 percent of India’s 1.2 billion people, still burns wood and dung for cooking, which emits soot and other particles. NASA data show that across South Asia emissions of nitrogen dioxide from vehicles, power plants, and industries rose steeply from 2005 to 2014, with the highest increases over the Indo-Gangetic plain. Sulfur dioxide levels doubled in the same period because of the growing number of coal-fired power plants. Geography also plays a major role in this Indo-Gangetic pollution hotspot. The plain is landlocked, with the Himalayas largely blocking air pollution from escaping to the north, creating a valley effect, says Sachchidanand Tripathi, senior scientist at the Indian Institute of Technology in Kanpur. Seasons bring their own influence. In spring, dust blows in from the Thar Desert, adding large natural particles to the mix. In winter, everything converges to create peak pollution. Fine particulates from crop fires and industries dominate. Cold winds from the northwest lower temperatures. The air stills and the boundary layer — the part of the atmosphere closest to the ground — drops, confining pollution to the earth’s surface. Poor people begin to burn wood and trash to stay warm, further fueling the haze. “Every city is a plume, trapped,” says Tripathi.

A thick river of haze over the Indo-Gangetic Plain in January 2016, the result of a combination of urban and industrial pollution, agricultural and cooking fires, and a meteorological phenomenon known as a temperature inversion. NASA

In the past, government action has focused on curbing vehicular and industrial pollution. But of late, other sources have come under scrutiny. One is the burning of garbage and leaf litter, a practice that is ubiquitous across Indian cities. Roadside garbage fires in India produce a “rainbow” of toxins due to the variety of organic and plastic material, according to a 2016 study by Duke University researchers. Another study, published in October, found that municipal waste burning was the major contributor to the particulates settling on and discoloring the surface of the Taj Mahal. Widespread biomass burning by farmers produces some of the same toxic gases as industrial and vehicular emissions, says Vinayak Sinha, associate professor of earth sciences and chemistry at the Indian Institute of Science Education and Research. “Fossil fuel combustion is at least treated before release, there are scrubbers,” he says, “But with open burning, there is no real loss between what is emitted and what is transferred to the atmosphere.” Monitoring the air for two years near the fields of the Punjab region, near Delhi, Sinha and his team found that post-harvest fires led to a near-doubling in daily average concentrations of a category of gases known as volatile organic compounds, such as benzene and toluene. Annual concentrations of benzene, a known carcinogen, exceeded safe limits, the study found, increasing cancer risks for children and adults. In a separate study, the team found that the crop fires also led to higher levels of surface ozone. The substance is damaging to respiratory health; it’s also bad news for agriculture in a region known as the “bread basket of India.” A 2014 study from the Indian Institute of Tropical Meteorology estimated that in 2005 the top-producing agricultural states — including the North Indian states — lost 3.5 million tons of wheat and 2 million tons of rice due to damage from ozone. Losses from ozone-induced crop damage, the study estimated, would be enough to feed 94 million people.

Urban emissions contribute to surface ozone, too, of course, highlighting the interconnected nature of the problem. “While the focus is mostly on cities, emissions from surrounding areas influence the city,” says Prakash Bhave, senior air quality specialist at the International Center for Integrated Mountain Development, an intergovernmental think-tank in Kathmandu. “And emissions from cities across the plains influence the entire region.” For instance, a significant portion of the air pollution at Lumbini in Nepal, the birthplace of Buddha and a major tourist attraction, comes from India, a study found last year. In that case, says Bhave, “reducing local emissions is only going to do so much.” Emerging research also points to an intriguing, if tentative, link between pollution and winter fog. Studies by Tripathi’s lab at IIT Kanpur suggest that high levels of fine particulate matter could be increasing the frequency and density of fog. Pollution particles tend to form finer fog droplets that scatter more light, reducing visibility. But fog also seems to be changing these particles to make them absorb more moisture, says Tripathi, resulting in the formation of more fog droplets.

Research suggests that aerosols in the haze over this region are altering how much it rains and where it falls.