GENEVA » Almost everyone in large cities in poor and middle-income countries faces excessively high air pollution, a growing problem that is killing more than 3 million people prematurely each year and “wreaking havoc on human health,” the World Health Organization said today.

The U.N. health agency says more than four out of five city dwellers worldwide live in cities that don’t meet WHO air quality guidelines — 98 percent in poorer countries and 56 percent even in high-income countries.

“Ambient air pollution, made of high concentrations of small and fine particulate matter, is the greatest environmental risk to health, causing more than 3 million premature deaths worldwide ever year,” WHO said.

The findings are part of WHO’s third Global Urban Ambient Air Pollution Database, which examines outdoor air in 3,000 cities, towns and villages — but mostly cities — across 103 countries. It’s based on country reports and other sources for the period from 2008 to 2013, although some nations, including many in Africa, don’t contribute data.

An accompanying U.N. news release said global urban air pollution levels rose 8 percent over that time span “despite improvements in some regions,” and noted that people face a higher risk of strokes, heart disease, lung cancer and respiratory diseases as air quality worsens.

“Urban air pollution continues to rise at an alarming rate, wreaking havoc on human health,” said Dr. Maria Neira, a WHO director for environment and public health. “At the same time, awareness is rising and more cities are monitoring their air quality. When air quality improves, global respiratory and cardiovascular-related illnesses decrease.”

The agency’s database named Zabol, Iran, as the city with the highest annual mean concentration of particulate matter of less than 2.5 microns in diameter — a key measure of air pollution that could damage health. By that measure, India stood out as home to more than half of the 21 most polluted cities on the WHO list.

New Delhi, which had previously topped the list, dropped to No. 11. The Indian capital has managed to decrease its annual average concentration of particulate matter by about 20 percent from 2013 to 2015. The change coincides with a series of air-clearing measures including banning older cars and cargo trucks from the city, introducing steep fines for construction pollution or garbage burning, and shutting down an old coal-fired power plant.

“New Delhi has succeeded in arresting the trend, which shows that if you take action, you will see results,” said Anumita Roychowdhury of the Delhi-based Center for Science and Environment.

But India overall is still struggling. The WHO data showed four other Indian cities — Gwalior, Allahabad, Patna and Raipur — surpassing New Delhi to rank within the world’s top ten polluted cities — coming in second, third, sixth and seventh, respectively.

In Europe, the Bosnian city of Tuzla had the worst air on the continent, although its pollution level was far less than in much larger cities in India, Pakistan and China. The worst air pollution in a U.S. city was in California’s Visalia-Porterville area, but it too ranked far lower — 1,080th — than many developing-world cities.

Paris came in at 1116th most polluted, London at 1,389th and the New York-Northern New Jersey-Long Island region at 2,369th.

The database’s cleanest town was Sinclair, Wyoming, ranking 2,973th with a particulate matter (PM 2.5) of 3, compared to 217 for the most polluted city, Zabol.

WHO guidelines urge a PM 2.5 rating of 10 or less. But WHO technical officer Sophie Gumy cautioned against drawing too many comparisons between cities from different continents, saying the data from various sources doesn’t always line up exactly.

Some countries publish official air pollution statistics, but others don’t. Some of WHO’s data comes from academia. Some monitoring stations could be near highways, which could skew the results. Some data includes night air pollution, which tends to be less than during rush hour, but other national reports do not. And rainy seasons can rinse away air pollution, while drier climates can let the pollution linger.

The agency praised efforts by policymakers to promote cleaner transportation, more efficient energy sources and better waste management.

“More than half of the monitored cities in high-income countries and more than one-third in low- and middle-income countries reduced their air pollution levels by more than 5 percent in five years,” the agency said.